WBCS

WBCS Preliminary(Biology): Endocrine System

noreply@blogger.com (WBCSguru) — Sat, 04 Sep 2010 14:53:00 +0000

Glands are of two types:
1. Exocrine glands: Exocrine glands are those which pour their secretions into a duct. For example, sweat glands, tear glands, etc.
2. Endocrine glands: Endocrine glands are those which are richly supplied with blood vessels and pour their secretions directly into the blood vessels. The secretions reach their target through blood. These glands are called the ductless glands as they do not have ducts. For example, thyroid, adrenal, etc.
Hormones: The secretions of the endocrine glands are called hormones. Hormones are grouped into three classes based on their structure:
1. Steroids: Steroids are lipids derived from cholesterol. e.g. Testosterone is the male sex hormone. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
2. Peptides: Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.
3. Amines: Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla.
The human endocrine system consists of the following glands:
- Hypothalamus
- Pineal
- Thyroid
- Parathyroid
- Pituitary:
- Thymus
- Adrenal
- Pancreas
- Ovary in female
- Testes in male
Hypothalamus: Forms a part of fore - brain, secretes neurohormones, which effect the release of hormones from pituitary.
Pituitary gland: Also called Master Gland. It is the smallest endocrine gland. It is a pea shaped gland that is located below the hypothalamus in the brain. It is under the control of the hypothalamus and in turn controls many functions in the body. Hormones Produced by Pituitary:
- Thyroid stimulating hormone (TSH): Stimulates thyroid gland to produce thyroxine.
- Growth Hormone (GH): Stimulates overall growth of the body. Its deficiency causes dwarfism and over-production causes gigantism.
- Adrenocorticotrophic hormone (ACTH): Influences the secretion from the cortex of adrenal glands.
- Follicle stimulating hormone (FSH): It is secreted in males and females both. In males, it stimulates spermatogenesis and development of seminiferous tubules. In females, it stimulates formation and growth of ovarian follicle in ovary.
- Luteinizing hormone (LH): In females only. Final maturation of ovarian follicleand ovulation takes place by LH only.
- Lactogenic Hormone : Initiates milk production in the pregnant females.
- Prolactin maintains the pregnancy and stimulates the secretion of milk.
- Oxytocin helps in contraction of uterus during delivery.
Pineal : It is a small round gland in the brain. It secretes melatonin that regulates the sexual cycle.
Thyroid Gland: It is the largest endocrine gland located in the neck between the trachea and larynx. It is a butterfly-shaped, bilobed gland that is situated at the base of the larynx. The two lobes are joined by an isthmus. The hormone secreted is thyroxine. Controls BMR (Basal Metabolic Rate).
- Thyroxine : Regulates physical, mental and sexual development. A diet, poor in iodine, which is insufficient for the synthesis of thyroxin, leads to simple goitre. Oversecretion results in exophthalmic goitre in which the person shows a marked increase in metabolic rate, protrusion of eyes, rapid heart rate and shortness of breath.
- Thyrocalcitonin : It controls the amount of calcium in the body.
Parathyroid Gland: Secretes Parathormone, which is also known by the name of Collip's Hormone. Deficiency of parathormone causes brittle bones. Oversecretion of parathormone softens the teeth and bones.
Thymus Gland: Situated near the heart. It secretes hormone called thymosin. Thymosin helps in the production of lymphocytes.
Pancreas: It is a narrow gland present at the junction of stomach and duodenum. It is an exocrine as well as an endocrine gland. Its 3 types of cells secrete 3 different hormones. Beta cells secrete Insulin which controls the amount of sugar in the blood. Its hyposecretion leads to Diabetes Mellitus. Alpha cells secretes Glucagon which increases blood sugar level. The delta cells secrete somatostatin that inhibits the secretion of insulin and glucagon.
Adrenal: The adrenal glands are present on top of the kidneys and appear cap-like on top of each kidney. Consists of 2 Distinct Parts : Outer cortex and inner medulla.
- Adrenal Medulla secretes Adrenaline or epinephrine which effects liberation of glucose from glycogen stored in liver and increases the rate of metabolism. Its over - secretion leads to increased saliva flow, tears, bile and sweat, quickens heart beat, speeds up respiratory activities.
- The adrenal cortex secretes hormones like the glucosteroids, mineralocorticoids and cortisones. Glucosteroids increase the blood sugar level in times of stress by converting protein into glucose.
Testes produce testosterone which produces the secondary sexual characteristics like moustache and beard.
Ovaries along with the egg-production, secrete oestrogen from the mature follicle that produces the secondary sexual characteristics like enlargement of breasts. After ovulation, another hormone, progesterone is produced from corpus luteum that maintains the pregnancy.
Plant hormones or phytohormones: The phytohormones have been put in five different categories based on their actions.
- Auxins: Auxins are phytohormones that are mainly concerned with cell enlargement. It is produced by shoot apex, young leaves and roots (to some extent). They only move in upward direction through phloem or xylem.
- Gibberellins: Gibberellins are plant hormones that are mainly responsible for cell elongation. They cause the cells to grow in length. They are synthesised in embryos, young leaves, root tips, buds and seeds. They move up or down in the plant body through xylem or phloem.
- Cytokinins: They are phytohormones that induce cell divisions even in mature tissues. There are many types of cytokinins present. For example, zeatin, a cytokinin present in maize grains. Cytokinins are synthesized in the fruits and seeds where rapid cell division takes place.
- Abscissic Acid (ABA): It is a growth inhibitor that results in dormancy and abscission. It is synthesized in stem, leaves, fruits and seeds.
- Ethylene:Ethylene is a gaseous growth regulator that speeds up the ripening process. It is a gas produced by most of the plant organs. Chemically, ethylene (ethene) is an unsaturated hydrocarbon

WBCS(EXECUTIVE) ETC, EXAMINATION, 2011

noreply@blogger.com (WBCSguru) — Sat, 21 Aug 2010 07:50:00 +0000

Advertisement of WBCS(Executive) ETC. EXAMINATION, 2001 has been published by the Public Service Commission, West Bengal on 21/08/2010 on leading newspapers.
Brief Information:

QUALIFICATIONS:
1. A Degree of Rrecognised University
2. Ability to read, write and speak in Bengali ( not required for recruitment in the case of Nepali speaking candidates from hill areas of Darjeeling).
AGE:
1. On First January, 2011-for candidates competing for Group(s) A, C and D- not below 21 years but not more than 32 years that is born not earlier than 02/01/1979 and not later than 01/01/1990.
2. For Group B- not below 20 years and not more than 32 years that is born not earlier than 02/01/1979 and not later than 01/01/1991.
3. Upper age limit is relaxable by 5 years for SC/ST candidates of West Bengal and by 3 years for BC candidates of West Bengal and upto 45 years of age for persons with disabilities having Physical Disability of 40% and above.
4. SC/ST/BC candidates not belonging to the state of West Bengal shall be trated as General Candidates.
The Preliminary Examination will be held at various centres in West Bengal in January, 2011 or thereabout.
The Main Examination will be held in Kolkata only in June, 2011 or thereabout.
For details click on PSC's WEBSITE: pscwb.org.in

WBCS Preliminary (Biology): Nervous System

noreply@blogger.com (WBCSguru) — Mon, 16 Aug 2010 15:41:00 +0000

The nervous system perceives the changes around us through our senses. It controls and coordinates all the activities of the muscles in response to the changes outside. It also conducts messages between different parts of the body.

The units of nervous system are specialised cells called the neurons. A neuron consists of dendrites, a cell body and an axon. Around the cell body are short sensory projections called the dendrons. The fine branches of dendrons are called the dendrites. These short fibres receive messages and pass them on to the cell body or the cyton. The axon carries an impulse transmitted to it by the cell body to another neuron or to an effector muscle or gland. There are three types of neuron sensory, motor and association. A synapse is the junction between two neuron. A bunch of neurons together make up a nerve.
In mammals these neuron comprise 2 types of nervous system i.e. Central nervous system and peripheral nervous system. The brain and spinal cord make up the central nervous system (CNS). The Central Nervous System (CNS) is composed of the brain and spinal cord.
Nerves are of three types based on the types of neurons they carry. They are:
1. Sensory Nerves or the Receptor Nerves: They are made up of only sensory neurons. For example, the cranial nerves that conduct impulses from the organs to the central nervous system.
2. Motor Nerves or the Effector Nerves: They are made up of only motor neurons. For example, the cranial nerves that conduct impulses from the central nervous system to the muscles and glands (effector).
3. Mixed Nerves : The nerves that are made up of both sensory and motor neurons. For example, all spinal nerves.
The human nervous system can be divided into three main parts:
1. Central nervous system: It is made up of the brain and the spinal cord which is the continuation of the brain. Brain and spinal cord are surrounded by membranes called the meninges.
  - The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The brain consists of gray matter (40%) and white matter (60%) contained within the skull. The brain has three main parts: the cerebrum, the cerebellum, and the brain stem (medulla). Cerebrospinal fluid (CSF) surrounds the brain. Although the brain is only 2% of the bodys weight, it uses 20% of the oxygen supply and gets 20% of the blood flow. Brain contained in the skull called cranium. The brain appears as three distinct but connected parts: the cerebrum, cerebellum and the brain stem a central core that gradually becomes the spinal chord.
  - Cerebrum: The cerebrum is the largest part of brain and makes up 85% of the brains weight. This is the thinking part of the brain.The cerebrum is made up of two halves, with one on either side of the head. The right half of the cerebrum controls the left side of your body, and the left half controls the right side. These two hemisphere is jointed by a nerve fibres known as corpus callosum. The surface of each cerebral hemisphere shows many convolutions called Gyri separated by sulci(depressions).
  - Thalamus: It is an area which coordinates the sensory impulses from the various sense organs - eyes, ears and skin and then relays it to the cerebrum.
  - Hypothalamus: The hypothalamus is composed of scattered masses of grey matter in white matter at the base of the brain. Although it is the size of only a pea (about 1/300 of the total brain weight), the hypothalamus is responsible for some very important functions. It is responsible for regulation of temperature, hunger, thirst and emotional reactions. The hypothalamus also controls the pituitary.
  - Mid Brain: It is a small portion of the brain that serves as a relay centre for sensory information from the ears to the cerebrum. It also controls the reflex movements of the head, neck and eye muscles. It provides a passage for the different neurons going in and coming out of the cerebrum.
  - Hind Brain : It consists of cerebellum, pons and medulla oblongata:
    - Cerebellum: The cerebellum is at the back of the brain, below the cerebrum. It is smaller than the cerebrum at only 1/8 of its size. It controls our balance, movement, and coordination (how your muscles work together). Because of our cerebellum, we can stand upright, keep your balance, and move around.
    - Pons: Pons literally means bridge. It serves as a relay station between the lower cerebellum and spinal cord and higher parts of the brain like the cerebrum and mid brain.
    - Medulla Oblongata :It is a small region of the brain. It is hidden as it is well protected because of its importance. It has the cardiovascular centre and the breathing centre. It also controls activites such as sneezing, coughing, swallowing, salivation and vomiting.
  - Spinal Cord: spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column. It is covered by the same meninges as the brain and its main function is conduct impulses to and from the brain and acts as a reflex centre.
2. Peripheral Nervous System: The peripheral nervous system is made up of nerves that connect the different parts of the body (peripheral tissues) to the central nervous system.
Reflex Actions: When the body instantly responds to an external stimulus, it is called reflex action. Reflex may be natural or conditioned. Natural reflex does not involve the brain whereas conditioned reflex involves the brain. The path along which the impulses travel in a reflex are called the reflex arc.
Animals have a nervous system for controlling and coordinating the activities of the body. But plants have neither a nervous system nor muscles.

WBCS Preliminary (Biology):Excretion

noreply@blogger.com (WBCSguru) — Wed, 04 Aug 2010 16:23:00 +0000

In living organisms energy is produced by metabolically burning food. But this is accompanied by the formation of a variety of by-products which are harmful to the organism. These wastes have to be eliminated. The removal of harmful and unwanted toxic waste products of metabolism is known as excretion. Different organisms use varied strategies to do this. Many unicellular organisms remove these wastes by simple diffusion from the body surface into the surrounding water and complex multi-cellular organisms use specialised organs to perform the same function.

Green plants in darkness or plants that do not contain chlorophyll produce carbon dioxide and water as respiratory waste products. Carbon dioxide released during respiration gets utilized during photosynthesis. Oxygen itself can be thought of as a waste product generated during photosynthesis. Plants can get rid of excess water by transpiration. For other wastes, plants use the fact that many of their tissues consist of dead cells, and that they can even lose some parts such as leaves. Many plant waste products are stored in cellular vacuoles. Waste products may be stored in leaves that fall off. Other waste products are stored as resins and gums, especially in old xylem. Plants also excrete some waste substances into the soil around them.
In lower animals carbon dioxide is eliminated directly into the environment through the body surface. In higher animals it is excreted out along with the exhaled air through the lungs. Excess water is excreted in the form of urine and sweat. Ammonia, Urea, Uric Acid , Amino Acids are the nitrogenous waste products produced by animals.
The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
Kidneys : Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.
- The kidneys are reddish brown, bean-shaped organs situated in the abdominal cavity, one on either side of the vertebral column in the lumbar region of the body. They lie asymmetrically, the right kidney being lower than the left as the right side of the abdominal cavity is occupied by the liver. Each kidney is 10 cm long, 6 cm wide and 4 cm thick and weighs 200 - 250 g in adults. A thin, tough, fibrous whitish capsule envelops each kidney. The outer surface of each kidney is convex while the inner surface is concave.
- The kidneys function as a pair of filters through which about one litre of blood circulates each minute. The entire blood in the body passes through them in 5 - 6 minutes. In a day it filters 1800 litres of blood which is 400 times the blood volume.
- A normal adult excretes 1 - 1.8 litres of urine per day.
- The nephron is the kidneys functional unit. Each kidney contains from one to two million nephrons. Each nephron has a network of capillaries called glomerulus which fits into a cup called the Bowman's capsule and a long tubule consisting of proximal convoluted tubule, Henle's loop, distal convoluted tubule and connecting tubule.
- How is urine produced? :The purpose of making urine is to filter out waste products from the blood. Just as CO₂ is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys.
- Hormones of the Kidneys: The human kidney is also an endocrine gland secreting two hormones: 1) Erythropoietin (EPO), Calcitriol, the active form of vitamin D as well as the enzyme renin.
Diseases of Kidney :
- Acute Renal Failure: No urine is formed by kidney and the wastes and water accumulates in the body. It can be treated by dialysis, by an artificial filtration of blood through semi permeable membaranes.
- Kidney stone: It is also known as renal calculi, kidney stones are the result of crystallization of certain substances found in urine, including calcium, phosphate, oxalic acid, and uric acid. Stones may form in the urine collecting area (pelvis) of the kidney, as well as the ureters (narrow tubes connecting the kidney to the urinary bladder).
- In case of kidney failure, an artificial kidney can be used. An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis. A matching kidney from another person may also be transplanted.
- Principle of dialysis Blood is made to flow into the dialysis machine made of long cellulose tubes coiled in a tank having a dialyzing solution. Waste substances diffuse out of lood into tank. The cleansed blood is pumped back into patient.
- Urinary Bladder:This is a large muscular storage sac that collects urine from both the kidneys through the ureters. As the urine gets drained into the bladder its volume increases. The mouth of the bladder is guarded by a tight ring of muscle called the sphincter which regulates the opening or closing of the bladder. When the sphincter relaxes, urine is released out through the urethra.
- Urethra:This is a short muscular tube that carries urine at intervals from the urinary bladder to the outside. The base of the urethra is also guarded by a sphincter which keeps the urethra closed except while passing urine.

