Chemistry Desk

Free Energy Change of Adsorption

2014-09-22T23:52:00.000+05:30

Since adsorption is Exothermic Reaction. Therefore for adsorption ΔH is always negative and favours the process. Also adsorption leads to decrease in entropy i.e. ΔS is negative because the adsorbate molecules gets lesser opportunity to move about on the surface of adsorbent. According to Gibbs - Helmholtz Equation.
ΔG = ΔH – TΔS
Since adsorption is Spontaneous Process, hence ΔG is negative or ΔH > TΔS. As adsorption continues, the difference between two opposing factors becomes lesser and lesser till they becomes equal i.e ΔH = TΔS or ΔG = 0. At this stage, Equilibrium called Adsorption Equilibrium is established and hence no adsorption takes place at this stage.

Applications of Adsorption

2014-09-22T23:40:00.000+05:30

The phenomenon of adsorption finds extensive application in field of Industry, laboratory and various other technical processes. A few important applications of adsorption are as under:-
1.     Manufacture of Gas Masks
              Activated charcoal used in gas masks removes all undesirable toxic and poisonous gases while purified air passes through its pores.
2.      Removing Of Colouring Matter From Sugar Juice & Vegetable Oils
                 Activated animal charcoal is used as decolourizer in removing colouring matter of sugar solution and other vegetable materials.
3.      Dehydration & Purification Of Gases
                  Silica gel and Alumina are used as good adsorbents for removing moisture and for controlling humidities in rooms which store delicate articles.
4.      Dyeing of Clothes
                  Modrants used in dyeing adsorb the colouring matter which does not attach to fabric otherwise.
5.      Creation Of High Vacuum In Laboratory
                  Finely divided coconut charcoal adsorbs all gases creating a vacuum as low as 10–4 mm.
6.      Heterogenous Catalysis
                 The action of heterogeneous catalysts like finely divided nickel, finely divided platinum, finely divided iron in hydrogenation of oils, conversion of SO2 to SO3 and manufacture of ammonia respectively is based on the phenomenon of adsorption.
7.      Ion-Exchange Resins
                  The organic high polymers containing groups such as –COOH, –SO3H or –NH2 possess the property of selective adsorption of ions from solution. These resins are largely used in Industrial softening of water and separation of rare earths from their mixture.
8.      Chromotography
                  There are a number of chromotographic techniques like adsorption chromotography, paper chromotography and vapour phase chromotography which are based on selective adsorption of different substances of the adsorbent.
9.      Qualitative Inorganic Analysis
                  The confirmatory lake test for Al3+ ions is based on the adsorption of colour of litmus by Al(OH)3.
10.    Drugs
         Various drugs get adsorbed on the tissues which then are heated. The germicidal action of medicines is also based on the property of adsorption.

Difference Between Physical Adsorption and Chemical Adsorption

2014-09-22T23:36:00.000+05:30

S.No.	PHYSICAL ADSORPTION	CHEMICAL ADSORPTION
1.	Molecules are held due to vanderwaal’s forces.	Molecules are held due to chemical bond forces.
2.	Heat of adsorption are in the range of 20 – 40KJ/Mol.	Heat of adsorption are in the range of 40 – 400 KJ/Mol.
3.	Activation energy are small.	Activation energy are appreciable.
4.	Usually occurs rapidly at low temperatures and decreases with increase in temperature.	Can occur at high temperature.
5.	Usually completely reversible.	Often irreversible.
6.	The extent of adsorption is approximately related to the ease of liquification of gas.	No such correlation is there.
7.	Not very specific.	Often highly specific.
8.	Form multilayer on the surface of adsorbent.	Forms unimolecular layer.
9.	No surface compound is formed	Surface compounds are formed.

Application of Colloids

2013-08-22T21:37:00.002+05:30

Some important applications of colloidal are as follows:-
1.      Rubber Plating
                  The negatively charged rubber particles from rubber sol are deposited on wares handles of different tools, rubber gloves, etc by electroplating.
2.      Sewage Disposal
                  Sewage water contains particles of dirt, rubbish etc which are of colloidal size carry and charge and therefore do not settle down easily. The particles can be removed by electrophoresis. Dirty water is passed through a tunnel fitted with metallic electrode which is maintained at high potential difference. The particles migrate to the oppositely charged electrode, lose their charge and are coagulated. The deposited matter is used as a manure and water left behind is used for irrigation.
3.      Cottrel Smoke Precipitator
                  Smoke is made free of colloidal particles by passing it through cottrel’s precipitator installed in the chimney of an industrial plant. It consists of two metal discs charged to a high potential. The charge on colloidal dust and carbon particles of smoke is neutralised and they are precipitated down while free gases come out through chimney.

4. Formation of Delta
River water is a sort of colloidal solution of clay in water. Sea water on other hand, contains large number of dissolved salts and is a sort of electrolyte. When river water comes in contact with sea water, coagulation of colloidal particles occurs. These coagulated clay particles settle down at a point of contact and gradually river bed starts rising. This lead river water to adopt different course and ultimately results in the formation of delta.

5.      Leather Industry
                  In leather industry, the tanning of leather is based on the mutual coagulation of oppositely charged colloids. The animal skin is positively charged colloidal system containing albumin and geletin in the colloidal state. When treated with negatively charged colloidal system of tanin, present in barks of trees, wood leaves etc, a mutual coagulation of two oppositely charged sols take place. As a result, the surface of leather becomes harder.
6.      Dyeing Industry
                  Dyeing is essentially an adsorption of dye by the colloidal particles of mordant used. The clothes like those of cotton, wool, silk etc are colloidal in nature and Mordants used are also colloidal in nature. The mordants like Al(OH)₃, SnCl₂, tanin etc are readily adsorbed by fibres. This is another typical case of colloidal state of matter.
7.      Artificial Rain
                  Artificial rain is caused by spraying oppositely charged colloidal dust or sand particles over the clouds by aeroplane. The colloidal water particles of cloud gets neutralised and coagulation causes artificial rain.
8.      Purification of Water
                  The colloidal particles present in water can be precipitated by addition of small amount of alum (K₂SO₄ . Al₂ (SO₄)₃ . 24 H₂O). The Al³⁺ ions furnished by alum helps coagulation of impurities which are present in colloidal form.
9.      Distinction of Natural from Artificial Honey
                  Natural honey is also colloidal in nature. The distinction between two is made by Ley’s Test. It consists of treating few drops of honey with ammonical AgNO₃ solution. In case of pure honey, metallic silver produced assumes reddish yellow colour due to the traces of Albumin or ethereal oils, which acts as protective colloids. The dark yellow or greenish precipitate is formed with artificial honey.

