Surface Chemistry : Colloids

A heterogeneous solution in which the diameter of the solute particles is in the range 1nm to 1000nm is called a colloid.

The solute and solvent are called dispersed phase and dispersion medium respectively in case of a colloid.

◆ Methods of preparation of colloids : Colloids are prepared by following methods-

1. Chemical methods : There are several methods to prepare colloids like oxidation, reduction, hydrolysis, double decomposition etc. All these methods can be called condensation methods because molecules aggregate together to form sols.

I. Double decomposition : As2O3 + 3H2S  → As2S3 (Sol)+ 3H2O

II. Hydrolysis : FeCl3 + 3H2O → Fe(OH)3 (Sol)+ 3HCl

III. Oxidation : SO2 + 2H2S → 3S(Sol) + 2H2O

IV. Reduction : 2 AuCl3 + 3 HCHO + 3H2O  → 2Au(Sol) + 3HCOOH

2. Electrical Disintegration or Bredig’s Arc Method : This method is used to prepare sols of metals like gold, silver, platinum etc. The colloid is formed by the involvement of both dispersion and condensation.

Process : In this method an electric arc is struck between metal electrodes immersed in the dispersion medium. The metal is first vaporised due to intense heat and then condenses to form parties of colloidal size. This, a colloid is prepared.

3. Peptisation : The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of an electrolyte is called peptisation.

The electrolyte used for this purpose is called the peptising agent.

Process : In this process the precipitate adsorbs one of the ions of the electrolyte on its surface. It causes the development of positive or negative charge on precipitates due to which precipitate particles break up into smaller particles of colloidal size. Thus, a colloid is formed.

Purification of colloidal solution : While forming a colloidal solution some impurities enter into it. Sometimes electrolytes are in the excess. Due to these impurities a colloidal particles may coagulate and the solution may be spoiled.

Properties of colloids : 1. Colligative properties :

2.Charge on colloidal particles : There is charge on each particle of the dispersed phase in a colloidal solution. The reason behind it are as follows:

I. Frictional electrification :  One the reasons responsible for charge on the particles of the dispersed phase is the rubbing of the particles of the dispersed phase with those of the dispersion medium.

II. Dissociation of molecules :

III. Selective adsorption of ions :


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