WBCS Preliminary (Biology): Circulatory System

noreply@blogger.com (WBCSguru) — Sat, 31 Jul 2010 15:41:00 +0000

Plants take water and mineral nutrients from the soil through the roots and transport it to the leaves. The leaves prepare food for the plant, using water and carbon dioxide during photosynthesis. Plants absorb water and minerals by the roots. The roots have root hair. The root hair increase the surface area of the root for the obsorption of water and mineral nutrients dissolved in water. The root hair is in contact with the water present between the soil particles.
Plants have pipe-like vessels to transport water and nutrients from the soil. The vessels are made of special cells, forming the vascular tissue. The vascular tissue for the transport of water and nutrients in the plant is called the xylem. The xylem forms a continuous network of channels that connects roots to the leaves through the stem and branches and thus transports water to the entire. Two types of xylem cells are involved in transport of water - tracheids and vessels. They are dead cells with lignified walls. They are joined end to end forming a capillary system to draw water up the plant.
Leaves synthesise food. The food has to be transported to all parts of the plant. This is done by the vascular tissue called the phloem. Phloem is a living tissue. Sieve tubes and companion cells are the phloem cells involved in the transport of food. Other than sucrose, phloem also transports hormones (from the site of synthesis to the site of action) and some of the mineral ions (from the leaves about to fall to the other regions). The transport of soluble substances like the sugars, amino acids and hormones by the phloem is called translocation.
Transpiration is the loss of water from the aerial parts of the plant, mainly through the stomata of the leaves. In tall trees transpiration pulls water up the xylem. The rate of transpiration is affected by many factors such as light, temperature, availability of soil water and atmospheric humidity.
Animals in general have a higher metabolic rate than the plants and thus require a more efficient transport system. There are two types of blood circulatory systems:
- Open circulatory system: In open circulatory system the blood vessels are open-ended as they open into the common cavities called the haemocoel. It is seen in insects.
- Closed circulatory system: In closed circulatory system the blood always remains inside the blood vessels and never comes in direct contact with the cells. It is seen in mammals including man.
Blood is an alkaline fluid that consists of the liquid portion called plasma and three types of corpuscles. Plasma is yellow coloured fluid consisting of water (92%), proteins(6-9%) and 1% minerals. Plasma transports red and white blood cells and platelets throughout the body. It also delivers nutrients to cells and picks up cell waste products.
RBC: One type of cells are the red blood cells (RBC) which contain a red pigment called haemoglobin. Haemoglobin bind with oxygen and transports it to all the parts of the body and ultimately to all the cells. It will be difficult to provide oxygen efficiently to all the cells of the body without haemoglobin. The presence of haemoglobin makes blood appear red. The red blood cells are synthesised in the bone marrows of ribs, sternum and vertebrae at the rate of 1.2 million cells per second. The life span of the cells is only about 120 days. They are destroyed in the liver. The iron part is retained and the pigment is excreted in the bile juice as bilirubin.
WBC: The blood also has white blood cells (WBC) which fight against germs that may enter our body. They are also called leucocytes. They lack haemoglobin and are therefore colourless. They are nucleated and amoeboid. The WBCs are involved in the production of antibodies that either neutralise, kill or poison the germs. The WBCs can be induced to produce antibodies with the help of vaccinations thus preparing the body for an attack.
Platelets: The clot is formed because of the presence of another type of cells in the blood, called platelets. They are also called the thrombocytes. They number 250,000 to 400,000 per cubic mm of blood. Their life span is 8 to 14 days.
Blood clotting: It is internal mechanisms of animals to prevent blood loss at the time of injury. During the blood clotting, the blood platelets produce an enzyme called thromokinase which forms prothrombin protein in the plasma. It combines with calcium ion to form thrombin. It convert the soluble plasma protein(fibrinogen) to form fibrin threads. corpuscles get entangled in these threads and forms clot.
Blood Groups: There are four types of blood among the humans. The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies. The antigens are located on the red blood cells and the antibodies are in the blood plasma. based on this, four blood groups are identified. They are denoted by the letter A,B, AB, O (null). The blood that has antigen A belongs to group A, the blood that has antigen B belongs to group B, the blood that has both A and B belongs to group AB and the blood which has neither belongs to group O. The presence of these antigens is determined genetically. Thus, there are four types of blood groups A, B, AB and O.
The blood also contains antibodies which act against these antigens. The antibodies react to the presence of proteins in the foreign bodies. There are two types of antibodies, corresponding to the types of antigens. The two antibodies are : (a) antibody a and (b) antibody b. Anti-body a reacts to the antigen A and antibody b reacts to the antigen b. Thus, the blood of A group that contains antigen A will contain antibody b and not antibody a. The blood of B group that contains antigen B will contain antibody a and not antibody b. The blood of AB group that contains antigens A and B will not contain either antibody a or antibody b. The blood of O group that does not contain either antigen A or antigen B will contain both antibody a and antibody b.
Blood transfusion: Transfusion is the replacement of lost blood of a person with the blood of another person. During transfusion, the person receiving blood is called recipient and the person donating blood is called donor. The general rule for blood transfusion is the donors red cells must be compatible with recipients plasma. Anti A plasma agglutinates A red cells, and anti B plasma agglutinates B red cells. People with O blood group can give blood to any group because they do not contain A and B anti bodies. So they are called universal donors. People with AB blood group are called universal recipients because they can accept blood from any group. So they are known as universal receipients.
Rhesus Factor: About 85% of people also have a so called Rh factor on the red blood cells surface. This is also an antigen and those who have it are called Rh+. Those who have not are called Rh-. Rh-ve blood can be safely given to Rh+ve person. However, the transfusion of Rh+ve blood to Rh-ve blood needs to be monitored. During the first transfusion, the Rh factor in the Rh+ve blood will induce the production of anti-Rh antibodies in the Rh-ve blood.
Lymph: It is a yellowish fluid carried in the lymphatic system. Lymph is like blood but it contains white blood cells and red blood cells are absent. It fights against germs.
In human circulatory system the blood is flows in closed blood vessels called the arteries, veins and their capillaries. Arteries and veins have elastic and muscular walls. Capillaries lack muscles.
- Arteries: carry oxygen-rich blood from the heart to all parts of the body. Since the blood flow is rapid and at a high pressure, the arteries have thick elastic walls. The two main arteries leaving the heart are: Aorta- The branches of which supply blood to different parts of the body and Pulmonary Artery- The artery that takes blood to the lungs.
- Veins: are the vessels which carry carbon dioxide-rich blood from all parts of the body back to the heart. The veins have thin walls. There are valves present in veins which allow blood to flow only towards the heart.
Every beat of the heart involves a sequence of events called the cardiac cycle. This consists of three major stages: the atrial systole, the ventricular systole, and the complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood in to the ventricles. Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the sound of the valves closing which produces the familiar beating sounds of the heart. The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood has left, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves the relaxation of the atria and ventricles in preparation for new blood to enter the heart.
Heart: The heart is an organ which beats continuously to act as a pump for the transport of blood, which carries other substances with it. Human heart is four chambered. There are two receiving chambers, the auricles and two pumping chambers, the ventricles. Oxygen-poor blood enters the right atrium of the heart (via veins called the inferior vena cava and the superior vena cava). The blood is then pumped into the right ventricle and then through the pulmonary artery to the lungs, where the blood is enriched with oxygen (and loses carbon dioxide). The oxygen-rich (oxygenated) blood is then carried back to the left atrium of the heart via the pulmonary vein. The blood is then pumped to the left ventricle, then the blood is pumped through the aorta and to the rest of the body. This cycle is then repeated. Every day, the heart pumps about 7,600 liters of blood, beating about 100,000 times.
Heart contains valves which prevents mixing of blood in the four chambers. The wall of the heart is very muscular and does not tire. These muscles contract and relax rhythmically. This rhythmic contraction followed by its relaxation constitute a heartbeat.
Blood Pressure: The force that blood exerts against the wall of a vessel is called blood pressure. This pressure is much greater in arteries than in veins. The pressure of blood inside the artery during ventricular systole (contraction) is called systolic pressure and pressure in artery during ventricular diastole (relaxation) is called diastolic pressure. The normal systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg. Blood pressure is measured with an instrument called sphygmomanometer. High blood pressure is also called hypertension and is caused by the constriction of arterioles, which results in increased resistance to blood flow. It can lead to the rupture of an artery and internal bleeding.
Heart Attack: Heart attack is clinically Also called coronary thrombosis. The blood vessel supplying blood to the heart is blocked. This affects/stops the functioning of the heart resulting in heart attacks. If the blocked artery is one of the arteries supplying to the brain, it causes a condition called stroke. Stroke is the inactivation of a certain region of the brain which controls a particular activity. It may be sight, limb movements, etc.

WBCS Preliminary(Biology): Respiration

noreply@blogger.com (WBCSguru) — Thu, 29 Jul 2010 15:11:00 +0000

Respiration is essential for survival of living organisms. It releases energy from the food. Respiration is defined as the catabolic biochemical process during which organic compounds break down in order to release energy. The organic compounds that are broken down are called the substrates. Glucose is the most common substrate. The general equation for respiration is: C₆H₁₂+ 6O₂--->6CO₂+6H₂O+38ATP (Adenosine Triphosphate).
Breathing provides the mechanism necessary to take in oxygen and give out carbon dioxide that is a waste gas. It is a physical process and part of respiration. Respiration is a biochemical and physical process.
Respiration takes place in the following three stages:
1. External Respiration: The exchange of gases between the environment and the body is called external respiration or gaseous exchange.
2. Internal Respiration: The exchange of gases between the body spaces or fluids and the cells is called internal respiration or tissue respiration. The area over which this exchange takes place is called the respiratory surface.
3. Cellular Respiration: The process of breakdown of food in the cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cells of all organisms.
In lower plants (and also protozoans) exchange of gases takes place through the general body surface as they are not highly modified or specialised. Also, the body surface allows the diffusion of gases. Gaseous exchange in higher plants takes place through the stomata in the leaves, lenticels in the stem and general surface of the roots.
- Stomata are openings generally present on the lower surface of the leaves through which the gases and water vapour diffuse in and out easily. The oxygen diffuses in through the stomata and then enters the leaf cells. Similarly, the carbon dioxide produced by the leaf cells diffuses out through the stomata.
- In woody stems, the entire surface is covered by bark which is impervious to gases or water. However, there are certain openings or pores in the layer of bark. These are called the lenticels. They are visible slightly more raised than the general surface of the stem. At the base of the lenticels are loosely arranged cells which allow the diffused gases to pass through them.
- Gases diffuse in and out of the general surface of the roots. The gases are found in the soil surrounding the roots. Plants which grow in salty water show specialised roots called the pneumatophores. These are roots growing out of the surface of water with numerous pores on their surface.
In lower animals, gas diffusion takes place through a moist surface membrane, as in flatworms; through the thin body wall, as in earthworms; through air ducts, or tracheae, as in insects; or through specialized tracheal gills, as in aquatic insect larvae. In the gills of fish the blood vessels are exposed directly to the external (aquatic) environment. Oxygen–carbon dioxide exchange occurs between the surrounding water and the blood within the vessels; the blood carries gases to and from tissues. In higher vertebrates, oxygen-poor, carbon dioxide–rich blood from the right side of the heart is pumped into the lungs and flows through the net of capillaries surrounding the alveoli, the cup-shaped air sacs of the lungs.
Human Body - Respiratory system: The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the respiratory systems means of getting oxygen to the blood. Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adults lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale.
- The total lung capacity of an adult human is 5.0 litres
- Tidal air : Volume of air entering and leaving the lungs during normal breathing (.5 litres)
- Vital capacity : Volume of air exhaled after a forceful breathing (3.5 litres)
- Residual air : Volume of air which remains after forceful expiration (1.5 litres)
- Lung volume and other respiratory volumes can be measured by spirometer.
The oxygen is transported to the different parts of the body through the circulatory system. The red blood cells in the blood have pigments called haemoglobin which transport oxygen to the tissues. From the tissues, most of the carbon dioxide is transported as bicarbonate ions in the plasma (outside red blood cells) of the blood.
We know Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is trapped in the form of ATP (adenosine triphosphate) for use by all the energy-consuming activities of the cell. The process occurs in two phases:
1. glycolysis, the breakdown of glucose to pyruvic acid
2. the complete oxidation of pyruvic acid to carbon dioxide and water. This process called aerobic respiration. When the respiration takes place with out oxygen it is known as anaerobic respiration. The end product of anaerobic respiration is carbon dioxide and chemical such as lactic acid and ethanol. When the micro organisms respire anaerobically we call it as fermentation. e.g. fermentation of yeast in the preparation of ethanol.
3. Breathing Mechanism: The physical movements associated with the gaseous exchange are called breathing. They are controlled by the respiratory centre of the medulla oblongata in the human brain. Thus, the breathing movements are involuntary to a large extent. However, we can control the rate of breathing and the extent of breathing but not for a long time. The respiratory centre is stimulated by the carbon dioxide concentration of the blood. There are two types of physical movements associated with the gaseous exchange. They are:
  - Inspiration or inhalation: During inspiration, the outer intercostal muscles contract, which raises the chest cavity or the ribs. This is accompanied by the lowering of the diaphragm. Together these movements serve to increase the area of the thoracic cavity, which reduces the pressure. The air from outside rushes into the lungs.
  - Expiration or exhalation: The inner intercostal muscles contract bringing the ribs back to the original position and the diaphragm is also raised back by the action of the abdominal muscles. This reduces the space in the chest cavity and increases the pressure. This expels the air out of the lungs.

WBCS Preliminary (Biology): Nutrition

noreply@blogger.com (WBCSguru) — Tue, 27 Jul 2010 16:44:00 +0000

All organisms take food and utilise it to get energy for the growth and maintenance of their bodies.

Plants are the only organisms that can prepare food for themselves by using water, carbon dioxide and minerals.
The mode of nutrition in which organisms make food themselves from simple substances is called autotrophic (auto = self; trophos = nourishment) nutrition. Therefore, plants are called autotrophs. Animals and most other organisms take in ready made food prepared by the plants. They are called heterotrophs (heteros = other).
PHOTOSYNTHESIS: Chlorophyll, sunlight, carbon dioxide and water are necessary to carry out the process of photosynthesis. The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. This energy is used to synthesise (prepare) food from carbon dioxide and water. Since the synthesis of food occurs in the presence of sunlight, it is called photosynthesis (Photo: light; synthesis : to combine).
Besides leaves, photosynthesis also takes place in other green parts of the plant — in green stems and green branches.
Complex chemical substances such as carbohydrates are the products of photosynthesis. During the process oxygen is released. The carbohydrates ultimately get converted into starch. The carbohydrates are made of carbon, hydrogen and oxygen.
The mode of nutrition in which organisms take in nutrients in solution form from dead and decaying matter is called saprotrophic nutrition. Plants which use saprotrophic mode of nutrition are called saprotrophs.
Some organisms live together and share shelter and nutrients. This is called symbiotic relationship. For example, certain fungi live in the roots of trees. The tree provides nutrients to the fungus and, in return, receives help from it to take up water and nutrients from the soil.
There are some plants which do not have chlorophyll. They cannot synthesise their food. They use the heterotrophic mode of nutrition. It takes readymade food from the plant on which it is climbing. The plant on which it climbs is called a host. Since it deprives the host of valuable nutrients, it is called a parasite.
Animals get their food from plants, either directly by eating plants or indirectly by eating animals that eat plants.
The components of food such as carbohydrates are complex substances. These complex substances cannot be utilised as such. So they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion.
The human digestive system consists of the alimentary canal and secretory glands. It consists of the (i) buccal cavity, (ii) oesophagus, (iii) stomach, (iv) small intestine, (v) large intestine ending in rectum and (vi) anus. The main digestive glands which secrete digestive juices are (i) the salivary glands, (ii) the liver and (iii) the pancreas. The stomach wall and the wall of the small intestine also secrete digestive juices.
Digestion is a complex process involving: (i) ingestion, (ii) digestion, (iii) absorption, (iv) assimilation and (v) egestion.
Digestion of carbohydrates, like starch, begins in the buccal cavity. The digestion of protein starts in the stomach. The bile secreted from the liver, the pancreatic juice from the pancreas and the digestive juice from the intestinal wall complete the digestion of all components of food in the small intestine. The digested food is absorbed in the blood vessels in the small intestine.
Food is taken into the body through the mouth. The process of taking food into the body is called ingestion. We chew the food with the teeth and break it down mechanically into small pieces. Our mouth has the salivary glands which secrete saliva. The saliva breaks down the starch into sugars.
The tongue is a fleshy muscular organ attached at the back to the floor of the buccal cavity. It mixes saliva with the food during chewing and helps in swallowing food.
The swallowed food passes into the foodpipe or oesophagus. Food is pushed down by movement of the wall of the foodpipe. Actually this movement takes place throughout the alimentary canal and pushes the food downwards.
The stomach is a thick-walled bag. Its shape is like a flattened U and it is the widest part of the alimentary canal. It receives food from the food pipe at one end and opens into the small intestine at the other. The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach. The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic. The digestive juices break down the proteins into simpler substances.
The small intestine is highly coiled and is about 7.5 metres long. It receives secretions from the liver and the pancreas. Besides, its wall also secretes juices.
The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder . The bile plays an important role in the digestion of fats.
The pancreas is a large cream coloured gland located just below the stomach. The pancreatic juice acts on carbohydrates and proteins and changes them into simpler forms.
Partly digested food now reaches the lower part of the small intestine where the intestinal juice completes the digestion of all components of the food. The carbohydrates get broken into simple sugars such as glucose, fats into fatty acids and glycerol, and proteins into amino acids.
The digested food can now pass into the blood vessels in the wall of the intestine. This process is called absorption. The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi (singular villus).
Each villus has a network of thin and small blood vessels close to its surface. The surface of the villi absorbs the digested food materials. The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by the body. This is called assimilation. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released. The food that remains undigested and unabsorbed then enters into the large intestine.
The large intestine is wider and shorter than small intestine. It is about 1.5 metre in length. Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and remains there as semi-solid faeces. The faecal matter is removed through the anus from time-to-time. This is called egestion.
The grazing animals like cows, buffaloes and deer are known as ruminants.They quickly swallow the grass and store it in a separate part of the stomach called rumen. Here the food gets partially digested and is called cud. But later the cud returns to the mouth in small lumps and the animal chews it. This process is called rumination and these animals are called ruminants.
Ruminants have a large sac-like structure between the small intestine and large intestine. The cellulose of the food is digested here by the action of certain bacteria which are not present in humans.