Micelles

2013-08-22T21:27:00.000+05:30

Certain substances like soap and detergents behave as electrolytes at low concentration (the molecules of soap or detergent are smaller than colloidal particles). However, at high concentrations they constitute colloidal sol, which is known as Associated Colloids. The colloidal behaviour of such substances is due to formation of aggregates or clusters in solutions. Such clusters are of colloidal dimensions and are known as Micelles. Thus, micelles may be defined as the cluster or aggregated particles formed by associated colloids in solutions. The formation of micelles takes place above certain concentration called Critical Micellisation Concentration (CMC). Every micelle system has specific value of CMC.
Mechanism of Micelle Formation
Micelles are generally formed by the specific type of molecules which have lyophilic as well as lyophobic ends. Such molecules are known as Surface Active Molecules or Surfactant Molecules. Sodium oleate, C17 H33 COO^– Na⁺ is an example of such molecules.

The long hydrocarbon part of oleate radical (C₁₇H^-₃₃) is lyophobic end while COO^– is lyophilic end. When the concentration of the solution is below CMC (3 x 10^–3 Mol L^–1) sodium oleate behaves as an electrolyte and ionises to Na⁺ and C₁₇ H₃₃ COO^– ions. As the concentration exceeds CMC, the lyophobic part starts moving away from solvent (it is solvent repelling) and are made to approach each other. However the polar COO^– part tends to interact with water (solvent). This ultimately lead to formation of cluster having dimensions of the colloidal particles. In each such cluster a large number of oleate groups (usually 100 or more) are clumped together in a spherical manner so that their hydrocarbon part interact with one another to reduce contact with solvent but COO^– part remains projected in water.

Emulsions

2013-08-21T22:36:00.000+05:30

“An Emulsion is a colloidal solution in which both dispersion medium as well as dispersed phase are liquids. (The liquids should be Immiscible)”.
Preparation
Emulsions prepared by mere shaking of two liquids or by passing the mixture through a colloidal mill, called the Homogenizer, are unstable and tend to separate into two layers on standing. Thus, in order to get a stable emulsion, small quantities of certain other substances are added during preparation. The substances thus added to stabilize the emulsions are called Emulsifiers or Emulsifying Agents. The commonly used emulsifying agents are soaps, proteins, long chain sulphonic acids, basic sulphates of Fe, Cu, Ni etc.

Role of Emulsifier

                  The role of emulsifier can be explained by taking the example of soap as an Emulsifier. Soaps are sodium or potassium salts of higher fatty acids. A molecule of soap consists of two parts, the long hydrocarbon part (R–) which is oil soluble and the polar group COO^- Na⁺ which is water soluble. Thus when oil and water mixture in presence of soap is shaken vigrously, the oil drop is surrounded by soap solution, the R – part of soap remains in oil and COO^- Na⁺ part remains in water. Thus soap molecules are concentrated over the surface of oil drop. As a result interfacial tension between water and oil decreases and hence they do not coagulate or separate.

Types of Emulsion
Emulsion is colloidal solution of two immiscible liquids. Generally, one of the two liquids is water and the other which is immiscible with water, is designated as oil. Either of two liquids can constitute the dispersed phase. Thus there are two types of Emulsions.
i).      Oil-in-water type (o/w type)
ii).     Water-in-oil type (w/o type)
         i).      Oil-In-Water Emulsions
In this type of emulsions, oil acts as disperse phase and water acts as dispersion medium. Some examples of this type of emulsion are milk, vanishing cream etc.
         ii).     Water-In-Oil Emulsions
In this type of emulsions, water acts as dispersed phase and oil acts as dispersion medium. For example, butter, cod liver oil, cold cream etc.

Identification of Emulsions
The following tests may be employed to distinguish between two types of emulsions;
(i).     Dye Test
To the emulsion some oil soluble dye is added. If the background becomes coloured, the emulsion is water-in-oil type and if the droplets becomes coloured the emulsion is oil-in-water type.
(ii).    Dilution Test
If the emulsion can be diluted with water, this indicates that water is dispersion medium and the emulsion is of oil-in-water type. In case the water added forms a separate layer than emulsion is water-in-oil type.

Applications of Emulsions
Some important applications of emulsions are as follows:-
(1).    Cleansing Action of Soap
Washing action of soap is due to emulsification of grease, dirt and taking it away in water.
(2).    Digestion of Fats
Digestion of fats in the intestines is aided by emulsification. A little of fat forms a sodium soap with alkaline solution in the intestines and this soap emulsifies the rest of fat. It is easier for digestive enzymes to carry out their functions on emulsified fat.
(3).    In Medicines
A wide variety of pharmaceutical preparations are emulsions. For example: cod liver oil, disinfectant phenyl results in emulsion when poured in water.
(4).    In Metallurgical Operations
Emulsions play an important role in industry. The metal ores are concentrated with froth-floatation process which involves the treatment of the pulversied ore in emulsion of pine oil.

Gold Number

2013-08-21T22:09:00.002+05:30

The lyophilic colloids differ widely in their power of protection. The protective action of different colloids is measured in terms of the Gold number introduced by Zsigmondy. The gold number is defined as:
“The number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10ml of standard gold sol (containing 0.5 to 0.6gm of gold per litre) when 1 ml of 10% sodium chloride solution is added”.
The precipitation of gold sol is indicated by a colour change from red to blue. (When the particle size increases colour changes). The smaller the gold number of lyophilic colloid, the greater is its protecting power. Geletin has small gold number and is an effective protective colloid.

The gold number of few protective colloids is as follows:

Geletine = 0.005 – 0.01
Haemoglobin = 0.03 – 0.07
Egg Albumin = 0.08 – 0.10
Gum Arabic = 0.10 – 0.15
Potato Starch = 25
Casein = 0.01 – 0.02

In the preparation of ice cream, geletin is used as protecting agent to the colloidal particles of ice. If the ice particles coagulate, the smooth texture of ice cream is lost.

