By Sunil Bhardwaj

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Colloids can be classified on:

1. The basis of physical state of dispersed phase and dispersion medium

Each of the dispersed phase and the dispersion medium can be a solid or liquid or gas. There are eight different combinations possible.

#Dispersed PhaseDispersion MediumNameExample
1SolidSolidSolid SolColoured glass, alloys. gems.
2SolidLiquidSolPaints, inks. white of egg, colloidal Au,
3SolidGasAerosolSmoke. dust.
4LiquidSolidGelCurds, cheese, jelly, pudding.
5LiquidLiquidEmulsionMilk, butter. oil in water.
6LiquidGasAerosolMist, fog, clouds.
7GasSolidSolid foamCake, bread, pumice stone.
8GasLiquidFoamSoap lather, aerated water.

A colloidal solution with both the components in the gaseous state is not possible as according to Grahams Law of diffusion, they form a homogeneous mixture.

2. Classification on the basis of appearance:

#SolGel
1When a colloidal solution appears a fluid, it is termed as sol. Sols are named after the dispersion medium.When a colloid has a solid like rigid appearance it is called gel. The rigidity of a gel varies from substance to substance. Some substances may occur both as sols as well as gels. This depends upon the relative concentrations of the dispersed phase and dispersion medium.
2For example when dispersion medium is water, they are called hydrosols, if dispersion medium is alcohol they are called alcosols and if it is benzene, they are called benzosols.A gel is formed by interlocking of the dispersed phase particles. Gelatine dissolves in warm water to form a colloidal solution which sets into a gel.

3. Classification on the basis of solvent affinity

#lyophilic Sol (solvent loving):lyophobic sol (solvent hating):
1When the dispersion medium exerts an attraction on the dispersed phase then we get lyophilic Sol (solvent loving). When the attraction between the dispersion medium and the dispersed phase is very little then the sol is called lyophobic sol (solvent hating).
2Examples of lyophilic (reversible) sols: Protein, starch, glue, gelatine and agar-agar give reversible colloidal system with water. Rubber in benzene is also a reversible colloidal system.Typical examples of lyophobic sols (non-reversible): Metals, sulphur, sulphides, silver halides, egg albumen, silicic acid and ferric hydroxide give non-reversible colloidal systems with water.