WBCS Preliminary (Biology): Animal Kingdom

noreply@blogger.com (WBCSguru) — Sun, 25 Jul 2010 17:47:00 +0000

There are 10 major phyla in the animal kingdom from Protozoa to Chordata.

Protozoa : This most primitive unicellular organisms which reproduce by fission, budding,spores or sexually. e.g. Amoeba, Entamoeba, paramecium, vorticella, plasmodium, Euglena & trypanosoma. They move a variety of ways. The ameba has a false foot that extends as it moves. The paramecium is covered with hairs and the euglena has a whip-like tail(flagella) to move. A protozoa takes in oxygen through the cell membrane and gives off carbon dioxide through the cell membrane. Some protozoans are harmful to man as they can cause serious diseases.
Porifera (Sponges): These are most primitive of multi cellular animals. They live in water bodies. Body with a large number of incurrent pores called ostia leading into a spongocoel through a system of canals, Spongocoel opening out by one or two large excurrent pores called oscula. Respiration and excretion by simple diffusion.
Cnidaria (Coelenterata): These are mostly marine species except hydra which is fresh water form. Body has a mouth at the oral end which leads into a spacious cavity called gastrovascular cavity or coelenteron. Respiration and excretion by simple diffusion. Important coelentrates are coral, hydra, jellyfish, sea anemone, Portuguese man of war.
Platyhelminths(Flat worms): Flat worm is a common name for soft bodied, usually parasitic animals, the simplest of animals possessing heads. Respiration by simple diffusion. They are bilaterally symmetrical and somewhat flattened and are elongated. E.g. Tape worm, flukes.
Nemathelminthes(round worms): These are roundworms which are usually found in soil, water, plants and in animals as a parasite. Body is long, cylindrical, fusiform (pointed at both the ends). Respiration by simple diffusion. A roundworm has a definite digestive system that runs the length of their bodies. It has a mouth, pharynx, intestine and anus. e.g. Ascaris(round worm),oxyuris(pinworm), ancylostoma(hook worm),wucheria(Filaria worm).
Annelida: Free-living, terrestrial or aquatic form (freshwater or marine). Body is long, cylindrical and metamerically segmented. Body wall consists of cuticle, epidermis and musculature. Respiration is either through skin or through gills. Body has a true coelom. E.g. earth worms, leech and clamworm.
Arthropods: This phylum is the largest in the animal kingdom comprising of more than 75% of the animal species that have been identified. Body is elongated and segmented, usually distinguished into regions like head, thorax and abdomen. The arthropods are the first animal group to have jointed legs. Circulatory system is of open type. Blood flows freely in the body cavity (hemocoel). Respiration through gills, or trachea or book lungs. The major groups are:
- Insects : The largest class of arthropods. They have six legs and three body parts, a head, a thorax and an abdomen. All insects grow from eggs. A insect has a circulatory system that carries food, but not oxygen throughout its body. Since it does not carry oxygen, insect blood is green, not red like mammal blood. Example- butterfly, bee, ant, beetle, dragonfly, termite, grasshopper and true bug etc.
- Arachnids: The annelids are similar to insects. However, they have eight legs, wings are different and they have no antenna. The arachnids are spiders, scorpions, etc
- Crustaceans: These are predominantly aquatic arthropods with protective carapace and compound eyes. E.g. Prawns, Lobsters and Crabs.
- Myriapods: include millipedes and centipedes. The centipedes have two pairs of legs per segment while millipedes have one pair.
Mollusca : It is the common name for members of a phylum soft-bodied animals with a hard external shell (so called shell fishes). It is the second largest kingdom after arthropods. The three major groups are gastropods, bivalves and cephalopods. Respiratory organs are in the form of gills called ctenidia. Example-snails, slugs, clam, cockle, mussel, oyster, scallop, shipworm, squid, octopus, and nautilus.
Echinoderm: It consists of phylum of marine animals such as starfish, brittle stars, sea urchins, sand dollars and sea cucumbers. Body is represented by a central disc covered by ossicles with spines called pedicellaria. Disc may bear extensions called arms. Tube feet are present for locomotion and respiration. Tube feet are extended and retracted by variation in hydraulic pressure of the fluid in them and contraction of their muscles. A Echinoderm is a male or female. The males and females discharge their eggs and sperm into the water where they are fertilized. A female can release one hundred million eggs at once.
Chordate: Presence of a solid supporting structure on the dorsal side of the body called notochord lying above the gut and beneath a single hollow dorsal nerve cord. The Phylum chordate has been divided into Proto chordate and Vertebrate.
- Proto Chordate: They are primitive lower chordates possess notochord but lacks vertebral column or back bone.
- Vertebrate: It represents the largest group of chordates.
  - Agnatha: They are fish like forms with the absence of Jaws and scales. The best examples of this type are lamprey and the hagfish.
  - Gnathostomata: These are vertebrates with jawed mouth. These are subdivided into fishes, amphibians, reptiles, birds and mammals.
    1. Fishes: These are diverse group of cold blooded animals that live and breathe in water. Most fish have gills for breathing, two chambered heart, fins for swimming, scales for protection, and a streamlined body for moving easily through the water.
    2. Amphibians: Amphibian means both sides of life. it begins its life in the water and then finishes it mainly on land. It uses gills in larval stage (tadpole) and lungs and skins in the adult stage. Their heart is three chambered. Skin is often kept moist for gas exchange. These are cold blooded animals and have to be near water to complete their life cycle.Example--Frogs, toad, Salamander, caecilians, sirens etc.
    3. Reptiles: Reptiles are cold blooded animal with tough, dry skin covered with horny scales adapted for life in dry places. Some forms are aquatic. Reptiles breathe air with lungs alone. They have teeth except in tortoises and turtles. They have three chambered heart (crocodile has a four chambered heart). The females eggs are fertilized in her body by the male. The eggs are laid in a shell that has a leathery covering to protect it in the wilds. E.g. turtles, lizards, snakes, crocodiles, gavial (Gharial),alligators and dinosaurs.
    4. Birds: They are unique in the fact that they are covered with feathers and fly. They are descended from reptiles, feathers are modified limbs and their eggs resemble reptilian eggs. But like mammals they are warm blooded and have four chambered heart. The bones of many adult birds are hollow rather than filled with marrow, making them lighter and enabling them to disperse heat in flight.
    5. Mammals: Mammals are warm blooded animals having distinctive characteristics such as milk producing mammary glands, hair on their body, external ears, sweat glands, give birth to young rather laying eggs and nurse their young.

WBCS Preliminary (Biology): Plant Kingdom

noreply@blogger.com (WBCSguru) — Sat, 24 Jul 2010 18:04:00 +0000

Plants are multicellular photosynthetic producers of biosphere. With the help of protists and fungi, plants provide the oxygen we breathe. The plant kingdom consists of 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees etc.

The land plant belongs to two major divisions Bryophytes & Tracheophytes
1. Bryophytes: Bryophytes are non vascular embryo bearing plants consisting of three plant divisions: Bryophyta (Mosses), the Hepatophyta(liverwort) and the Anthocerophyta(Hornworts).
2. Tracheophytes (Vascular Plants): The characteristic organs of vascular plants are roots, stems and leaves and have vascular tissue xylem, which conducts water and minerals from ground to stems and leaves and phloem which conducts food produced in the leaves to stems, roots and storage and reproductive organs. Vascular plants are divided into Ferns (vascular plants without seed), gymnosperm ( seeds without fruit) and angiosperms(fruit & seed).
Photosynthesis:
- It is a process by which green plants and certain other organisms use light energy to convert carbon dioxide and water to simple sugar glucose. In doing so, it provides the basic energy sources for all organisms and releases oxygen on which most organisms depend.
- Photosynthesis takes place within cellular organelles known as chloroplasts.
- Photosynthesis takes place in two stages:
  1. The chloroplast traps light energy and covert it into a chemical energy contained in nicotinamide adenine dinucleotide phospate (NADPH) and adenosine triphosphate(ATP)
  2. In the second stage, called the light independent reaction, NADPH provides the energy for this and other reactions used to synthesize glucose.
  3. The overall equation can be represented as shown below:6CO₂+12H₂O—CHLOROPHYLL & LIGHT---> C₆H₁₂O₆+6H₂O+6O₂
A plant has two organ systems: 1) the shoot system, and 2) the root system. The shoot system is above ground and includes the organs such as leaves, buds, stems, flowers. The root system includes those parts of the plant below ground, such as the roots, tubers, and rhizomes.
1. Root: The major functions of roots are hold a plant in the ground, to absorb water and minerals from the soil, in some cases store food and in rare cases produce a new plant. There are two main types of root. In one type, the taproot system, a single large root grows straight down. In the other type, the fibrous root system, there are several main roots with many smaller branching roots of almost equal diameter.
2. Stem: It is the portion of vascular plants that commonly bears leaves and buds. It is usually is aerial, upright and elongate, but may be highly modified in structure. Stems that grow above ground are called aerial stems and below the ground are called Subterranean stems. There are two classes of aerial stems – herbaceous and woody. Herbaceous stems are slender, greenish and comparatively soft. The plants with herbaceous stems are called herbs. Woody stems are thicker, taller and harder than herbaceous stems. These may be either trees or shrubs. A tree has a thick main stems called the trunk, which branch abundantly. In shrub there are a number of comparatively slender main stems which branch abundantly.
3. Leaf: It is part of plant that serves primarily as the plant’s food making organ and takes part in transpiration and respiration. It may store food and water and provide structural support. A leaf is an extension of a plants stem. Some plants whose leaves change colour and lose their leaves in the autumn called deciduous. Those plants such as laurels and pines, the leaves do not change colour and do not fall off in autumn and are called evergreens. The outer surface of the leaf has a thin waxy covering called the cuticle, this layer's primary function is to prevent water loss within the leaf. (Plants that leave entirely within water do not have a cuticle). Directly underneath the cuticle is a layer of cells called the epidermis . The vascular tissue, xylem and phloem are found within the veins of the leaf. Veins are actually extensions that run from to tips of the roots all the way up to the edges of the leaves. The outer layer of the vein is made of cells called bundle sheath cells , and they create a circle around the xylem and the phloem. One the picture, xylem is the upper layer of cells and is shaded a little lighter than the lower layer of cells - phloem . Recall that xylem transports water and phloem transports sugar (food). Within the leaf, there is a layer of cells called the mesophyll. Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata .
4. Plant tissues: Xylem-- These cells conduct water and minerals from roots to leaves. Phloem-- It transports food from the leaves to rest of the plant.

WBCS Preliminary( Biology): Microorganism

noreply@blogger.com (WBCSguru) — Fri, 23 Jul 2010 02:39:00 +0000

Microorganisms or microbes are too small and are not visible to the unaided eye. They may be unicellular or multicellular.
- Microorganisms include bacteria, fungi, protozoa and some algae. Viruses, though different from the above mentioned living organisms, are considered microbes. Viruses are quite different from other microorganisms. They reproduce only inside the host organism; bacterium, plant or animal cell.
- They can live in all kinds of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. Microorganisms are found in air, water and in the bodies of plants and animals.
- Some microorganisms are useful for commercial production of medicines and alcohol. In agriculture they are used to increase soil fertility by fixing nitrogen.
Virus:
- A complete virus particle, known as a virion , is little more than a gene transporter, consisting of nucleic acid either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) surrounded by a protective coat of protein called a capsid. A capsid is composed of proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally coded protein units called protomers will self-assemble to form the capsid, requiring no input from the virus genome - however, a few viruses code for proteins which assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid.
- Viruses are dead outside the cells multiply only inside living cells and moves along blood or Phloem sap in animals and plants respectively. The nucleic acid of the virus enters the cell and control the host cell to produce identical virus nucleic acid and protein coat and thus it multiplies. Continuous multiplication of virus, exhaust the protoplasmic contents and it finally the cell ruptures and realises virus. This process is known as lysis of the host cell. The new viruses repeat the entire multiplication process.
Bacteria:
- Bacteria are microorganisms that lack a nucleus and have a cell wall composed of peptidoglycan, a protein-sugar molecule. Bacteria are the most common organisms on earth and are intimately connected to the lives of all organisms. The common structural forms are:
  1. Spherical or ovoid (coccus )
  2. rod shaped or cylindrical (bacillus )
  3. spiral or screw (spirillum) Many forms of bacteria are not capable of independent movement,. Some Bacteria which live in liquid often have thread like projections called flagella (e.g Salmonella bacterium)
- Reproduction in bacteria is largely by binary fission i.e. it splits into two. In some case due to some extreme environmental conditions, they form tiny structures is called Spores. It is formed by condensation of protoplasm into a spherical or egg shaped body and they germinate under favourable conditions. Some bacteria exhibit a type of sexual reproduction.
- Vaccination or Immunization or inoculation is a method of stimulating resistance in the human body to specific diseases using microorganisms -bacteria or viruses-that have been modified or killed. These vaccines do not cause diseases but stimulates the production of antibodies in its blood. It build a defense mechanism that continuously guards against the disease.
- Serum is a preparation from blood of an animal that has been inoculated with bacteria. This contains antibodies that formed as a consequence of the disease. The important antibodies produced with the help of bacteria are streptomycin, Aureomycin, Terramycin.
Fungi: The fungi (singular fungus) are a kingdom of eukaryotic organisms. Fungi lack chlorophyll; consequently they cannot synthesize their own food. In order to feed fungi release digestive enzymes that break down food outside their bodies. The fungus then absorbs the dissolved food through their cell walls. It is a simple plant body that has no roots, stems, flowers and seeds. It includes mushrooms, molds, yeasts, truffles etc. The branch of biology involving the study of fungi is known as mycology.
Algae: These are chiefly plant like organisms found usually in water bodies and moist environments that are not subjected to direct sunlight. They make their own food by photosynthesis but they lack roots, leaves and other structures typical of true plants. They capture more of sun’s energy than all plants combined and form the foundation of most aquatic food webs.