Protective Colloids

2013-08-21T21:54:00.000+05:30

Lyophobic sols like those of metals (Au, Ag etc) are unstable and are easily coagulated by addition of electrolyte. However, it is observed that when certain lyophilic colloids such as gum, geletin, Agar – Agar etc. are added to a lyophobic sol, the stability of the lyophobic colloids is markedly increased i.e. the addition of small amount of electrolytes does not cause coagulation of lyophobic colloids. This action of lyophilic colloids to prevent the coagulation of lyophobic colloid by addition of electrolyte is called Protection of Colloidal Sol and the lyophilic colloid is called Protective Colloid.

Explanation

The Particles of protective colloid is believed to be adsorbed by lyophobic colloidal particles and thus forms a covering over the surface of lyophobic colloidal particles. The lyophobic colloid thus behaves as lyophilic colloid and is precipitated less easily by electrolytes.
It may be noted that protective colloid not only increases stability of lyophobic colloids but it also makes them reversible.

Hardy-Schultz Rule

2013-08-20T00:54:00.000+05:30

     The coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum amount of an electrolyte (millimoles) that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called the Coagulation or Flocculation Value of the Electrolyte. Thus smaller is the flocculation value of electrolyte, greater is its coagulation or precipitating power.
Different electrolytes have different coagulation values. The coagulation behaviour of various electrolytes was studied by Hardy and Schultz. They gave a generalisation known as Hardy – Schultz Law, which states,
“Greater the valency of oppositely charged ions of the electrolyte being added, the faster is coagulation”.
So, for coagulation of sols carrying negative charge Al³⁺ ion is more effective than Ba²⁺ions or Na⁺ ions. Similarly for coagulation of positively charged sols ion is more effective than or ions.
Thus in case of positively charged sol the coagulating power of anions is in the order of

and in case of negatively charged sols, the coagulating power of cations is in the order of Al³⁺ > Ba²⁺ > Na⁺

Coagulation or Flocculation of Colloidal Solutions

2013-08-20T00:45:00.000+05:30

The colloidal sols are stable by the presence of electric charges on colloidal particles. Because of electric repulsion the particles do not come close to one another and hence do not aggregate. The removal of charge by any means will lead to the aggregation of particles and hence precipitates immediately.
The process by means of which the particles of the dispersed phase in a sol are precipitated is known as Coagulation or Flocculation.
The coagulation can be effected by different methods as follows:-
         1.      Heating or Cooling
                        In some cases, heating the sol results in coagulation. E.g. When egg is boiled, the albumin gets coagulated. Similarly, in some cases, cooling the sol results in the coagulation. E.g. Coagulation of milk i.e. on cooling milk, fat starts flowing on the surface.
2.      Mutual Action of Sols
                        When two sols carrying opposite charges are mixed together, their charges get neutralized and they get coagulated. Thus when negatively charged arsenic sulphide sol is added to the positively charged ferric hydroxide sol, in suitable proportions, coagulation of both sols take place simultaneously.
3.      Electrophoresis
When an electric current is passed through colloidal dispersion, particles migrate towards oppositely charged electrode and their charges get neutralized due to opposite charge on electrode. They thus get deposited on the electrode or are coagulated. Example: Rubber cloth can be prepared by electrophoresis of rubber latex.
4.      By Prolonged Dialysis
The stability of a colloidal sol is due to presence of small electrolyte. On prolonged dialysis the electrolyte is completely removed. As a result the colloidal sol becomes unstable and gets coagulated.
5.      Addition of Electrolyte
Presence of small concentration of appropriate electrolyte is necessary to stabilize the colloidal solutions. However, when large amount of electrolyte is added the colloidal particles get neutralized by the opposite charged ion of the electrolyte. Due to the neutralization of colloidal particles they aggregate and are coagulated.
For example: When aluminium sulphate or barium chloride is added to arsenic sulphide sol, it takes up barium ion or aluminium ion while the positively charged particles of Ferric hydroxide takes up chloride ion or sulphate ion, as the case may be. This causes neutralization of the charge on colloidal particles and lowers the stability of the sols, leading to coagulation.

Electrical Properties of Colloidal Solution (Electrophoresis and Electro-Osmosis)

2013-08-20T00:40:00.002+05:30

(I). Electrophoresis

When a potential difference (electric field) is applied across two platinum electrodes immersed in a colloidal solution, the particles of dispersed phase move towards either the positive or negative electrode. This observation was first discovered by Rauss in 1807 and was investigated later by Linder and Picton.

The movement of colloidal particles under the action of electric field is known as Electrophoresis.
If the colloidal particles move towards the positive electrode (Anode) they carry negative charge. On the other hand if the sol particles migrate towards negative electrode (Cathode), they are positively charged. From the direction of movement of colloidal particles it is possible to find out the charge on colloidals.

Demonstration of Electrophoresis
The demonstration of electrophoresis is as follows:-
Take a colloidal sol say AS2S3 sol in a U – tube. Place an electrolyte, having density less than that of solution (say distilled water). The electrolyte provides distinct boundary between electrolyte and colloidal sol.
Place two platinum electrodes in two arms of U – tube such that they dip in the colloidal sol. When a high potential difference of about 100 volts is applied across the two platinum electrodes, it is observed that the level or Boundary of colloidal solution falls on the negative electrode side and rises up on positive electrode side. On reaching the positive electrode, the colloidal particles get discharged. As a result of neutralisation of charge, the colloidal particles aggregate and settle down at the bottom.

Electro-Osmosis
A colloidal solution as a whole is electrically neutral in nature i.e., dispersion medium carries an equal and opposite charge to that of the particles of dispersed phase. When the movement of dispersed phase of colloidal solution is prevented by suitable means, the dispersion medium can be made to move under the influence of an applied electric field or potential. This phenomenon is referred to as Electro-Osmosis. Thus electro-osmosis may be defined as the movement of the dispersion medium under the influence of an applied electric field when the particles of dispersed phase are prevented from moving.

Demonstration of Electro-Osmosis

The phenomenon of electro-osmosis can be demonstrated experimentally as follows:-
The demonstration of electro-osmosis is carried out in a specially designed apparatus. The apparatus consists of a bigger tube having two side tubes T and T/ attached to its ends. The bigger tube is divided into three compartments A, B and C by means of two semi-permeable membranes. A tube carrying a stop-cock is attached to the central compartment A. Two platinum electrodes are inserted in the outer compartments B and C.
A colloidal dispersion is placed in the central compartment A and the outer compartments B and C are filled with water. The water in compartments B and C also extends to the side tube T and T/. The function of membrane is to prevent the movement of colloidal particles. Therefore, when a potential difference is applied across the electrodes held close to the membranes in the compartment B and C, dispersion medium begins to move.