WBCS Preliminary (Biology): Tissues

noreply@blogger.com (WBCSguru) — Wed, 21 Jul 2010 14:33:00 +0000

A group of cells of the same type or of a mixed type having a common origin and performing similar functions are called tissues.
Plant tissues are of two main types –Meristematic Tissues (Meristems) and Permanent.
Meristematic tissues are composed of cells that divide continuously. As the cells of this tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles. Found in growing tips of root and shoot. The main function of meristematic tissue is to continuously form a number of new cells and help in growth.
Permanent tissues are derived from meristematic tissues. They have lost the power of dividing, having attained their definite form and size. In their earlier stages the cells are more or less similar in structure but slowly they become specialized and form permanent tissues. They can be classified into simple and complex tissues.
Parenchyma, collenchyma and sclerenchyma are three types of simple tissues. Xylem and phloem are types of complex tissues.
Parenchyma, a type of permanent tissue, consists of relatively unspecialised cells with thin cell walls. They are live cells. They are usually loosely packed,so that large spaces between cells (intercellular spaces) are found in this tissue. This tissue provides support to plants and also stores food. In some situations, it contains chlorophyll and performs photosynthesis,and then it is called chlorenchyma.In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.
The flexibility in plants is due to another permanent tissue, collenchyma. It allows easy bending in various parts of a plant (leaf, stem) without breaking. It also provides mechanical support to plants.
Yet another type of permanent tissue is sclerenchyma. It is the tissue which makes the plant hard and stiff. The cells of this tissue are dead.
Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex tissues. They are both conducting tissues and constitute a vascular bundle.
Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. The cells have thick walls, and many of them are dead cells. Tracheids and vessels are tubular structures. This allows them to transport water and minerals vertically. The parenchyma stores food and helps in the sideways conduction of water. Fibres are mainly supportive in function.
Phloem is made up of four types of elements: sieve tubes, companion cells, phloem fibres and the phloem parenchyma. Sieve tubes are tubular cells with perforated walls. Phloem is unlike xylem in that materials can move in both directions in it. Phloem transports food from leaves to other parts of the plant. Except for phloem fibres, phloem cells are living cells.
Animal tissues can be epithelial, connective, muscular and nervous tissue.
The covering or protective tissues in the animal body are epithelial tissues. Epithelium covers most organs and cavities within the body. Depending on shape and function, epithelial tissue is classified as squamous, cuboidal, columnar, ciliated and glandular.
The different types of connective tissues in our body include areolar tissue, adipose tissue, bone, tendon, ligament, cartilage and blood.
Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement. Striated, unstriated and cardiac are three types of muscle tissues.
The brain, spinal cord and nerves are all composed of nervous tissue. Each nerve cell is called a neuron. These are highly specialized cells. They have the ability to receive stimulus from within or outside and send impulses to different parts of the body. Each cell consists of three parts, the cyton or the cell body, the dendrons which are short processes arising from the cyton and further branch into thin dendrites and the axon which is a single long cylindrical process forming fine branches terminally. Dendrites receive impulses and the axon takes impulses away from the cell body.

WBCS Preliminary (Biology): Cell-Unit of Life

noreply@blogger.com (WBCSguru) — Mon, 19 Jul 2010 17:59:00 +0000

All organisms are made of smaller parts called organs.
Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
The word cell is derived from the Latin word “cellula” which means “a little room”.
Cells were first observed in cork by Robert Hooke in 1665.
Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time.
Cells exhibit variety of shapes and sizes.
Number of cells also varies from organism to organism.
Man is estimated to have about 100 trillion (10¹⁴) cells in number.
Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
Some organisms are single-celled, while others contain large number of cells.
The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
Cells are enclosed by a plasma membrane composed of lipids and proteins.
The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
- Cell Membrane: The cell membrane also called the plasma membrane is an active part of the cell. It regulates the movement of materials between the ordered interior of the cell and the outer environment. The plasma membrane is porous and allows the movement of substances or materials both inward and outward.
- Cell Wall: Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
- Nucleus: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus.
- In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring.
- Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
- Gene: Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring.
- Cytoplasm : It is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, golgi bodies, ribosomes, etc.
- Vacuoles: Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
- Most plant cells have large membranous organelles called plastids, which are of two types – chromoplasts and leucoplasts.
- The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification and packaging of substances manufactured in the cell.
- Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis.The primary function of leucoplasts is storage.
- The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.
- The nucleus of the bacterial cell is not well organised like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes (pro :primitive; karyon : nucleus). Examples are bacteria and blue green algae. Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles.
- The cells, like onion cells and cheek cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes. (eu : true; karyon: nucleus).
Cell Division: Every living organism reproducing sexually is derived from a single cell, the zygote, which divides again and again to produce a large number of body cells. This division is accomplished in phases, the division of nucleus called mitisis and the cytoplasmic division called meiosis.
- Mitosis
  - A parent cell will copy of all its internal components, divide them equally, and then split in half to form 2 daughter cells,
  - The daughter cells formed are identical to each other.
  - In single-celled, eukaryotic organisms this is the way that they form new individuals.
  - In multi-cellular organisms, mitosis is used for growth and repair of damaged tissue.
  - Mitosis is tightly controlled by the cell cycle to ensure that mitosis happens only when it is needed.
  - A normal cell completes the cycle with-in 24 hours.
  - Main steps during mitosis:
    1. Interphase :DNA is replicated along with organelles and other cellular components and the cell prepares for division. During the replication process, the DNA changes from one double helix (unreplicated) to two double helices (replicated).The two helices in the replicated chromosomes are joined to one another in the special region of the chromosome called the centromere – at this point, the two double helices are called are sister chromatids
    2. Prophase : (preparation phase) The DNA recoils and the chromosomes condense; the nuclear membrane disappears, and the mitotic spindles begin to form.
    3. Metaphase : (organizational phase). The chromosomes line up the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome.
    4. Telophase :The chromosomes, along with the cytoplasm and its organelles and membranes are divided into 2 portions
    5. Cytokinesis: The actual splitting of the daughter cells into two separate cells is called cytokinesis and occurs differently in both plant and animal cells as is illustrated below.
  - Animal cells: The cell pinches in on both sides to form a cleavage furrow. This will gradually move toward the center to split the parent cell into 2 daughter cells.
  - Plant cells: The cell forms a cell plate, which starts in the center and moves towards the outer edges to split the parent cell into two daughter cells.
- Meiosis: Meiosis is used in sexual reproduction for the formation of gametes (egg and sperm cells). It creates genetic diversity. The gametes that are produced as an end result of meiosis are haploid cells, they contain half the genetic material of the parent cell. This will ensure that when gametes combine during fertilization that the new zygote will contain the normal amount of chromosomes instead of double the genetic material.
  - Just like in mitosis, during INTERPHASE, the DNA is replicated before the cell undergoes the division process.
    1. Prophase I : The chromosomes coil up and condense and the nuclear membrane disappears. The cell prepares for separation. During this process, the chromosomes of each homologous pair lie so close to one another that the arms can get tangled up. The lining up of homologous chromosomes in pairs is known as synapsis. This can lead to exchange of genetic material between the homologous pairs known as crossing-over. This will serve to create genetic diversity.
    2. Metaphase I : The homologous pairs line up in the center of the cell. During this time, the chromosomes obey the law of independent assortment – this says that each homologous pair arranges itself independently of the others.
    3. Anaphase I :The homologous pairs are separated and pulled opposite poles of the cell by the spindle fibers.
    4. Telophase I : The parent cell separates into 2 daughter cells with the division of the cytoplasm, organelles, and membranes. The new daughter cells now have the number of chromosomes of the parent cell but the chromosomes are still replicated. The two daughter cells are no longer identical to the parent cell or each other.
  - Meiosis II: This is essentially the same as mitosis – we have to split the replicated chromosomes so they are no longer in a replicated state. Remember that this is taking place now in both of the two new daughter cells we have just created.
    1. Prophase II: This stage resembles that of mitotic prophase – the chromosomes recoil and condense and any nuclear membrane that has formed will now disappear.
    2. Metaphase II: The chromosomes line up in the middles attached to the spindle fibers and prepare for separation – again, remember this is happening in two different cells simultaneously.
    3. Anaphase II: The sister chromatids are pulled apart to opposite ends of the cells by the spindle fibers.
    4. Telophase II: The cells split into 2 daughter cells, and cytokinesis occurs. The end result is 4 haploid daughter cells that are not identical to one another.

WBCS Preliminary(Chemistry): Some Common Elements & Compounds

noreply@blogger.com (WBCSguru) — Thu, 15 Jul 2010 18:17:00 +0000

Hydrogen: Symbol H, formula H₂. The first element in the periodic table and the most basic and common of all elements in the universe. Over ninety percent of all the atoms in the universe are hydrogen atoms and they are the lightest of all elements. The name hydrogen comes from the Latin word "hydro" which means water. Scientists use the letter "H" to represent hydrogen in all chemical equations and descriptions.
- Hydrogen atom has one electron in its valence shell like alkali metals.
- Hydrogen generally shows + 1 valency like alkali metals.
- Hydrogen is a good reducing agent like other alkali metals.
- The isotopes of hydrogen: Protium has an atomic number 1, and mass number 1, Deuterium, has an atomic number 1, and mass number 2 and Tritium has an atomic number 1, and mass number 3.
- It has a vapour density of 1, which is 14.4 times lighter than air.
Carbon: The sixth element in the periodic table. It is a very stable element. Because it is stable, it can be found in many naturally occurring compounds and by itself. Scientists describe the three states of carbon as diamond, amorphous, and graphite.
- Carbon exhibits allotropy and shows maximum catenation.
- Normal valency of carbon is four due to the presence of four valence electrons. Thus all four bonds are generally covalent.
- Carbon occurs both in free state as diamond, coal etc. and also in the combined form as CO₂.
- Diamond is one of the allotropic forms of carbon and is the purest form of natural carbon. It is the hardest natural substance. Diamond is a giant framework that forms a rigid structure with no free electrons to conduct electricity.
- Graphite is also an allotropic form of carbon, which is very soft and slippery. Graphite has a mobile cloud of electrons on the horizontal planes, which makes it a good conductor of electricity.
- Apart from diamond and graphite, which are crystalline forms of carbon, all other forms of carbon are amorphous allotropes of carbon. Destructive distillation of coal gives products like coal gas, gas carbon, coal tar and ammonical liquor.
- Lamp Black is also known as Soot. Soot is obtained by the incomplete combustion of carbonaceous, fuels, especially oil fuels, in limited supply of air. The soot settles on the cooler parts of the chamber, and can be collected by scrapping it.
- Wood charcoal is obtained by the destructive distillation of wood. The chief products formed are wood charcoal, wood tar, pyroligneous acid and wood gas .
- Sugar charcoal can be obtained by dehydrating cane sugar, either by treating it with concentrated sulphuric acid or by heating it in the absence of air.
- Bone charcoal is a black powder called as 'ivory black'. It is porous and can adsorb colouring matter. It is mostly used in sugar industry to decolourise sugar.
Nitrogen: It is the seventh element of the periodic table located between carbon and oxygen. Almost eighty percent of Earth's atmosphere is made of nitrogen gas. Nitrogen is a clear gas that has no smell when it is in its pure form. It is not very reactive when it is in a pure molecule, but it can create very reactive compounds when combined with other elements including hydrogen (ammonia). There are 7 electrons in a nitrogen atom.
- Nitrogen has 5 electrons in its valence shell. It has a valency of 3 with respect to hydrogen and a valency upto 5 with respect to oxygen.
- In the laboratory nitrogen is prepared by the action of heat on a mixture of ammonium nitrite and ammonium chloride. Nitrogen is collected by the downward displacement of water and is called chemical nitrogen.
- Nitrogen is a neutral gas and is neither combustible nor a supporter of combustion.
Oxygen :Symbol O, formula O₂. Alone, oxygen is a colorless and odorless compound that is a gas at room temperature. Oxygen molecules are not the only form of oxygen in the atmosphere; you will also find oxygen as ozone and carbon dioxide. There are 8 electrons in an oxygen atom. In the laboratory oxygen is usually obtained by heating a mixture of potassium chlorate and manganese dioxide. Manganese dioxide facilitates the decomposition of potassium chlorate, but it itself remain unchanged in mass and composition and hence acts as a catalyst in the reaction. Oxygen is non-combustible but a good supporter of combustion. An oxide is a compound of two elements, one of which is oxygen. It can be liquefied and solidified. It is employed in welding process and also used in hospitals for artificial respiration. Oxygen shows a valency of -2.
Chlorine: Chlorine belongs to group VII A. Members of this group are called halogens which means 'salt producers'. Chlorine has seven electrons in its outer most shell and so has a valency of 1. Chlorine is prepared by the oxidation of concentrated hydrochloric acid using oxidising agents like manganese dioxide, lead dioxide, trilead tetra oxide, potassium permanganate and potassium dichromate. Chlorine is a non combustible gas but supports the burning of certain metals and non-metals. Chlorine is highly reactive. It reacts with hydrogen, other non metals and metals to form the corresponding chlorides. Chlorine being an acidic gas turns moist blue litmus paper to red and then bleaches it.
Water( H₂O):
- Water is the only substance that can exist simultaneously in all the three states of matter, i.e., solid, liquid and gaseous on this earth.
- Pure water is a colourless, odourless and tasteless liquid.
- The density of water is 1 g cm^-3at 4^oC.
- The boiling point of water is 100^oC at a pressure of 760 mm of Hg. The melting point of ice is 0^oC at a pressure of 1 atmosphere.
- Ice has a relative density of 0.92. The specific heat capacity of water is 1 cal/g at 15^oC.
- Water is called the "Universal Solvent". Almost all substances dissolve in water to a certain extent. Hence, it known as a universal solvent. Because of this property, it is impossible to get chemically pure water on the earth.
- Metals such as gold, silver, copper, tin, etc. do not react with water. Ordinary iron gets rusted and aluminium gets tarnished.
- Water is described as being 'hard' if it does not lather readily with soap. 'Soft water', on the other hand, is described as the one, which lathers readily with soap. Chemically, natural water is never pure and contains varying amounts of the dissolved impurities absorbed from the natural or man made environment. Temporary hardness and permanent hardness are the two types of hardness occurring in hard water: Water is said to be temporarily hard when it contains bicarbonates of calcium and magnesium (or hydrogen carbonates). This type of hardness can be easily removed by boiling. Water is said to be permanently hard when it contains sulphates and chlorides of calcium and magensium. Water becomes permanently hard when it passes over the rocks, which contain sulphates or chlorides of calcium and magnesium to form insoluble calcium bicarbonates or magnesium bicarbonates (or hydrogen carbonates). This hardness cannot be removed by boiling.
- Heavy water is prepared either by prolonged electrolysis or by fractional distillation of ordinary water. Heavy water(D₂O) is colourless, tasteless and odourless liquid. It has all higher values for physical constants than the corresponding values of ordinary water. Fission in uranium-235 is brought by slow speed neutron. Heavy water is used for this purpose in nuclear reactors as moderators.
Ammonia(NH₃):
- Ammonia is present in atmospheric air and in natural water in trace amounts. However in sewage water, it is present in greater proportion. Ammonia is present in the combined form as various ammonium salts. The two most popular salts are ammonium chloride and ammonium sulphate.
- Ammonia is generally obtained from Ammoniacal liquor obtained by the destructive distillation of coal, destructive distillation of nitrogenous organic matters such as horns, hoofs, bones etc. of animals, Ammonium salts.
- In the laboratory, ammonia is usually prepared by heating a mixture of ammonium chloride and slaked lime in the ratio of 2 : 3 by mass.
- Ammonia is a colorless gas. Its vapor density is 8.5. Hence it is lighter than air (vapor density of air = 14.4). When cooled under pressure ammonia condenses to a colorless liquid, which boils at -33.4^oC. When further cooled, it freezes to a white crystalline snow-like solid, which melts at -77.7^oC. Ammonia is one of the most soluble gases in water. At 0^oC and 760 mm of Hg pressure one volume of water can dissolve nearly 1200 volumes of ammonia. This high solubility of ammonia can be demonstrated by the fountain experiment. Ammonia is neither combustible in air nor does it support combustion. However it burns in oxygen with a greenish-yellowish flame producing water and nitrogen. Ammonia reacts with the acids to form their respective ammonium salts. Ammonia is highly soluble in water and forms ammonium hydroxide.
Hydrochloric Acid(HCL):
- Hydrochloric acid is prepared by dissolving hydrogen chloride gas in water. Hydrogen chloride is a covalent compound, but when dissolved in water it ionizes to form hydrogen ions and chloride ions
- Hydrochloric acid is produced along with the industrial preparation of caustic soda (sodium hydroxide). During the electrolysis of sodium chloride, large quantities of hydrogen and chlorine gas are obtained as by-products. These two gases are burnt to form hydrogen chloride gas. The hydrogen chloride gas so formed is dissolved in water to form hydrochloric acid. A saturated solution of the acid has a density of 1.2 g cm^-3. It contains about 40% by mass of hydrogen chloride.

Nitric Acid(HNO₃):
- Nitric acid is produced in large quantities in the atmosphere during thunder storms. It is manufactured by the Ostwald's Process by the reaction of ammonia and air in presence of platinum as catalyst at 700-800^o C.
- Nitric acid is colourless in pure form. Commercial nitric acid is yellowish due to the presence of dissolved nitrogen dioxide.
- Pure nitric acid is not very stable. Even at ordinary temperature, in presence of sunlight it undergoes slight decomposition. As the temperature increases, the rate of decomposition also increases. On strong heating it decomposes completely to give nitrogen dioxide, water and oxygen.
- Nitric acid is a strong monobasic acid. It ionizes in water readily.
- Nitric acid usually does not behave as an acid, with metals to form the corresponding salt and liberate hydrogen. However, magnesium and manganese are the only two metals, which react with cold and very dilute (1%) nitric acid to evolve hydrogen.
- Nitric acid is a strong oxidizing agent. When it undergoes thermal decomposition, it yields nascent oxygen
Sodium(Na):
- Sodium belongs to Group I in the periodic table. This group is otherwise known as the alkali metals group. Since the atomic number of sodium is 11, its electronic configuration is 2,8,1. Sodium easily loses the lone electron to attain the stable configuration of neon. Therefore alkali metals like sodium that are univalent can easily form ionic compounds.
- Since alkali metals like sodium are highly electropositive (tendency to lose an electron and become a cation), their carbonates and bicarbonates are highly stable to the action of heat.
- Some of the important sodium compounds are:
  1. Sodium Carbonate (Na₂CO₃): Popularly known as washing soda or soda ash, sodium carbonate is a commercially important compound.(a) Transparent crystalline solid with ten molecules of water per molecule. (b) Soluble in water. (c) Washing soda solution is alkaline due to hydrolysis.(d) Has detergent or cleansing properties. (e) Sodium carbonate is used as washing soda in laundry as a cleansing agent, for softening hard water, in manufacturing glass, paper, soap and caustic soda.
  2. Sodium Bicarbonate (NaHCO₃): Sodium bicarbonate is commonly called baking soda. Sodium bicarbonate is prepared in the laboratory by saturating a cold solution of sodium carbonate with carbon dioxide. (a) Sodium bicarbonate separates as white crystals. This is because it is very sparingly soluble in water. (b) Sodium bicarbonate is sparingly soluble in water.(c) Used in the preparation of carbon dioxide. (d) Used as a constituent of baking powder, and in effervescent drinks. Baking powder has sodium bicarbonate and tartaric or citric acid. When it is dissolved in water or heated carbon dioxide is produced. This carbon dioxide gas causes the puffiness and lightness of cakes, biscuits etc.(e) Sodium bicarbonate is used to extinguish fire as it produces carbon dioxide gas.
Calcium(Ca):
- The elements of Group II like calcium are called the alkaline earth metals. The atomic number of calcium is 20 and its configuration is 2,8,8,2. Calcium loses two electrons and becomes Ca²⁺ ion with the stable configuration of argon. Calcium is therefore bivalent in nature.

WBCS Preliminary (Chemistry): Organic Chemistry

noreply@blogger.com (WBCSguru) — Wed, 14 Jul 2010 16:24:00 +0000

Organic chemistry is that branch of chemistry which deals with the study of compounds of carbon with hydrogen (hydrocarbons), and their derivatives. Presently about five million organic compounds are known. Organic compounds were found to contain mainly hydrogen and carbon. Therefore, organic chemistry is defined as the study of hydrocarbons and their derivatives. Most atoms are only capable of forming small molecules. However one or two can form larger molecules. By far and away the best atom for making large molecules with is Carbon. Carbon can make molecules that have tens, hundreds, thousands even millions of atoms! The huge number of possible combinations means that there are more Carbon compounds that those of all the other elements put together! A single Carbon atom is capable of combining with up to four other atoms. We say it has a valency of 4. Sometimes a Carbon atom will combine with fewer atoms. The Carbon atom is one of the few that will combine with itself. In other words Carbon combines with other Carbon atoms. This means that Carbon atoms can form chains and rings onto which other atoms can be attached. This leads to a huge number of different compounds. Organic Chemistry is essentially the chemistry of Carbon. Carbon compounds are classified according to how the Carbon atoms are arranged and what other groups of atoms are attached.

Hydrocarbons: The simplest Organic compounds are made up of only Carbon and Hydrogen atoms only. Even these run into thousands! Compounds of Carbon and Hydrogen only are called Hydrocarbons.
1. Alkanes: In the alkanes, all four of the Carbon valency bonds are taken up with links to different atoms. These types of bonds are called single bonds and are generally stable and resistant to attack by other chemicals. Alkanes contain the maximum number of Hydrogen atoms possible. They are said to be saturated. The simplest Hydrocarbon is:
  - Methane: CH₄This is the simplest member of a series of hydrocarbons. Each successive member of the series has one more Carbon atom than the preceeding member.
  - Ethane: C₂H₆.
  - Propane –( heating fuel): C₃H₈.
  - Butane – (lighter / camping fuel): C₄H₁₀.
  - Pentane: C₅H₁₂.
  - Hexane: C₆H₁₄.
  Polythene is a very large alkane with millions of atoms in a single molecule. Apart from being flammable, alkanes are stable compounds found underground.
2. Alkenes: Another series of compounds is called the alkenes. These have a general formula: C_nH_2n. These compounds are named in a similar manner to the alkanes except that the suffix is -ene. Alkenes have fewer hydrogen atoms than the alkanes. The extra valencies left over occur as double bonds between a pair of Carbon atoms. The double bonds are more reactive than single bonds making the alkenes chemically more reactive. The simplest alkenes are listed in the table below:
  - Ethene ( used as an industrial starter chemical): C₂H₄.
  - Propene: C₃H₆.
  - Butene: C₄H₈.
  - Pentene: C₅H₁₀.
  - Hexene: C₆H₁₂.
3. Alkynes: A third series are the alkynes. These have the following formula: C_nH_2n-2. These highly reactive substances have many industrial uses. Again the naming of these compounds is similar to the alkanes except that the suffix is -yne. Alkynes have two carbon atoms joined by a tripple bond. This is highly reactive making these compounds unstable. Examples of alkynes are:
  - Ethyne - better known as acetylene which is used for welding underwater: C₂H₂
  - Propyne: C₃H₄
  - Butyne: C₄H₆
  - Pentyne: C₅H₈
  - Hexyne: C₆H₁₀
4. Carbon Rings: Alkanes, alkenes and alkynes all contain Carbon atoms in linear chains. When rings are combined with chains, the number of hydrocarbons is virtually infinite. There are also hydrocarbons arranged in rings. Some examples follow:
  - Cyclohexane - a saturated hydrocarbon with the atoms arranged in a hexagonal ring: C₆H₁₂
  - Benzene - an industrial solvent. The Benzene Ring is one of the most important structures in organic chemistry. In reality, its alternate double and single bonds are "spread around" the ring so that the molecule is symmetrical: C₆H₆
  - Toluene - an important solvent and starter chemical: C₇H₈
  - Naphthalene - used in moth balls. This can be depicted as two fused Benzene Rings: C₁₀H₈
Carbon, Hydrogen and Oxygen: When Oxygen atoms are added, the variety of compounds grows enormously. Here are some examples where each molecule has a single functional group.
1. Alcohols: Alcohols have the OH (hydroxyl) group in the molecule. A group of atoms that gives an organic series its distinctive character is called a functional group. These have a general formula: C_nH_2n+1OH. Examples: Methanol (wood alcohol) CH₃OH, Ethanol(drinking alcohol) C₂H₅OH, Phenol(carbolic acid - used as disinfectant) C₆H₅OH.
2. Ethers(Ethers have an O atom attached to two hydrocarbon chains) (C_nH_2n+1)₂O. Examples: Dimethyl Ether(a gas) (CH₃)₂O, Diethyl Ether (a liquid used as an anaesthetic) (C₂H₅)₂O
3. Ketones(Ketones have a CO group attached to two hydrocarbon chains) . These have a general formula: (C_nH_2n+1)₂CO.Example: Dimethyl Ketone (Also known as acetone: nail-varnish remover), CH₃COC H₃
4. Aldehydes(Aldehydes have a CHO group attached to a hydrocarbon chain). These have a general formula: C_nH_2n+1CHO. Example: Formaldehyde (preservative in labs) HCHO, Acetaldehyde- CH₃CHO.
5. Fatty Acids(Fatty Acids contain the CO₂H (or COOH) group attached to a hydrocarbon chain or ring). These have a general formula: C_nH_2n+1CO₂H. Example: Formic Acid(in ant bites and stinging nettles)- HCO₂H, Acetic Acid( vinegar)- CH₃CO₂H, Butyric Acid( the rancid butter smell)- C₂H₅CO₂H.
6. Esters (Esters are similar to Fatty Acids except that the H in the COOH group is another hydrocarbon chain. They are usually very sweet smelling liquids used in perfumes). These have a general formula: RCO₂R’( R and R’ are Hydrocarbon chain or rings). Examples: Methyl Methoate (essence of pear drops) - CH₃CO₂CH₃.
It is possible to have two or more functional groups on a molecule. These can be the same group (as in Oxalic Acid - a poison found in rhubarb leaves - which has two fatty acid groups) or different (as in Hydroxymethanoic Acid - which has a hydroxyl group and a fatty acid group): Oxalic Acid- (COOH)², Hydroxymethanoic Acid- CH₂OHCOOH.
The most famous compounds containing Carbon, Hydrogen and Oxygen are the Carbohydrates. An example is the common sugar, Sucrose (C₁₂H₂₂O₁₁).
Isomerism: An interesting phenomenon with organic molecules is called isomerism. Let us look at two compounds introduced earlier. Dimethyl Ether: (CH₃)₂O and Ethanol: C₂H₅OH. The first is a gas which will knock you out if inhaled. The second is common alcohol drunk in spirits. Both compounds contain 2 Carbon atoms, 6 Hydrogen atoms and 1 Oxygen atom. Even though the atoms are the same, they are arranged differently. This yields two different compounds with the same number of atoms. These compounds are isomers and the phenomenon is called Isomerism. Isomerism increases the number of Organic compounds. The more Carbon atoms in a compound, the more ways of arranging the atoms and the larger number of isomers.
Adding Nitrogen: Many very important organic compounds contain Nitrogen. This produces more series of compounds.
1. Amines (Amines have one or more of the Hydrogen atoms in Ammonia (NH3) replaced by a Hydrocarbon chain or ring). These have a general formula: C_nH_2n+1NH₂. Examples: Methylamine (a pungent, water soluble gas)- CH₃NH₂.
2. Cyanides (Cyanides have the CN group). These have a general formula: C_nH_2n+1CN. Examples: Methyl Cyanide- CH₃CN.
3. Amino Acids (Amino Acids have two functional groups: the amine (HN₂) group and the fatty acid (COOH) group. These have a general formula: C_nH_2nNH₂COOH. Examples: Glycine (the simplest amino acid)- CH₂NH₂COOH.
4. A famous compound containing Nitrogen is Trinitro Toluene (C ₆H₂CH₃ (NO)₃) - usually abbreviated to TNT). This is an artificially made explosive.
The vast majority of organic compounds contain Carbon, Hydrogen, Oxygen and Nitrogen. Other types of atoms can be included to form even more compounds. These can contain atoms like Phosphorus, Sulphur (e.g. Thiamine,), Chlorine (e.g. Chlorophyll-CHCl₃, Dichloro Diphenyl Trichloro Methane – DDT-C ₁₄H₉Cl₁₅) and Iron (e.g. Haemoglobin).

WBCS Preliminary (Chemistry): Chemical Bonding

noreply@blogger.com (WBCSguru) — Tue, 13 Jul 2010 18:07:00 +0000

Atoms are made up of three smaller particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus of the atom. Protons have a single positive charge. This is called the Atomic Number of an atom. The Atomic Number tells us the number of electrons that the atom contains. It is these electrons that determine the chemical properties of the atom and the way it combines with other atoms to form specific compounds. Electrons have a single negative charge. Normally, atoms are electrically neutral so that the number of electrons is equal to the number of protons.
Electrons orbit around the nucleus. Electrons cannot orbit the nucleus of an atom in any orbit. The electrons are restricted to specific paths called orbitals or shells. Each shell can only hold a certain number of electrons. When a shell is full, no more electrons can go into that shell. The key to the properties of atoms is the electrons in the outer shell. A complete outer shell of electrons is a very stable condition for an atom.
Valency: Hydrogen is the simplest element. It has one electron. Its outer shell only holds two electrons. Valency can be simply defined as the number of Hydrogen atoms that an element can combine with. The atoms with full electron shells (Helium, Neon, Argon) are chemically inert forming few compounds. The atoms don't even interact with each other very much. These elements are gases with very low boiling points. The atoms with a single outer electron or a single missing electron are all highly reactive. Sodium is more reactive than Magnesium. Chlorine is more reactive than Oxygen. Generally speaking, the closer an atom is to having a full electron shell, the more reactive it is. Atoms with one outer electron are more reactive than those with two outer electrons, etc. Atoms that are one electron short of a full shell are more reactive than those that are two short.
Chemical bonds are what hold atoms together to form the more complicated aggregates that we know as molecules and extended solids. The forces that hold bonded atoms together are basically just the same kinds of electrostatic attractions that bind the electrons of an atom to its positively-charged nucleus. chemical bonding occurs when one or more electrons are simultaneously attracted to two nuclei. .
Mainly 3 Types of bonds can be present in Chemical Compounds.
1. Electrovalent or Ionic Bond: It is formed by Transferring of Electrons between 2 Atoms. These types of bonds are mainly formed between Metals and Non - Metals. These compounds exist in solid form. These compounds have high boiling Point, Melting Point and thermal stability.
2. Covalent Bond: It is formed by equal sharing of Electrons between 2 Atoms. This type of bond is mainly formed between non - metals. These compounds may be solid, liquid or gas. These compounds have low boiling Point, Melting Point and thermal stability in comparison to Ionic Bond.
3. Co - Ordinate or Dative Bond : It is formed by unequal sharing of Electrons between 2 Atoms. This bond is also called as Semi - Polar bond since; it involves Electrovalency and Covalency both. These compounds may be solid, liquid or gas. These compounds are insoluble in H2O. These compounds do not conduct Electricity. These compounds have high B.P. than Covalent Compounds but less than Electrovalent Compounds.