If the particles carry positive charge, the dispersion medium would start moving towards the anode and the level of water in the side tube T would be seen to rise, indicating the presence of negative charge on the dispersion medium. If the particles carry negative charge, the dispersion medium would be seen to move towards cathode and water in the side tube T would start rising.
Electro osmosis is utilizing for dewatering moist clay and drying of dye pastes.

Mechanical Property (Brownian Movement)

2013-08-20T00:28:00.004+05:30

Robert Brown, a Botanist discovered in 1827 that pollen grains placed in water do not remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of colloidal particles when they were seen under the ultramicroscope. The particles were seen to be in constant motion in zig – zag path in all possible directions.
This zig – zag motion of colloidal particles is called Brownian Movement.

Cause of Brownian Movement

Brownian Movement arises because of impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impacts of the particles of dispersion medium on colloidal particles are unequal which ultimately causes its zig – zag motion. However, as the size of particle increases, the probability of unequal impacts decreases and the Brownian movement becomes slow. Ultimately, when the dispersed particles become bigger enough to acquire the dimensions of suspension, no Brownian movement is observed.
Significance of Brownian Movement
(i). Brownian Movement provides direct demonstration of ceaseless motion of molecules as postulated by Kinetic Theory.
(ii). It neutralizes the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols.

Optical Properties of Colloidal Solution (Tyndall Effect)

2013-08-20T00:18:00.000+05:30

Tyndall Effect
Tyndall in 1869, observed that if a strong beam of light is passed through a colloidal solution placed in a dark place and viewed at right angles to the direction of light beam, the path of light shows up as a hazy beam or cone. This is due to the fact that sol particles absorb light energy and then emit it in all directions in space. This emission of light known as Scattering of Light,illuminates the path of beam in colloidal dispersion.
“The phenomenon of scattering of light by the sol particles is called Tyndall Effect”. The illuminated beam or cone formed by the scattering of light by the sol particles is often referred as Tyndall Beam or Tyndall Cone. It is not shown by pure solvents.

Explanation
The Tyndall Effect is due to scattering of light by colloidal particles. The colloidal particles first absorb light and then a part of absorbed light is scattered from the surface of the colloidal particles. Maximum scattering intensity being in a plane at right angles to plane of incident light, the path becomes visible when seen from that direction. The particles of pure solvents or true solution are too small to scatter light.

General Properties of Colloidal Solutions

2013-08-20T00:07:00.001+05:30

         a).     Filterability
                        Colloidal particles can pass through ordinary filter paper (diameter of pores in ordinary filter paper is about 10-6m).
         b).     Molecular weight
                        Colloidal particles are formed by aggregation of a large number of small molecules, e.g starch (C6H10O5)n. Hence they possess very high molecular weights.
         c).     Osmotic Pressure
                        As colloidal particles is an aggregation of large number of particles, the number of particles in sol is very small. Therefore, osmotic pressure of sol is very low. The measurement of osmotic pressure has been used for determination of molecular weights of colloids.
         d).     Colour
                        The colour of sol depends on the wavelength of light. Scattered by colloidal particles which in turn depends on the size of the particles on which the wavelength of scattered light depends. Gold sol can be red, blue or purple depending upon size of particle.

Origin of Charge on Colloidal Particles

2013-08-19T23:57:00.001+05:30

The colloidal particles are known to carry positive or negative charge and all the particles of same sol carry same kind of charge. Various views have been put forward to explain the charge of colloidal particles.
(i). Frictional Electrification
The origin of charge on the colloidal particles may be due to frictional electrification. It is believed that frictional electrification due to rubbing the dispersed phase particles with those of dispersion medium results in some type of charge on colloidal particles of the sol. This view is not satisfactory.
(ii). Electron Capture by Colloidal Particles
It is believed that the colloidal solutions prepared by Bredig’s Arc Dispersion Method acquire a charge by electron capture. This view is not valid in all cases.
(iii). Dissociation Of Surface Molecules
The dissociation of surface molecules leads to electric charge on colloidal particles of the sol. For example, Consider an aqueous solution of soap which undergoes dissociation into ions.

Herein, the cation (Na⁺) passes into the solvent while the anions (C₁₅H₃₁ COO^-) have a tendency to form negatively charged aggregates due to weak attractive forces present in the long hydrocarbon chains. These aggregates are of colloidal dimensions and are negatively charged. This is not valid for colloidal solutions of non-electrolytes such as clay, smoke etc.
(iv). Presence of Some Acidic or Basic Groups in Colloidal Solution
Colloidal particles may acquire electric charge due to the presence of certain acidic or basic groups in colloidal solution. For instance, protein molecules give rise to formation of colloidal solutions. Thus the particles of protein sol either have positive charge or negative charge depending upon the PH of medium. A molecule of protein contains a carboxylic acid (COOH) group and also a basic amino (–NH2) group, it will form a positively charged particle in acidic medium and a negatively charged particle in basic or alkaline medium. This can be illustrated as follows:

At an intermediate PH point referred to as Iso-electric point, the protein will exist as uncharged molecules.
(v). Selective Adsorption of ions from Solutions
Colloidal particles adsorb preferentially positive or negative ions present in the dispersion medium. When two or more ions are present in the dispersion medium, the selective adsorption of ions common to the colloidal particles takes place resulting in the formation of positively charged or negatively charged particles in a colloidal solution. In simple words, the electric charge on the colloidal particles essentially originates by the selective adsorption of ions common to the colloidal particles from the dispersion medium.
For illustration, consider some typical examples of selective adsorption of ions.
(i). Positively charged Ferric hydroxide sol.
Fe(OH)₃ sol is prepared by shaking Ferric hydroxide precipitate with dilute solution of Ferric Chloride.

(ii). Positively charged silver chloride sol.
It is obtained by shaking silver chloride precipitate with dilute solution of silver nitrate.

(iii). Negatively charged Arsenious Sulphide Sol.
It is prepared by passing H₂S gas slowly through solution of AS₂O₃.