WBCS Preliminary (Chemistry):Periodic Classification of Elements

noreply@blogger.com (WBCSguru) — Mon, 12 Jul 2010 12:28:00 +0000

The grouping of elements with similar properties together and the separation of elements with dissimilar properties is known as classification of elements. The table, which classifies elements on the basis of their properties, is called the periodic table. Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves. Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.
Dobereiner's Triads arranged elements in an increasing order of atomic mass, in groups of three. The atomic mass of the middle element was the arithmetic mean of the other two elements of the triad.
Newland's law of octaves states that on arranging elements in increasing order of their atomic mass, the eighth element resembles the first in physical and chemical properties, just like the eighth node on a musical scale resembles the first note.
According to Mendeleev's periodic law, the physical and chemical properties of elements are periodic functions of their atomic mass. Mendeleev corrected the atomic masses of a few elements on the basis of their positions in the periodic table. Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.
Mendeléev’s Periodic Table contains vertical columns called ‘groups’and horizontal rows called ‘periods’. While developing the Periodic Table, there were a few instances where Mendeléev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass. The sequence was inverted so that elements with similar properties could be grouped together. Mendeleev's table could not assign a proper position to hydrogen or to the lanthanides and actinides and isotopes. Isotopes of all elements posed a challenge to Mendeleev’s Periodic Law. Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements — especially when we consider the heavier elements.
In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law.
The vertical columns are called groups, while the horizontal rows are called periods. There are 7 periods and 8 groups subdivided into 18 sub groups. The noble gases are on the extreme right of the table and on the table's extreme left, are the alkali metals. Transition elements are placed in the B subgroups in the middle of the table. The inner transition elements - lanthanides and actinides, are placed in two separate series at the bottom of the periodic table. Group number is number of electrons in the valence shell. Elements having the same valence number, are grouped together. The number of shells present in the atom gives period number.
Atomic size: The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.

WBCS Preliminary(Chemistry): Atomic Structure

noreply@blogger.com (WBCSguru) — Sat, 10 Jul 2010 01:32:00 +0000

An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.Atoms are made up of fundamental particles: electrons, protons and neutrons.
Dalton's Atomic Theory: John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:
- All substances are made up of tiny, indivisible particles called atoms.
- Atoms of the same element are identical in shape, size, mass and other properties.
- Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.
- Atom is the smallest unit that takes part in chemical combinations.
- Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.
- Atoms cannot be created, divided or destroyed during any chemical or physical change.
Representation of an Atom by a Symbol: Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. A symbol signifies a shorthand representation of an atom of an element. The symbol of any element is based on the English name or Latin name (written in English alphabets) and many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase). Examples are: (i) hydrogen- H (ii) aluminium- Al and not AL (iii) cobalt- Co and not CO. Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium. Therefore, each element has a name and a unique chemical symbol.
Size of the Atom/ Elements: Atoms are very small, they are smaller than anything that we can imagine or compare with. One hydrogen atom, the smallest atom known, is approximately 5 x 10-8 mm in diameter. Atomic radius is measured in nanometres. 1 m = 10⁹nm.
Atomic Mass: The mass of a particular atom is taken as a standard unit and the masses of other atoms are related to this standard. Hydrogen being the lightest element and being the smallest atom was chosen and assumed to have a mass of 1. An atom of hydrogen was assigned an atomic mass equal to one atomic mass unit (a.m.u). The number does not signify the mass of an atom in grams. It is just a pure number. The masses of atoms of other elements were compared to that of hydrogen, in order to find their atomic mass relative to it. If one atom of sulphur weighs as much as 32 atoms of hydrogen, then the relative atomic mass of sulphur is 32 a.m.u. This way of defining the mass of one atom of hydrogen has its difficulties. While the mass of one atom of hydrogen is considered as 1 atomic mass unit, hydrogen gas in its natural state has 3 isotopes of atomic mass 1, 2 and 3 respectively. Thus average mass works out to be 1.008 a.m.u rather than 1 a.m.u. This in turn complicates the atomic masses of all other elements. Later on, an atom of oxygen was preferred as standard by taking its mass as 16 units. However, in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly onetwelfth (1/12^th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12. It is equal to 1.66 x 10^-24 g.
Molecule: A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.
The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O₂. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a molecule is known as its atomicity.
Atoms of different elements join together in definite proportions to form molecules of compounds. Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion.
Chemical Formulae: The chemical formula of a compound is a symbolic representation of its composition. The chemical formulae of different compounds can be written easily.
The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. The valency of the atom of an element can be thought of as hands or arms of that atom.
The simplest compounds, which are made up of two different elements are called binary compounds. While writing the chemical formulae for compounds, we write the constituent elements and their valencies. Then we must crossover the valencies of the combining atoms.
The formulae of ionic compounds are simply the whole number ratio of the positive to negative ions in the structure .
Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions. Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.
Mole Concept: Since it is not possible to calculate the weight of particles individually, a collection of such particles called mole is taken for all practical purposes. It was discovered that the number of atoms present in 12g of carbon of ¹²C isotope is 6.023 x 10²³atoms. This is referred to as Avogadro number after the discoverer Avogadro. A mole of a gas is the amount of a substance containing 6.023 x 1023 particles. It is a basic unit of the amount or quantity of a substance. The substance may be atoms, molecules, ions or group of ions.
Mass of 1 mole of a substance is called its molar mass. One mole of any gas at STP will have a volume of 22.4 L. This is called molar volume.
Credit for the discovery of electron and proton goes to J.J. Thomson and E.Goldstein, respectively. J.J. Thomson proposed that electrons are embedded in a positive sphere.
Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model.
Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.
J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges.
The discovery of the electron, proton and neutron was the starting point of new avenues of research in science, which gave physicists an insight into the structure and nature of the atoms of matter. An atom is made up of three elementary particles, namely electrons, protons and neutrons. Electrons have a negative charge, protons have a positive charge and neutrons have no charge. Neutrons are neutral. Due to the presence of equal number of negative electrons and positive protons the atom as a whole is electrically neutral. Based on the above findings, one can say that the atom has two major divisions.
- The first is the centre of an atom, called its nucleus. The protons and neutrons are located in the small nucleus at the centre of the atom. Due to the presence of protons the nucleus is positively charged.
- The second are electrons, which revolve around the nucleus in different shells (or orbits). Shells of an atom are designated as K,L,M,N,….The space around the nucleus in which the electrons revolve, determines the size of the atom.
The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index, 1,2,3,…Hence the maximum number of electrons in different shells are as follows: first orbit or K-shell will be = 2 .1² = 2, second orbit or L-shell will be = 2 .2² = 8, third orbit or M-shell will be = 2 .3² = 18, fourth orbit or N-shell will be = 2 .4²= 32, and so on. The maximum number of electrons that can be accommodated in the outermost orbit is 8. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.
Valency: The electrons present in the outermost shell of an atom are known as the valence electrons. It is the decisive shell during a chemical reaction. The electrons of only this outermost shell are involved during chemical combinations; electrons are either given out from the outermost shell, or accepted into the outermost shell, or shared with the electrons in the outermost shell of another element. Elements having same number of valence electrons in their atoms possess similar chemical properties. The number of the valence shell in an atom determines its position in the Periodic Table i.e. the period to which the element belongs. Elements having 1, 2 or 3 electrons in the valence shell are metals. Exception is H and He. Elements having 4 to 7 electrons in their valence shell are non-metals. Valency is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom with an atomic number 6 has 4 valence electrons.
Calculation of Valency: The number of valence electrons is the valency of the element. The valency of an element can also be calculated by finding the number of electrons required to complete octet (8). If the outermost shell of an atom is completely filled, its valency = 0. The outermost shells of the noble gases helium, neon, argon, krypton etc. are completely filled. Hence their valency is zero. Such elements are very un-reactive and inert by nature.
Atomic Number: The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z.
Mass Number: The total number of protons and neutrons present in one atom of an element is known as its mass number. Mass number = number of protons + number of neutrons.
Isotopes: Isotopes are atoms of the same element, which have different mass numbers. It is interesting to note that atoms of a given atomic number can have different number of neutrons. For example, take the case of hydrogen atom, it has three atomic species, namely protium (¹₁ H), deuterium ((²₁ H or D) and tritium ((³₁ H or T). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively. All isotopes of an element have the same number of valence electrons thus have identical chemical properties. The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.
Isobars: Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. These have different number of protons but equal sum of number of protons and neutrons.
Isotones: The atoms of different elements, which have the same number of neutrons but different atomic numbers, are called isotones.
Radioactivity: Radioactivity is a nuclear phenomenon. It is the spontaneous emission of radiation from the nucleus. In 1899, the study of radioactivity was taken up by Ernest Rutherford. He placed a little radium at the bottom of a small lead box and subjected the rays that emerged from it to the action of a very strong magnetic field at right angles to their direction. He found that the rays separated into three distinct constituents. Rutherford called the three types of radiation alpha (α), beta (β) and gamma (g) rays. The α-rays were deflected in a direction opposite to that of β-rays and α-rays carried a positive charge, β-rays carried a negative charge and those which passed undeviated were neutral or uncharged were g-rays.

WBCS Preliminary (Chemistry): Properties of Gases

noreply@blogger.com (WBCSguru) — Fri, 09 Jul 2010 17:25:00 +0000

Properties of Gases:
- First, we know that a gas has no definite volume or shape; a gas will fill whatever volume is available to it. Contrast this to the behavior of a liquid, which always has a distinct upper surface when its volume is less than that of the space it occupies.
- The other outstanding characteristic of gases is their low densities, compared with those of liquids and solids. The most remarkable property of gases, however, is that to a very good approximation, they all behave the same way in response to changes in temperature and pressure, expanding or contracting by predictable amounts. This is very different from the behavior of liquids or solids, in which the properties of each particular substance must be determined individually.
- All gases expand equally due to equally due to equal temperature difference.
- Diffusion of gases: The phenomenon in which a substance mixes with another because of molecular motion, even against gravity- is called diffusion.
- The pressure of a gas: The molecules of a gas, being in continuous motion, frequently strike the inner walls of their container. As they do so, they immediately bounce off without loss of kinetic energy, but the reversal of direction (acceleration) imparts a force to the container walls. This force, divided by the total surface area on which it acts, is the pressure of the gas.
- The unit of pressure in the SI system is the pascal (Pa), defined as a force of one newton per square metre (1 Nm^–2= 1 kg m^–1 s^–2.)
- Temperature and Temperature Scales: Temperature is defined as the measure of average heat. Temperature is independent of the number of particles or size and shape of the object. The water boiling temperature is same for all type of containers.
- Thermometer: The device which is used to define the measure of temperature of an object is Thermometer.
- Temperature scale: A reference scale with respect to which the temperatures can be measured is known as 'scale of temperature'. Various scales of temperatures are in use. Important scales of temperature are:
  - Celsius scale
  - Kelvin scale
  - Fahrenheit scale
- To devise a scale of temperature, fixed reference points (temperature) are required, with respect to which all other temperatures are measured. For both Celsius and Fahrenheit Scales of temperatures, the fixed points are as follows:
  - Lower fixed point: Melting point of pure ice at normal atmospheric pressure is regarded as the lower fixed point.
  - Upper fixed point: Boiling point of pure water at normal atmospheric pressure is regarded as the lower fixed point.
- Celsius scale: In this scale the lowest fixed point is the freezing temperature of pure substance. The upper fixed point is the boiling point of water. The interval is divided into 100 divisions all are at equal distance. Every division being denoted as one degree Celsius(⁰C). The Celsius scale is also called as centigrade scale because the range of temperature is divided into 100 equal divisions.
- Kelvin scale: Another type of scale which is used to define the measure of temperature is Kelvin scale. The Kelvin scale is also known as absolute scale of temperature. The lowest fixed point is taken from the lowest temperature to which a substance to be cooled such as -273.15⁰C. According to the scale, a temperature is denoted by simply K .
- Absolute zero: The temperature at which a given mass of gas does not occupy any volume or does not exert pressure is called the “absolute zero”. Absolute zero i.e., 0K or -273oC is the lowest possible temperature that can be reached. At this temperature the gas has a theoretical volume of zero. In the Kelvin scale, the lowest possible temperature is taken as zero. This temperature is called as absolute zero.At the point absolute zero there is no molecular motion and there is no heat energy. At absolute zero all atomic and molecular motions stop. Hence the absolute zero is the lowest possible temperature which is denoted by 0K or -273.15⁰C.
- Fahrenheit Scale of Temperature: The lower and upper fixed points in this scale are considered as 32⁰F and 212⁰F respectively. The interval of 1800 F is divided into 180 equal parts. Each part is known as 1⁰F. This is widely used by doctors.
- The volume of a gas is simply the space in which the molecules of the gas are free to move. If we have a mixture of gases, such as air, the various gases will coexist within the same volume. In these respects, gases are very different from liquids and solids, the two condensed states of matter. The SI unit of volume is the cubic metre, but in chemistry we more commonly use the litre and the millilitre (ml). The cubic centimetre (cc) is also frequently used; it is very close to 1 milliliter (mL).
- Compressibility: Particles of a gas have large intermolecular spaces among them. By the application of pressure much of this space can be reduced and the particles be brought closer. Hence the volume of a gas can be greatly reduced. This is called compressing the gas.
Gas Laws – All gases, irrespective of their chemical composition, obey certain laws that govern the relationship between the volume, temperature and pressure of the gases. A given mass of a gas, under definite conditions of temperature and pressure, occupies a definite volume. When any of the three variables is altered, then the other variables get altered. Thus these Gas laws establish relationships between the three variables of volume, pressure and temperature of a gas.
- Boyle's Law: Robert Boyle (1627 - 1691) discovered this law in 1662 and it was named after him. It can be restated as "The product of the volume and pressure of a given mass of dry gas is constant, at constant temperature". P ∞1/ V( at constant temperature) or PXV= K (where K is constant).
- Charles' Law: "At constant pressure, the volume of a given mass of gas increases or decreases by 1/273 of its original volume at 32^oF, for each degree centigrade rise or lowering in temperature." Assume a given mass of gas has a volume of V1 at a temperature T1 Kelvin at a constant pressure, then, according to Charles' Law we can write: V ∞T or VT=K (Constant).
- Pressure Law: Volume remaining constant, the pressure of a given mass of gas increases or decreases by a constant fraction (=1/273) of its pressure at 0⁰C for each degree celsius rise or fall of temperature. If the pressure of a given mass of gas at 0⁰C be P_o; then for a rise or fall of temperature of T⁰C, its pressure P_t is given by P_t= P_o{1±(t/273)}
- Avogadro's Law : This is quite intuitive: the volume of a gas confined by a fixed pressure varies directly with the quantity of gas. Equal volumes of gases, measured at the same temperature and pressure, contain equal numbers of molecules. Avogadro's law thus predicts a directly proportional relation between the number of moles of a gas and its volume.
- Gay-Lussac’s Law: When different gases react with each other chemically to produce gaseous substances, then under the same condition of temperature and pressure, the volume of the reacting gases and product gases bear a simple ration among one another.
- Avogadro’s hypothesis: Under the same condition of pressure and temperature, equal volumes of all gases contain equal number of molecules.
- The molecular weight of an element or compound is the sum-total of the atomic weights of the atoms which constitute a molecule of the substance. Example: The molecular formula formula of nitric acid is HNO₃; hence its molecular weight = H+N+3XO=1+14+3X16=68(taking atomic weight of hydrogen as 1).
- Gram-Atomic Weight: A quantity of any substance whose mass in grams is numerically equal to its atomic weight, is called its Gram-Atomic Weight.
- Gram-Molecular Weight: A quantity of any substance whose mass in grams is numerically equal to its molecular weight, is called its Gram-Molecular Weight or mole.
- Molecular volume occupied by a mole of any gas is called the gram-molecular volume or molar volume. On the basis of Avogadro’s hypothesis, the gram molecular volume of any gas at normal temperature and pressure is 22.4 litres.
- Avogadro Number: From Avogadro’s hypothesis, we know equal volume of all gases contain equal number of molecules at normal temperature and pressure. Also we know that at normal temperature and pressure one mole of any gas occupies 22.4 litres. Combining the two, we can say that that, gram-molecular volume of all gases contain equal number of molecules at normal temperature and pressure. This number is known as Avogadro Number and is equal to 6.06X10²³.
- The Gas Equation: According to Boyle's Law, the volume of a gas varies inversely as the pressure, temperature remaining constant, i.e., V ∞1/ P and according to Charles' law, the volume of a gas varies directly as the absolute temperature, pressure remaining constant, i.e. V ∞T Both, these laws can be combined as: The volume of a given mass of a gas varies inversely with the pressure and directly with the temperature. V ∞(1/ P)XT or V ∞T/P or (PXV)/T = K(constant). In other words, For a given mass of a gas, if the initial conditions are P₁, V₁, and T₁, then the altered conditions are P₂, V₂, and T₂. Thus, (P₁X V₁)/ T₁ = (P₂X V₂)/ T₂
The ideal gas equation of state: If the variables P, V, T and n (the number of moles) have known values, then a gas is said to be in a definite state, meaning that all other physical properties of the gas are also defined. The relation between these state variables is known as an equation of state. By combining the expressions of Boyle's, Charles', and Avogadro's laws (you should be able to do this!) we can write the very important ideal gas equation of state: PV= nRT, where the proportionality constant R is known as the gas constant. This is one of the few equations you must commit to memory in this course; you should also know the common value and units of R.
An ideal gas is an imaginary gas that follows the gas laws and has 0 volume at 0 K i.e., the gas does not exist.