(iv). Negatively charged Silver Chloride Sol.
It is prepared by shaking silver chloride precipitate with dilute solution of hydrochloric acid.

Purification of Colloidal Solutions

2013-08-19T23:33:00.001+05:30

The colloidal solutions prepared by various methods contains appreciable amount of electrolyte and other soluble Impurities. These Impurities may destablise the colloidal sol. Hence, they are to be removed. A very important method of removal of soluble impurities from sol is Dialysis.

Dialysis
Animal membranes (bladder) or those made of parchment paper and cellophane sheet, have very fine pores. These pores permit ions (or small particles) to pass through, but not the large colloidal particles. Thus particles of true solution can pass through parchment membrane but colloidal particles cannot pass through these membranes. A bag made of such membrane is filled with the colloidal solution and is then suspended in fresh water (distilled water). After some time whole of crystalloid in solution passing out leaving the colloidal behind. The distilled water is renewed frequently to avoid accumulation of the crystalloid as otherwise they may start diffusing back into the bag. “The process of separating the colloidal particles from those of crystalloids by diffusion of mixture through a parchment or animal membrane is known as dialysis”.
The above process can be quickened if an electric field is applied around the membrane, the process is then known as Electrodialysis.

Ultrafiltration
In this method, colloidal solutions are purified by carrying out filtration through special type of graded filters called Ultra – filters. These filter papers allow electrolytes to pass through them but do not allow colloidal particles. These filter papers are made from ordinary filter papers impregnating them with colloidal particles. In order to quicken the filtration increased pressure or suction is required.
Ultra-Centrifugation
In this method, the colloidal solution is place in a high-speed centrifugal machine. On centrifuging, the colloidal particles settle down. The impurities remain in the centrifugate and are removed. The settled colloidal particles are mixed with water to form the colloidal solution again.

Preparation of Colloids

2013-08-12T00:12:00.001+05:30

Lyophilic sols may be prepared by simply warming the solid with liquid dispersion medium. E.g. Starch with water. On the other hand, lyophobic sols have to be prepared by special methods. These methods fall into two categories;

Condensation or Aggregation Methods.
Dispersion Methods.

Condensation or Aggregation Methods

Double Decomposition:

₂

₃

₂

₃

₂

₃

₂

₃

₂

Oxidation:

₂

Reduction:

₃

₂

₄

₃

Hydrolysis:

₃

₂

₃

The important physical methods for preparing lyophobic sols are:

By Exchange of Solvent:
By Excessive Cooling:

₃

Dispersion Methods

Mechanical Dispersion:
Bredig’s Arc Method:

Peptisation:

Cause Of Peptisation

₃

Difference Between Lyophilic and Lyophobic Colloids

2012-10-29T22:48:00.000+05:30

S.No.	PROPERTY	LYOPHILIC COLLOIDS	LYOPHOBIC COLLOIDS
1.	Mode of preparation.	Forms easily by mere shaking or warming the dispersed phase with dispersion medium E.g. Gelatine in water.	Forms only by special methods. Example: AS₂S₃ in water.
2.	Size of particle	The particles are true molecules and are just bigger in size (Molecular colloids).	The particles are aggregates of thousands of molecules (Associated colloids).
3.	Nature	Reversible and once precipitated can readily pass into colloidal state by direct contact with dispersion medium because particles are solvent loving.	Irreversible and once precipitated can not easily pass into colloidal state because particles are solvent hating.
4.	Visibility	The particles are not easily detected even under ultra microscope.	The particles are easily detected under ultra microscope.
5.	Charge	The charge of particles depends upon P_H of medium and it may be positive, negative or neutral.	The particles carry positive or negative charge.
6.	Stability	These are stable and are self – stabilized.	These are unstable and hence require traces of electrolyte for stabilisation.
7.	Action of electrolytes	Co-agulation can be brought about by the addition of large amount of electrolyte.	Co-agulation can be brought about by small amount of electrolyte.
8.	Viscosity	It is much higher than that of dispersion medium.	It is about same as that of dispersion medium.
9.	Surface	It is lower than dispersion medium	It is about same as that of dispersion medium.
10.	Tyndall effect	Less distinct	More distinct.
11.	Electrophoresis	The particles may or may not show electrophoresis.	The particles show electrophoresis.
12.	Hydration	The particles are heavily hydrated due to love for solvent.	The particles are not appreciably hydrated due to hate for solvent.
13.	Conc. Of dispersed phase	Higher concentrations of dispersed phase are possible.	Only low concentrations of dispersed phase are possible.
14.	Colligative property	They have relatively high osmotic pressure, depression in freezing point and high lowering in vapour pressure.	They have high osmotic pressure, less depression in freezing point, less elevation in boiling point and less lowering of vapour pressure.

Classification of Colloids

2012-10-29T22:44:00.004+05:30

Since the colloidal solutions consist of dispersed phase and dispersion medium, the components of these phases have certain specific characteristics of their own. In view of these observations, the colloidal solutions can be classified into the various types as under;

(I). CLASSIFICATION BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM

Based on the physical state of dispersed phase and dispersion medium, there are eight types of colloidal solutions. Either these phases may be solid, liquid or gas. Since gases gives homogeneous mixture in all proportions, hence colloidal solution can not be obtained in which dispersed phase and dispersion medium are gases.

S.No.	Dispersed Phase	Dispersion Medium	Name of Colloidal Solution	Examples
1.	Gas	Liquid	Foam	Soap lather, whipped cream, soda water, froth on beer.
2.	Gas	Solid	Solid foam	Cake, Bread, Pumice stone lava.
3.	Liquid	Gas	Aerosol	Mist, fog, Clouds.
4.	Liquid	Liquid	Emulsion	Milk, cream, butter, oil in water
5.	Liquid	Solid	Gel.	Curd, Cheese, Jellies, boot polish.
6.	Solid	Gas	Solid Aerosol	Smoke, dust.
7.	Solid	Liquid	Sols	Paints, gold sol, inks.
8.	Solid	Solid	Solid Sols	Alloys, coloured glass, gem stones.

(II). CLASSIFICATION BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE

Based on the nature of charge on the dispersed phase, there are two types of colloidal solution;

(a). Positive Colloids

When dispersed phase in a colloidal solution carries a positive charge, it is known as Positive Colloid.

Examples :- Metal hydroxides like Fe(OH)3, Al(OH)3, TiO₂, Methylene blue sols etc.

(b). Negative colloids

When dispersed phase in a colloidal solution carries a negative charge, it is known as Negative Colloid.

Examples: AS2S3 Sol, Cu or Ag Sol.

(III). CLASSIFICATION BASED ON APPEARANCE OF THE COLLOIDS

On this basis, the colloidal solutions can be divided into two types;

(a). Sols

When a colloidal solution appears as fluid it is referred to as Sol. The sols are generally named after dispersion medium. For Instance, when the dispersion medium is water, the sol is known as Hydrosol or Aquosol. When the dispersion medium is alcohol and benzene, the sols are called Alcosol and Benzosol respectively.

(b). Gels

When a colloidal has solid like appearance it is called Gel. The rigidity of gels varies from substance to substance. Some examples of gels are: Jelly, butter, cheese, curd.

(IV). CLASSIFICATION BASED ON MOLECULAR SIZE OF THE DISPERSED PHASE

On the basis of molecular size the colloids can be divided into three types:

(a). Multimolecular Colloids

In these colloids, the Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10–7cm. For instance a gold sol consists of particles of various size containing several atoms. Likewise, a sulphur sol contains particles each having eight sulphur atoms (S8). In these colloids, the particles are held by weak vanderwaal’s forces.

(b). Macro Molecular Colloids:

In these colloids, the particles of dispersed phase are sufficiently large in size enough to be of colloidal dimensions. These are called Natural Polymers. Some typical examples are starch, cellulose and proteins. Some artificial or synthetic polymers are polythene, polystyrene, nylon and plastics.

(c). Associated Colloids

These colloids behave as normal electrolytes at low concentrations but behave as colloids at higher concentration. These associated colloids are also referred as Micelles. Sodium Stearate behaves as electrolyte in dilute solution but as colloid at higher concentration. Some other examples of associated colloids are; Soaps, higher alkyl sulphonates, polythene oxide etc.

(V) CLASSIFICATION BASED ON THE INTERACTION OR AFFINITY OF TWO PHASES

The colloidal solutions can also be classified in terms of the attraction between the dispersed phase and dispersion medium as:

(a). Lyophilic Colloids

Colloidal solutions in which the dispersed phase has a great affinity or love for the dispersion medium are termed as Lyophilic Colloids. Such substances have a tendency to pass directly into colloidal solution when brought in contact with dispersion medium. The lyophilic colloids are also called Intrinsic Colloids.

If the dispersion medium is water, they are called Hydrophilic or Emulsoids. The lyophilic colloids are generally self-stablised, reversible in nature and are heavily hydrated. The typical examples of lyophilic colloids are starch, geletin, rubber, protein etc.

(b). Lyophobic Colloids

Colloidal solutions in which the dispersed phase has no affinity or hates the dispersion medium are called Lyophobic Colloids. These are also referred as Extrinsic Colloids. Such substances have no tendency to pass into colloidal solution when brought in contact with dispersion medium. The lyophobic colloids are relatively unstable and readily tend to change into a suspension. They are irreversible by nature and are stablised by adding small amount of electrolyte. They are poorly hydrated.

If the dispersion medium is water, the lyophobic colloids are called Hydrophobic or Suspensoids. The important typical instances of lyophobic colloids are sols of metals like Au, Ag, sols of metal hydroxides like Al(OH)3, Fe(OH)3 etc and sols of metal sulphides like AS2S3.

Colloids

2012-10-29T22:40:00.000+05:30

Selmi (1843) was first to study colloids systematically. He prepared colloidal solutions of sulphur, prussian blue, casein etc. According to him, these colloids were not true solutions but suspensions of extremely small particles in water. Later on Thomas Graham in 1861 studied the process of diffusion of dissolved substances through a Parchment membrane. He observed that some substances like glue, geletin starch etc in solution form diffused very slowly and rates of diffusion were found to be very small in comparison to those other substances like sugar, salt and urea in water.

In view of his observation, Thomas Graham divided substances into two distinct classes as under:-

(i). Crystalloids

(ii). Colloids

(I) Crystalloids:- Substances which diffuse readily in solution and can easily pass through a parchment membrane or animal membrane are known as Crystalloids. For instance, inorganic acids, bases, salts and organic compounds like sugar, urea etc.

(II) Colloids:- Substances which diffuse very slowly in solution and cannot pass through parchment paper or animal membrane are called Colloids. For instance, starch, glue, geletin, albumin etc.

In recent years, Graham’s classification of substances into Crystalloids and Colloids has undergone major change because it has been observed that every substance irrespective of its nature can be a colloidal under suitable conditions. For instance, NaCl a typical crystalloid in water behaves like a colloid in benzene. Soap, a typical colloid in water, behaves crystalloid in alcohol. Similarly insoluble metal like copper, silver and gold can be converted into colloidal form by suitable methods.

COLLOIDAL STATE OF MATTER

A colloidal is not a substance but it is a particular state of substance which depends on the particle size. Any substance can be brought into colloidal state by suitable methods. There are three distinct types of solutions of substances based on the size of particle. These are:–

1. True Solution :-

It is a homogeneous solution which contains small solute particles like molecules or ions dispersed throughout the solvent. The solute particle in a true solution are invisible even under microscope and these particles pass through a filter paper or an animal membrane. The particle size is 0.1 nm – 1 nm.

2. Suspension :-

It is a heterogeneous mixture which contains large insoluble particles. The particles of a suspension can be seen by naked eye and these particles can neither be passed through filter paper nor through animal membrane. The size of particles is greater than 100 nm.

3. Colloidal solution :-

It is a heterogeneous solution which contains solute particles of intermediate size. The particles of colloidal solution can pass through ordinary filter paper but not through animal membrane. These particles cannot be seen with a naked eye but can be seen under ultramicroscope. The particle size is 1nm – 100 nm.