WBCS Preliminary (Chemistry): Chemical Reactions and Equations

noreply@blogger.com (WBCSguru) — Thu, 08 Jul 2010 15:29:00 +0000

Atoms and Molecules, Elements and Compounds: There are about a hundred different types of atoms in the Universe. Substances made up of a single type of atom are called Elements. Some elements are made up of single atoms: Carbon©, Helium(He), Sodium(Na), Iron(Fe) etc. He, Fe, and Na are the Chemical Symbols of the elements.
Some elements are made up of groups of atoms: Oxygen(O₂), Ozone(O₃ ), Chlorine(Cl₃ ) etc. These groups of atoms are called molecules.
Molecules can also be made up of combinations of different types of atoms. These substances are called compounds: Common Salt(NaCl), Methane(CH₄), Ammonia(NH₃) etc. O₂, CH₄, NH₃ are the Chemical Formulas of Oxygen, Methane and Ammonia respectively. CH₄ means that a single molecule of methane contains one atom of Carbon and four atoms of Hydrogen. This chemical formula could have been written but the C₁ H₄ is never written. Similarly, a molecule of Ammonia (NH₃) contains one atom of Nitrogen and three atoms of Hydrogen.
A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The change may conveniently be represented by a chemical equation.
Chemical reactions occur when different atoms and molecules combine together and spit apart. For example, if Carbon (C) is burnt in Oxygen (O₂) to form Carbon Dioxide, a Chemical Reaction occurs. This reaction can be written: C + O₂--> C O₂. This is called a Chemical Equation. The substances on the left hand side of the equation are called the Reactants. The substances on the right hand side are called the Products.
There is one very important rule with chemical equations: The number of individual atoms on each side of the equation must be the same. On the left had side, there is an atom of Carbon and a molecule of Oxygen (containing two atoms). On the right hand side there is a molecule of carbon dioxide (containing one atom of carbon and two atoms of Oxygen). The number of atoms on the left hand side is equal to the number of atoms on the right hand side. All that has changed is the arrangement of the atoms. In a chemical reaction atoms are re-arranged; no atoms are destroyed or created.
Hydrogen gas is mixed with Oxygen gas. If the mixture is sparked, it explodes to form water. This chemical reaction can be expressed as: H₂ + O₂--> H₂O. On the left had side, there is a molecule of Hydrogen (containing two atoms) and a molecule of Oxygen (also containing two atoms). On the right hand side there is a molecule of water (containing two atoms of Hydrogen and one atom of Oxygen). The left hand side has one extra atom of Oxygen. This is not allowed by the Law of Conservation of Matter. Both sides must contain the same number of atoms. To make the equation conform, we must balance the equation. It is not possible to change the chemical formulas of the reactants or products. Water will always be H2O. Balancing the equation is achieved by changing the number of molecules involved. The balanced form of the above equation is: 2H₂ + O₂--> 2H₂O. Now, on the left had side, there are two molecules of Hydrogen (each containing two atoms making four atoms) and a molecule of Oxygen (containing two atoms). On the right hand side there are two molecule of water (each containing two atoms of Hydrogen and one atom of Oxygen making a total of four atoms of Hydrogen and two of Oxygen). The equation is now balanced. In summary, when Hydrogen reacts with Oxygen, two molecules of Hydrogen react with one molecule of Oxygen to give two molecules of water.
The reaction goes in both directions. While the Nitrogen and Hydrogen are combining to form Ammonia, Ammonia splits to form Hydrogen and Nitrogen. A mixture of all three substances results. This type of reaction is called an Equilibrium and is represented by arrows going in both directions. N₂ + 3H₂--> 2NH₃.
It is possible to push the reaction in one direction by adding a Catalyst. A catalyst is a substance that helps a reaction without being used up. If Ammonia is removed from the equilibrium mixture, the reaction will move to produce more Ammonia so that equilibrium is attained.
The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction.
During a chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Actually, chemical reactions involve the breaking and making of bonds between atoms to produce new substances.
In a combination reaction two or more substances combine to form a new single substance.
Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.
Reactions in which heat is given out along with the products are called exothermic reactions.
Reactions in which energy is absorbed are known as endothermic reactions.
When an element displaces another element from its compound, a displacement reaction occurs.
Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
Precipitation reactions produce insoluble salts.
Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. The substance that brings about oxidation and is itself reduced is termed as oxidizing agent and the substance that brings about reduction and is itself oxidized is referred to as reducing agent.There are a number of oxidation-reduction reactions that are of industrial use. The production of metals from their ores invariably involves these two processes.

WBCS Preliminary (Chemistry):Acid, Base and Salts.

noreply@blogger.com (WBCSguru) — Wed, 07 Jul 2010 15:55:00 +0000

ACID:
- The word 'acid' is derived from a Latin word, which means "sour". The sour taste of most of the fruits and vegetables is due to various types of acids present in them. The digestive fluids of most of the animals and humans also contain acids.
- An acid is a compound, which on dissolving in water yields hydronium ions (H₃O⁺) as the only positive ions. The characteristic property of an acid is due to the presence of these hydronium ions.
- Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H₂SO₄; Nitric, HNO₃). However, not all compounds that contain Hydrogen are acids (Water, H₂O; Methane, CH₄). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen.
- Acids can be classified in various ways, depending on the factors mentioned below:
  1. Classification Based on the Strength of the acid.
  2. Classification Based on the Basicity of the Acid.
  3. Classification Based on the Concentration of the acid.
  4. Classification Based on the presence of Oxygen.
- The strength of an acid depends on the concentration of the hydronium ions present in a solution. Greater the number of hydronium ions present, greater is the strength of acid. However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.
- Strong Acid: An acid, which dissociates completely or almost completely in water, is classified as a strong acid. It must be noted that in these acids all the hydrogen ions (H+) combine with water molecule and exist as hydronium ions (H3O+). Examples of strong acids are: hydrochloric acid, sulphuric acid, nitric acid etc.
- Weak Acid:An acid that dissociates only partially when dissolved in water, is classified as a weak acid. Most of the molecules remain in solution in molecular form itself in such acid. Examples are: acetic acid, formic acid, carbonic acid etc.
- Acids are generally sour in taste. Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.
- The most commonly used natural indicator is litmus. It is extracted from lichens . It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper.
- The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
- Acids are corrosive and can burn flesh and dissolve metal.
Bases and Alkalis:
- A Base is a substance that gives OH^- ions when dissolved in water. Bases are usually metal hydroxides (MOH). Examples include Sodium Hydroxide, NaOH, Calcium Hydroxide, Ca(OH)₂. The solution of a base in water is called an alkali.
- Bases and acids neutralize each other, therefore another way to define a base is 'a compound which reacts with an acid to give salt and water only'. Like acids, alkalis can be strong or weak. The more hydroxide ions they produce, the stronger the alkali.
- The acidic property of an acid is due to the presence of hydrogen ions (H+) while that of a base or alkali, is due to the presence of hydroxyl (OH-) ions in them. When an acid and base (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.
- The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water.
  1. Strong Base : A base that dissociates completely or almost completely in water is classified as a strong base. The greater the number of hydroxyl ions the base produces, the stronger is the base. Examples: Sodium hydroxide: NaOH , Potassium hydroxide: KOH , Calcium hydroxide: Ca(OH) ₂.
  2. Weak Base: A base that dissociates in water only partially is known as a weak base. Examples: Magnesium hydroxide: Mg(OH) ₂, Ammonium hydroxide: NH₄OH.
- Bases are bitter to taste. They are soapy and slippery to touch. Strong alkalis like sodium hydroxide and potassium hydroxide are highly corrosive or caustic in nature. Sodium hydroxide and potassium hydroxide are commonly called caustic soda and caustic potash respectively. Organic tissues like skin, etc. get completely corroded by these two alkalis. However, the other alkalis are only mildly corrosive.
pH:
- A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali. Generally paper impregnated with the universal indicator is used for measuring pH. One such paper is shown in .
- There are chemicals that change colour at different pH values. These are called indicators. One of the most famous is Litmus. This substance turns red when the pH is less than 7 (acidic) and turns blue when the pH is greater than 7 (basic).
Salts:
- A Salt results when an acid reacts with a base. Both are neutralised. The H+ and OH- ions combine to form water. The non metalic ions of the acid and the metal ions of the base form the salt.
- Important salts used in everyday life and industrial applications are Sodium chloride (NaCl), Sodium carbonate, (Na ₂ CO₃), Sodium Bicarbonate, (NaHCO₃), Sodium Hydroxide (NaOH)
- The salt ions normally stay in solution. The salt crystalizes out when the water is removed. Some salts are insoluble. They will precipitate out when the acid and base are added together.
- Salts of a strong acid and a strong base are neutral with pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.

WBCS Preliminary (Chemistry):Matter and Its Nature

noreply@blogger.com (WBCSguru) — Tue, 06 Jul 2010 16:33:00 +0000

MATTER AND ITS NATURE:
- Anything that possesses mass, occupies space, offers resistance and can be perceived through one or more of our sense is called matter.
- Matter is made up of particles. Particles of matter have space between them and are continuously moving and attract each other.
- Matter can exist in three states-
  1. Solid
  2. Liquid
  3. Gas.
- Solid has a definite shape, distinct boundaries and fixed volumes, Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
- Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid.
- A gas has no definite volume or shape. gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles.
- The forces of attraction between the particles(inter-molecular force) are maximum in solids, intermediate in liquids and minimum in gases. The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.
- The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.
- In spite of above differences all kinds of matter have a common property, the property of having a mass.
- The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure.
- On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
- The process of melting, that is, change of solid state into liquid state is also known as fusion.
- During the melting, the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat.
- The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
- The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
- Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
- Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa.
- Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state. The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed. Evaporation causes cooling.
- During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.
- Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
- Properties such as shape, size, colour and state of a substance are called its physical properties. A change , which does not involve any alteration in composition of the substance is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
- Some substances can be obtained in pure state from their solutions by crystallisation.
- A change that alters the composition of a substance or substances taking part in the change is termed a chemical change. A chemical change is also called a chemical reaction.All new substances are formed as a result of chemical changes.
- Burning of coal, wood or leaves is a chemical change. Explosion of a firework is a chemical change. If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting. The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O2, from the air)water (H₂O) → rust (iron oxide- Fe₂O₃) For rusting, the presence of both oxygen and water (or water vapour) is essential. It is a chemical change.
- Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. Another way is to deposit a layer of a metal like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanisation.
- Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
- Changes attended with absorption of heat are called endothermic changes, while those which occur with evolution of heat are called exothermic changes. The reactions in which heat is absorbed are known as endothermic reactions, while chemical reactions which evolve heat are called exothermic. The compounds formed from their elements with absorption of heat are called endothermic compounds, whilst those formed from their elements with evolution of heat are called exothermic compounds.
Classifications:
- A pure substance is one that contains one kind of materials throughout its body. A substance cannot be separated into other kinds of matter by any physical process. Mixtures are constituted by more than one kind of pure form of matter, known as a substance. Mixtures can be separated into pure substances using appropriate separation techniques like filteration, sublimation, decantation, chromatography, crystallization, etc.
- A substance is said to be homogeneous if it has one and the same composition and properties in all its parts. On the other hand, if the composition and properties are not identical throughout the body the substance is heterogeneous. A pure substance must be homogeneous.
- Pure substance are classified into elements and compounds.
  Elements: An element is a form of matter that cannot be broken down by chemical reactions into simpler substances.Robert Boyle was the first scientist to use the term element in 1661. Elements can be normally divided into metals, non-metals and metalloids.
- Compound: A compound is a substance composed of two or more different types of elements, chemically combined in a fixed proportion. Properties of a compound are different from its constituent elements.
- Symbols: The symbol is an abbrevaiation for the full name of an element. In many cases the initial capital letter of the common name of element is used as abbrevaiation for it. H stands for Hydrogen, N for Nitrogen, etc. Two letters are used in cases of two or more elements having the same initial letter. A second prominent letter ( small) from its name is added to the initial letter. Al stands for Aluminium, Cl stands for chlorine, etc. In some cases the symbols are derived by taking letter or letters from the Latin name of the element. Cu stands for Copper ( Latin name Cuprum), Au stands for Gold ( Latin name Aurum), etc.
- Symbol represents one atom and naturally stands for a perfectly definite amount of the element concerned. Every substance is an aggregate of its molecules, and the symbolic representation of a molecule of the substance is called its formula. The number of atoms per molecule of the element is known as the atomicity of the molecule. If the molecule of an element contains one atom, then the molecule is represented by the symbol only, i.e., in such a case symbol represents also the formula.
- Valency: The number of chemical substances, except the element themselves, are composed of two or more of these elementary materials combined together. The valency of an element is the combining capacity of an atom of the element and is measured by the number of hydrogen atoms with which it can be combined. Hydrogen is chosen as the standard of referrence because the combining capacity of hydrogen is least. Though the combining capacity of an atom of the element is by and large fixed, valancy may vary; some elements exhibit different valancies. The highest valancy known being 8, the valancies range between 0 and eight. Helium, argon, etc., the so-called inert gases have no combining capacity and hence they are regarded as zero valent element. Valancy is always a whole number.
- Compounds too like elements are represented by molecular formula. To build up the formula of a compound the symbols of the constituent elements are written side by side and the number of atoms of each is indicated by putting numerals to the lower right of the symbols. But the subscript one is not written in formula.
Solution:
- A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute. Lemonade, soda water etc. are all examples of solutions. We can also have solid solutions (alloys) and gaseous solutions (air).
- The particles of a solution are smaller than 1 nm (10^-9 metre) in diameter. So, they cannot be seen by naked eyes. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.
- The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution/solvent.
- Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture.
Alloys:
- Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.
- Non-homogeneous systems, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.
- Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium.
METALS AND NON-METALS: Elements can be normally divided into metals, non-metals and metalloids. Metals usually show some or all of the following properties:
- They have a lustre (shine).Exception: Mercury, though a metal is liquid.
- They have silvery-grey or golden-yellow colour.
- They conduct heat and electricity. Silver is the best while copper stands second.
- They are ductile (can be drawn into wires).Gold is the most ductile metal.
- They are malleable (can be hammered into thin sheets). Exception: Metals like antimony and bismuth are brittle.
- They are sonorous (make a ringing sound when hit).
- Metals have high melting points. Exception:Gallium and Caesium have very low melting points.
- Metals can form positive ions by losing electrons to non-metals. In electrolysis metals get deposited at the negative electrode(cathode).
- Metals combine with oxygen to form basic oxides. Aluminium oxide and zinc oxide show the properties of both basic as well as acidic oxides. These oxides are known as amphoteric oxides.Different metals show different reactivities towards oxygen. Metals such as potassium and sodium react so vigorously that they catch fire if kept in the open. Hence, to protect them and to prevent accidental fires, they are kept immersed in kerosene oil.
- Different metals have different reactivities with water and dilute acids.Metals above hydrogen in the Activity series can displace hydrogen from dilute acids and form salts.
- Metals occur in nature as free elements or in the form of their compounds.The extraction of metals from their ores and then refining them for use is known as metallurgy.
- The surface of some metals, such as iron, is corroded when they are exposed to moist air for a long period of time. This phenomenon is known as corrosion.