DISTINGUISHING CHARACTERISTICS

TRUE SOLUTION, COLLOID AND SUSPENSION

S.No.	PROPERTY	TRUE SOLUTION	COLLOIDAL SOLUTION	SUSPENSION
1.	*Nature*	Homogenous	Heterogeneous	Heterogeneous
2.	*Size of Particle*	0.1 nm – 1nm.	1nm – 100nm.	> 100 nm
3.	*Filterability*	Passes through ordinary filter paper as well as animal membrane.	Passes through filter paper but not through animal membrane.	Does not passes through filter paper or animal membrane.
4.	*Separation*	It can not be separated by ordinary filter paper nor by ultra filtration.	It can not be separated by ordinary filter paper but can be separated by ultra filtration.	It can be separated by both.
5.	*Appearance*	Transparent	Generally transparent	Opaque.
6.	*Setting*	Does not settle.	Settles in centrifuge.	Settles under the action of gravity.
7.	*Ionisation*	May be ionised.	Not ionised.	Not ionised.
8.	*Diffusion*	Diffuses readily	Diffuses slowly	Does not diffuse.
9.	*Tyndal Effect*	Does not show Tyndal effect.	Shows Tyndal effect.	Does not show Tyndal effect.
10.	*Brownian Movement*	Does not show Brownian movement.	Shows Brownian movement.	May show.
11.	*Electrical Charge*	Uncharged	Charged either positively or negatively	Uncharged.

PHASES OF COLLOIDAL SOLUTION

A colloidal solution is heterogeneous in nature. It consists of two phases, dispersed phase and dispersion medium.

1. DISPERSED PHASE

In a colloidal solution, the component present in smaller proportion (solute) is known as Dispersed Phase. Example: In colloidal solution of silver in water, the component silver acts as dispersed phase.

2. DISPERSION MEDIUM

In a colloidal solution, the component present in larger proportion (solvent) is called Dispersion Medium. Example: In colloidal solution gold in water, the component water acts as a dispersion medium.

Types of Adsorption

2012-10-27T19:00:00.000+05:30

On the basis of nature of forces which hold molecules of adsorbate to the surface of the adsorbent, adsorption is classified into two types as under:-

I. Physical Adsorption

II. Chemical Adsorption

I. PHYSICAL ADSORPTION OR PHYSISORPTION

In this type of adsorption, the molecules of adsorbate are held by the weak vanderwaal’s forces of attraction. The heat of adsorption in this type is of order of 20 to 40 KJ/Mol. If the temperature is raised, the kinetic energy of the gas molecules increases and they leave the surface of the adsorbent. Thus, rise in temperature lowers the extent of adsorption. Moreover, in physical adsorption, the equilibrium (between adsorption and desorption) is reversible and is established rapidly. Physical adsorption does not depend upon the chemical nature of substance which is adsorbed. Physical adsorption increases with increase in pressure.

II. CHEMICAL ADSORPTION OR CHEMISORPTION

Unlike physical adsorption chemical adsorption involves formation of surface compound i.e. chemical linkage is formed between adsorbed molecule and the surface of adsorbent. Thus it is highly selective. It is found that only particular type of molecules are adsorbed by a solid in chemisorption. In other words, this type of adsorption depends upon the chemical properties of gas and the adsorbent. Moreover, chemisorption is accompanied by much higher heat changes 40–400KJ Mol^–1.

Unlike physical adsorption, it is not reversible. In many cases it is found that physical adsorption takes place at low temperature. But as the temperature is raised, it changes into chemical adsorption. Chemical adsorption is often called as Activated Adsorption.

DIFFERENCE BETWEEN Physical Adsorption AND Chemical Adsorption

S.No.	PHYSICAL ADSORPTION	S.No.	CHEMICAL ADSORPTION
1.	Molecules are held due to vanderwaal’s forces.	1.	Molecules are held due to chemical bond forces.
2.	Heat of adsorption are in the range of 20 – 40KJ/Mol.	2.	Heat of adsorption are in the range of 40 – 400 KJ/Mol.
3.	Activation energy are small.	3.	Activation energy are appreciable.
4.	Usually occurs rapidly at low temperatures and decreases with increase in temperature.	4.	Can occur at high temperature.
5.	Usually completely reversible.	5.	Often irreversible.
6.	The extent of adsorption is approximately related to the ease of liquification of gas.	6.	No such correlation is there.
7.	Not very specific.	7.	Often highly specific.
8.	Form multilayer on the surface of adsorbent.	8.	Forms unimolecular layer.
9.	No surface compound is formed	9.	Surface compounds are formed.

Applications of adsorption

The phenomenon of adsorption finds extensive application in field of Industry, laboratory and various other technical processes. A few important applications of adsorption are as under:-

1. Manufacture Of Gas Masks

Activated charcoal used in gas masks removes all undesirable toxic and poisonous gases while purified air passes through its pores.

2. Removing Of Colouring Matter From Sugar Juice & Vegetable Oils

Activated animal charcoal is used as decolourizer in removing colouring matter of sugar solution and other vegetable materials.

3. Dehydration & Purification Of Gases

Silica gel and Alumina are used as good adsorbents for removing moisture and for controlling humidities in rooms which store delicate articles.

4. Dyeing of Clothes

Modrants used in dyeing adsorb the colouring matter which does not attach to fabric otherwise.

5. Creation Of High Vacuum In Laboratory

Finely divided coconut charcoal adsorbs all gases creating a vacuum as low as 10–4 mm.

6. Heterogenous Catalysis

The action of heterogeneous catalysts like finely divided nickel, finely divided platinum, finely divided iron in hydrogenation of oils, conversion of SO2 to SO3 and manufacture of ammonia respectively is based on the phenomenon of adsorption.

7. Ion-Exchange Resins

The organic high polymers containing groups such as –COOH, –SO3H or –NH2 possess the property of selective adsorption of ions from solution. These resins are largely used in Industrial softening of water and separation of rare earths from their mixture.

8. Chromotography

There are a number of chromotographic techniques like adsorption chromotography, paper chromotography and vapour phase chromotography which are based on selective adsorption of different substances of the adsorbent.

9. Qualitative Inorganic Analysis

The confirmatory lake test for Al3+ ions is based on the adsorption of colour of litmus by Al(OH)3.

10. Drugs

Various drugs get adsorbed on the tissues which then are heated. The germicidal action of medicines is also based on the property of adsorption.

Factors Affecting The Adsorption of Gases by Solids

2012-10-26T22:04:00.000+05:30

Following are the factors which influence the adsorption of gases by solids.

1. Surface area

2. Nature of gas

3. Temperature

4. Pressure

1. Surface Area

Adsorption being a surface phenomenon, the extent of adsorption depends upon the surface area. Increase in the surface area of the adsorbent, increases the total amount of gas adsorbed. Thus finely divided metals (nickel, platinum) and porous substances (Charcoal, silica gel) provides large surface area and are best solid adsorbents.

2. Nature of Gas

The amount of gas adsorbed by a solid depends upon the nature of gas. In general, more easily liquefiable a gas is (i.e. higher its critical temperature), the more readily will it be adsorbed. Thus 1gm of activated charcoal adsorbs 380 ml of sulphur dioxide (critical temperature 157°C), 16 ml of methane (critical temperature –83°C) and 4.5 ml of hydrogen (critical temperature –240°C). This is valid for physical adsorption only.

Since Chemical Adsorption is Specific in nature, it occurs only if the gas can form a chemical bond with the solid.

3. Tempreture

The process of Adsorption is an Exothermic Reaction. Thus according to Le-chatlier’s Principle, the magnitude of adsorption should increase with decrease in temperature. Infact it is found to be so in case of physical adsorption because vanderwaal’s forces are strong at low temperatures. However, the chemisorption first increases with rise in temperature and then starts decreasing. The initial increase shows that like chemical reactions, chemisorption also needs activation energy.

If a plot is drawn between amount of gas adsorbed (x/m) and temperature at constant equilibrium pressure, then curve obtained for physical adsorption shows there is a regular decrease in adsorption with temperature rise. While for chemisorption it first increases and then shows regular decrease. Such curves are known as Adsorption Isobars.

4. Effect of Pressure

In order to understand the effect of pressure on adsorption of gas on some solid we must keep in mind that physical adsorption is reversible in nature and is accompanied by decrease in pressure. Therefore, it is expected that extent of adsorption increases with increase in pressure and decrease in pressure causes desorption.

The extent of adsorption is generally expressed as x/m where ‘m’ is mass of adsorbent and ‘x’ is mass of adsorbate when equilibrium has attained. The graph between extent of adsorption (x/m) and the pressure ‘P’ of gas at constant temperature is called Adsorption Isotherm.
This is a simple type of adsorption isotherm in which at equilibrium pressure P_s, reaches its maximum value and no more adsorption takes place even if the pressure is further increased. This state is also called SaturationState

and the corresponding pressure (P_s) is called Saturation Pressure. Such isotherms are obtained in cases where adsorbing gases forms unimolecular layers on the surface of adsorbent and adsorbing gas behaves ideally in vapour phase.

Heat (Enthalpy) of Adsorption

2012-10-26T21:29:00.001+05:30

Adsorption is a surface phenomenon and adsorbate molecules are held to adsorbent due to attractive interactions. Since energy is always released during attractive interactions, adsorption is an exothermic reaction. The amount of heat evolved when 1 mole of an adsorbate gets adsorbed on the surface of an adsorbent is called Molar Heat or Molar Enthalpy of Adsorption.

In Chemisorption, molecules of adsorbate are held to surface of adsorbent by chemical bonds. On the other hand, in Physisorption, adsorbate molecules are held by the weak Vanderwaals forces. Hence, more energy is released during Chemisorption as compared to Physisorption.

Free Energy Change of Adsorption

Since adsorption is Exothermic Reaction. Therefore for adsorption DH is always negative and favours the process. Also adsorption leads to decrease in entropy i.e. DS is negative because the adsorbate molecules gets lesser opportunity to move about on the surface of adsorbent. According to Gibbs - Helmholtz Equation.

DG = DH – T DS

Since adsorption is Spontaneous Process, hence DG is negative or DH > TDS. As adsorption continues, the difference between two opposing factors becomes lesser and lesser till they becomes equal i.e DH = TDS or DG = 0. At this stage, Equilibrium called Adsorption Equilibrium is established and hence no adsorption takes place at this stage.

Adsorption

2012-10-26T21:04:00.001+05:30

When a solid surface is exposed to a gas or a liquid, molecules of the gas or the solution phase accumulate or concentrate at the surface.

“The phenomenon of higher concentration of molecular species (gases or liquid) at the surface of a solid than in bulk is called Adsorption”. The solid substance on the surface of which adsorption occurs is called Adsorbentwhile the substance which gets adsorbed on the solid surface is called Adsorbate. The adsorbent may be a solid or a liquid and the adsorbate may be gas or a solute in some solutions. For example, activated charcoal (Finely divided charcoal) adsorbs a number of gases like ammonia, phosgene etc. Here, charcoal is adsorbent and the gases are adsorbate. Water vapours are adsorbed by silica gel. In this case silica gel is adsorbent and water vapours are adsorbate.

The term adsorption must be carefully distinguished from another like sounding term absorption. Adsorption implies concentration at surface only i.e it is surface phenomenon whereas in Absorption, the substance assimilated is uniformly distributed throughout the body of solid or liquid. For example, a chalk crayon dipped in ink is adsorbed and on breaking a chalk it is found white from inside. On the other hand water is absorbed by a sponge and is distributed throughout the sponge uniformly.

Some times adsorption and absorption takes place simultaneously and is difficult to distinguish the two, such process is termed as Sorption(Introduced by MC Bain).

(The adsorption of gases on the surfaces of metals is called Occulsion)

Explanation for Adsorption

Molecules in the interior of a liquid are completely surrounded by other molecules on all sides and hence the intermolecular forces of attractions are exerted equally in all directions. However, a molecule at the surface of a liquid is surrounded by large number of molecules in the liquid phase and fewer molecules in the vapour phase i.e., in the space above the liquid phase. As a result, these molecules lying at the surface, experience some net inward forces of attraction which cause surface tension. Similar inward forces of attraction exist at the surface of solids. Whereas in case of certain solids such as transition metal like Ni, there are unutilized free valencies at the surface.

Because of unbalanced inward forces of attraction or free valencies at the surface, liquids and solids have property to attract and retain the molecular species which comes in contact to these surfaces.

VAN ARKEL Method

2012-08-11T17:21:00.003+05:30

Ultra pure metals are being prepared by the Van Arkel Method. Crude metal is heated with a suitable substance so that the pure metal present in it may be converted into stable volatile compound leaving behind impurities. The compound so formed is then decomposed by heating to get the pure metal. Van Arkels method is used to purify crude titanium metal. It is heated with iodine to about 500K to form volatile compound. Til₄ leaving behind the impurities .Til₄ is further heated to 1700K when it decomposes to give pure titanium.