WBCS Preliminary ( Physics): Space Science

noreply@blogger.com (WBCSguru) — Tue, 08 Jun 2010 18:23:00 +0000

The limitless expanse of space around us consisting of solar system, galaxies, stars and planets etc, is called universe. The age of the universe is estimated to be (1-2)X10¹⁰ years.
The vast collection of billions of stars along with the vast amount of hydrogen and dust in an isolated in the universe is called galaxy. There are nearly 10¹⁰ galaxies which are the building block of the vast universe. Galaxies are not fixed in the universe but are moving outwards.
Our Solar System is a part og thr galaxy called the "milky Way". The nearest galaxy to our own galaxy is Andromeda galaxy. The Milky way has three main parts: a nuecleus, a disc and a halo. It contains about 100,000 milion stars.The diameter of the Milky way is neraly 120,000 light years.
Nebula, which appear in the sky as bright spots, are actually clusters of stars and gaseous clouds. The gases in a nebula gradually gather together into spinning balls. They spins more and more quickly, untill they get amazingly hot and a big blast, called a nuclear reaction, begins. When this happens, a baby star begins to glow. Stars are mostly made of two gases, hydrogen and helium.
A group of a few stars whose arrangement can be compared to the figure of some animal or any other known thing is called costellations. There are in all 89 constellations. The largest of this is Hydra.
One of the most famous constellations which you can see during summer time in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi.There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.
Orion is another well-known constellation that can be seen during winter in the late evenings.It also has seven or eight bright stars. Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.
Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.
Solar System: The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.
There are eight planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars , Jupiter, Saturn, Uranus and Neptune.
Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system.
- The Sun: The Sun is 150 milion kilometer away from the earth. Its diameter is 1.4X10⁶ km and is approximately 3.0X10⁵ times heavier than earth. The core of the sun is made of mostly hydrogen gas at extremely high pressure and its temperature in the centre can be as high as 15 milion degree celsius. The atmosphere of sun consists three parts:
  1. Corona at a temperature of 1.7 X 10⁶ degree celsius.
  2. Chromosphere at a temperature of 27800 degree celsius.
  3. Photosphere at a temperature of 6000 degree celsius.
- Planets: The planets look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. A planet has a definite path in which it revolves around the Sun. This path is called an orbit. All the planets revolve round the sun in elliptical orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun. Besides revolving around the Sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation.
  1. Mercury (Budh):The planet mercury is nearest to the Sun. It is the smallest planet of our solar system.Mercury has no satellite of its own.
  2. Venus (Shukra): Venus is earth’s nearest planetary neighbour. It is the brightest planet in the night sky.Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.
  3. The Earth:The Earth is the only planet in the solar system on which life is known to exist.The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The average diameter of earth is about 12.800 kilometer or its radius is about 6400 kilometer. The earth takes 365 days and 6 hours to complete one revolution around sun. It also rotates on its own axis once in 24 hours. The Earth has only one satellite(Moon).
  4. Mars (Mangal):The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
  5. Jupiter (Brihaspati):Jupiter is the largest planet of the solar system. Jupiter has a large number of satellites. It also has faint rings around it. Ganymede which is the sattelite of jupiter is the largest satellite of the solar system.
  6. Saturn (Shani):Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eye. Saturn also has a large number of satellites. Titan of saturn is the second largest sattellite of the solar system. One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.
  7. Uranus:The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit. Uranus rotates east to west. Uranus' rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side.Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble and Keck observations showed that the outer rings are brightly colored. Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.
  8. Neptune: Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth.The main axis of Neptune's magnetic field is tipped over by about 47 degrees compared with the planet's rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth.Neptune has six known rings. Voyager 2's observations confirmed that these unusual rings are not uniform but have four thick regions (clumps of dust) called arcs. The rings are thought to be relatively young and short-lived.Neptune has 13 known moons, six of which were discovered by Voyager 2. Triton, Neptune's largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold - temperatures on its surface are about -235 degrees Celsius (-391 degrees Fahrenheit).
- The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets(terrestial planets).The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets( Jovian planets).
- Asteroids: There is a large gap in between the orbits of Mars and Jupiter . This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids.
- Comets : Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun.One such comet is Halley’s comet, which appears after nearly every 76 years.
- Meteors and Meteorites: A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly.These are commonly known as shooting stars, although they are not stars.Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite.
- The Moon: Any celestial body revolving around another celestial body is called its satellite.The Earth has only one satellite, moon.The day on which the whole disc of the moon is visible is known as the full moon day.Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.The moon completes one rotation on its axis as it completes one revolution around the Earth. The moon revolves around the earth once in about 27 days and 8 hours. The moon’s surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. Some of these are as high as the highest mountains on the Earth.The moon has no atmosphere. It has no water. On July 21, 1969 (Indian time) the American astronaut Neil Armstrong landed on the moon for the first time followed by Edwin Aldrin

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WBCS Preliminary ( Physics): Atomic Physics

noreply@blogger.com (WBCSguru) — Mon, 07 Jun 2010 18:58:00 +0000

An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.
Dalton’s atomic theory , which suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory.
Thomson proposed that:
1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model.
Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.
J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges. The mass of an electron is about 1/2000 times the mass of an hydrogen atom. The mass of a proton and a neutron is taken as one unit each.
We know that protons are present in the nucleus of an atom. It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess.
Mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons. Therefore, the mass of an atom resides in its nucleus.
Isotopes are atoms of the same element, which have different mass numbers.
Isobars are atoms having the same mass number but different atomic numbers.
To bind a nucleus together there must be a strong attractive force of a totally different kind. It must be strong enough to overcome the repulsion between the (positively charged) protons and to bind both protons and neutrons into the tiny nuclear volume. This force is called Nuclear Force.
The nuclear force is much stronger than the Coulomb force acting between charges or the gravitational forces between masses.The nuclear force between neutron-neutron, proton-neutron and proton-proton is approximately the same. The nuclear force does not depend on the electric charge.
Radioactivity occurs when an atomic nucleus breaks down into smaller particles. There are three types of nuclear radiation: alpha, beta, and gamma. Alpha particles are positively charged, beta particles are negatively charged, and gamma particles have no charge. The radiations also have increasing levels of energy, first Alpha, then Beta, and finally Gamma, which is the most energetic of all these. Alpha and Beta are particles, but Gamma is a wave.
When a radioactive nucleus changes, the remaining nucleus (and atom) is not the same as it was. It changes its identity. The term half-life describes the time it takes for half of the atoms in a sample to change, and half to remain the same.
There is even a radioactive isotope of carbon, carbon-14. Normal carbon is carbon-12. C-14 has two extra neutrons and a half-life of 5730 years. Scientists use C-14 in a process called carbon dating. This process is not when two carbon atoms go out to the mall one night. Carbon dating is when scientists try to measure the age of very old substances. There are very small amounts of C-14 in the atmosphere. Every living thing has some C-14 in it. Scientists measure the amount of C-14 in the things they dig up to estimate how old they are. They rely on the half-life of 5730 years to date the object.
Fission is the splitting of an atom. Not all atoms will go through fission; as a matter of fact, very few do under normal circumstances.
In a nuclear reaction, scientists shoot a whole bunch of neutrons at uranium-235 atoms. When one neutron hits the nucleus, the uranium becomes U-236. When it becomes 236, the uranium atom wants to split apart. After it splits, it gives off three neutrons and a lot of energy. Those neutrons hit three other U atoms in the area and cause them to become U-236. Each cycle, the reaction gets three times bigger. A reaction that, once started, continues by itself, is called a chain reaction.
Fusion is the process of two small atomic nuclei coming together to make a larger nucleus which is stable. The simplest nuclei to use are deuterium and tritium (isotopes of hydrogen).

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WBCS Preliminary(Physics): Sound

noreply@blogger.com (WBCSguru) — Sun, 06 Jun 2010 15:09:00 +0000

Sound is a form of energy and like all other energies, sound is not visible to us. It produces a sensation of hearing when it reaches our ears. Sound can not travel through vacuum.
Sound is produced due to vibration of different objects.The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Sound moves through a medium from the point of generation to the listener.
In longitudinal wave the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves. Sound travels as successive compressions and rarefactions in the medium. In sound propagation, it is the energy of the sound that travels and not the particles of the medium.
There is also another type of wave, called a transverse wave. In a transverse wave particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels. Thus a transverse wave is the one in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. Light is a transverse wave but for light, the oscillations are not of the medium particles or their pressure or density – it is not a mechanical wave.
To and fro motion of an object is known as vibration. This motion is also called oscillatory motion.
Amplitude and frequency are two important properties of any sound.
The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.
The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.
The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.
The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.
The number of complete oscillations per unit time is called the frequency (ν), ν =(1/T). The frequency is expressed in hertz (Hz).
Larger the amplitude of vibration, louder is the sound. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.
The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher, we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch.
A sound of single frequency is called a tone whereas a sound of multiple frequencies is called a note. Of the several frequencies present in a note, the sound of the lowest frequency is called the fundamental tone. Besides the fundamental, other tones present in a note are known as overtones. Of the overtones, those which have their frequencies simple multiple of fundamental frequency, are known as harmonics. All harmonics are overtone but all overtones are not harmonics.
The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time. speed, v = distance / time =(λ/T). Here λ is the wavelength of the sound wave. It is the distance travelled by the sound wave in one time period (T) of the wave. Thus, v = λ ν[Q(1/T)= ν]. or v = λ ν, That is, speed = wavelength X frequency. The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.
Sound propagates through a medium at a finite speed. The speed of sound depends on the properties of the medium through which it travels. The speed of sound in a medium depends also on temperature and pressure of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature the speed of sound increases. Experiment shows that the velocity of sound in air at 0 ⁰C is about 332 metres per second.
The velocity of sound through a gas is inversely proportional to the square root of the density of the gas.
The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface and the three lie in the same plane.
If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. The sensation of sound persists in our brain for about 0.1 second. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 second. If we take the speed of sound to be 344 m/s at a given temperature, say at 22 ⁰C in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) × 0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m. This distance will change with the temperature of air. Echoes may be heard more than once due to successive or multiple reflections.
The phenomenon of prolongation of sound due to successive reflections of sound from surronding objects is called reverberation.
Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.
The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s). Children under the age of five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz).
Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.
Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by dolphins, bats and porpoises.
Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are generally used in construction of big structures like buildings, bridges, machines and also scientific equipment. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect.
Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echocardiography’.
Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body. A doctor may image the patient’s organs such as the liver, gall bladder, uterus, kidney, etc. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs. In this technique the ultrasonic waves travel through the tissues of the body and get reflected from a region where there is a change of tissue density. These waves are then converted into electrical signals that are used to generate images of the organ. These images are then displayed on a monitor or printed on a film. This technique is called ‘ultrasonography’.
The acronym SONAR stands for SOund Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.Sonar consists of a transmitter and a detector and is installed in a boat or a ship. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound. Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through seawater be v. The total distance, 2d travelled by the ultrasound is then, 2d = v × t. The above method is called echo-ranging. The sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc.
Again if the speed of any substance, specially of an air-craft, be more than the speed of sound in air, then the speed of the substance is called supersonic speed. The ratio of the speed of a body and that of sound in air is, however, called the Mach number of the body. If the Mach number of a body is more than 1 , it is clear that the body has supersonic speed.

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WBCS Preliminary (Physics): Light

noreply@blogger.com (WBCSguru) — Tue, 01 Jun 2010 17:12:00 +0000

To understand light you have to know that what we call light is what is visible to us. Visible light is the light that humans can see. Other animals can see different types of light. Dogs can see only shades of gray and some insects can see light from the ultraviolet part of the spectrum.
As far as we know, all types of light move at one speed when in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second.
Any medium through which light can travel is an optical medium. If this medium is such that light travels with equal speed in all directions, then the medium is called a homogeneous medium. The homogeneous media through which light can pass easily, are called transperant media. The media through which light cannot pass, are called opaque media. Again the media through which light can pass partly, are called translucent media.
LIGHT TRAVELS ALONG A STRAIGHT LINE.
Light is reflected from all surfaces. Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
After striking the surface, the ray of light is reflected in another direction. The light ray, which strikes any surface,is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray.
The angle between the normal and incident ray is called the angle of incidence . The angle between the normal and the reflected ray is known as the angle of reflection.
Two laws of reflection are:
1. The angle of incidence is equal to the angle of reflection.
2. Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. On the other hand reflection from a smooth surface like that of a mirror is called regular reflection.
When rays of light coming from a point of source, after reflection or refraction, actually meet at another point or appear to diverge from another point, the second point is called the image of the first point. Images may be of two types, viz., (i) real and (ii) virtual.
An image which can be obtained on a screen is called a real image. An image which cannot be obtained on a screen is called a virtual image.
The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
The reflecting surface of a spherical mirror may be curved inwards or outwards. A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.
The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror. The pole is usually represented by the letter P.
The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C. Please note that the centre of curvature is not a part of the mirror. It lies outside its reflecting surface. The centre of curvature of a concave mirror lies in front of it. However, it lies behind the mirror in case of a convex mirror.
The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the mirror. It is represented by the letter R. You may note that the distance PC is equal to the radius of curvature.
Imagine a straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light. They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
Lenses are widely used in spectacles, telescopes and microscopes.Those lenses which feel thicker in the middle than at the edges are convex lenses. Those which feel thinner in the middle than at the edges are concave lenses. Notice that the lenses are transparent and light can pass through them.
A convex lens converges (bends inward) the light generally falling on it . Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens.
A convex lens can forms real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
A concave lens always forms erect, virtual and smaller image than the object.
The two surfaces of the lens are parts of two spheres. The straight line joining obtained by joining two centres of the spheres is called Principal axis. Generally we use lenses whose surfaces have equal curvature. In such lenses, if we take a point on the principal axis inside the lens equidistant from the two surfaces, the point is called the optical centre of the lens.
If a beam of parallel rays, travelling parallel to the principal axis of a convex lens, are refracted by the lens, the rays become converging and intersect each other at a particular point of the axis. The point is called the focus of the convex lens. The focal length of a lens is the distance between the optical centre and the focus of the lens.
The power of a lens is a measure of the degree of convergence( in the case of a convex lens) or divergence ( in the case of a concave lens). It is defined as the reciprocal of its focal length expressed in meters. The S.I. Unit of power of a lens is dioptre, the symbol being D. Thus, 1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m^–1. You may note that the power of a convex lens is positive and that of a concave lens is negative.
The phenomenon due to which a ray of light deviates from its path , at the surface of seperation of two media, when the ray of light is travelling from one optical medium to another optical medium is called refraction of light. When a ray of light travels from an optically rare medium to an optically denser medium, it bends towards the normal at the surface of seperation of two media.
When a ray of light travels from an optically denser medium to an optically rare medium, it bends away from the normal at the surface of seperation of two media.
When a ray of light strikes the surface of seperation of two media normally , it does not deviate from its original path. Some indexes of refraction are diamond (2.419), glass (1.523), and water (1.33).
Total internal reflection is the phenomenon which involves the reflection of all the incident light off the boundary. Total internal reflection only takes place when both of the following two conditions are met:(i) the light is in the more dense medium and approaching the less dense medium., and (ii) the angle of incidence is greater than the so-called critical angle. Total internal reflection will not take place unless the incident light is traveling within the more optically dense medium towards the less optically dense medium.
Dispersion of Light: It is the phenomenon of splitting of a beam of white light into its constituent colors on passing through prism. The order of colors from the lower end are violet, indigo, blue, green, yellow, orange and red. At one end of the band, there is red and at the other violet. The sequence of colours can be best remembered by the word VIBGYOR' which is formed by taking the initial letter of each colour.
A laser is just a really powerful beam of light. Laser isn't a word but an acronym. It stands for LIGHT AMPLIFICATION by STIMULATED EMISSION of RADIATION.

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