How are colloids classified on the basis of (i) physical states of components (ii) nature of dispersed phase a

Exercises · 5.14

Q. How are colloids classified on the basis of

(i) physical states of components

(ii) nature of dispersed phase and

(iii) interaction between dispersed phase and dispersion medium?

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Concept understanding — Colloids Classification

Colloids Classification – From Intuition to Precision

Imagine you drop a handful of sand into a glass of water. The sand sinks to the bottom — that's a suspension. Now imagine you stir a drop of milk into water. The milk spreads evenly and never settles. That's a colloid. A colloid is a mixture where one substance is finely dispersed in another, but the particles are too small to settle out and too large to be a true solution.

The key question is: how do we make sense of the endless variety of colloids? The answer is classification — by what is dispersed and what it is dispersed in, and by the nature of the particles themselves.

Classification by Dispersed Phase and Dispersion Medium

Every colloid has two parts: the dispersed phase (the substance that is broken into fine particles) and the dispersion medium (the substance in which those particles are spread). Both can be solid, liquid, or gas. That gives nine possible combinations, but only eight are stable — gas-in-gas is a true solution, not a colloid.

\text{Colloid type} = \frac{\text{Dispersed phase}}{\text{Dispersion medium}}

Here is the complete table:

Dispersed Phase	Dispersion Medium	Common Name	Example
Solid	Solid	Solid sol	Coloured gemstones (e.g., ruby — gold in glass)
Solid	Liquid	Sol	Paint, ink, muddy water (fine clay)
Solid	Gas	Aerosol (solid)	Smoke, dust in air
Liquid	Solid	Gel	Jelly, cheese, butter
Liquid	Liquid	Emulsion	Milk, mayonnaise, cream
Liquid	Gas	Aerosol (liquid)	Fog, mist, cloud, hair spray
Gas	Solid	Solid foam	Pumice stone, foam rubber
Gas	Liquid	Foam	Whipped cream, froth on beer

Notice the pattern: the name often tells you the medium. "Sol" means a liquid medium, "gel" means a solid medium, "aerosol" means a gas medium, "emulsion" means both phases are liquid, and "foam" means gas dispersed in a liquid or solid.

Tip

To remember: Aerosol = particles in air (solid or liquid). Emulsion = liquid in liquid. Gel = liquid trapped in a solid network. Sol = solid particles in a liquid.

Classification by Nature of Particles

Colloids can also be grouped by how the dispersed particles interact with the dispersion medium. This is a deeper, chemical classification.

1. Lyophilic colloids (solvent-loving): The dispersed particles have a strong affinity for the dispersion medium. They form spontaneously — just shake gelatin in water and it disperses. These are reversible: if you evaporate the water, the gelatin can be redispersed. Examples: starch, gum, gelatin, proteins in water.

2. Lyophobic colloids (solvent-fearing): The particles have little or no attraction for the medium. They do not form spontaneously — you need special methods (like chemical reactions or mechanical grinding) to make them. They are irreversible: once dried, they cannot be redispersed. Examples: gold sol, silver sol, ferric hydroxide sol, sulphur sol. …

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Step 1 – (i) On the basis of physical states of the components. Combining the solid/liquid/gas states of the dispersed phase with those of the dispersion medium gives eight colloidal types: sol (solid–liquid), solid sol (solid–solid), aerosol (solid or liquid–gas), gel (liquid–solid), emulsion (liquid–liquid), foam (gas–liquid), solid foam (gas–solid). (Gas-in-gas is not a colloid.)

Step 2 – (ii) On the basis of the nature of the dispersed phase.

Multimolecular colloids: aggregates of many atoms/small molecules held by weak forces (e.g. gold sol, sulphur sol).
Macromolecular colloids: each particle is a single large molecule/polymer (e.g. starch, cellulose, proteins; nylon, polythene).
Associated colloids: electrolytes at low concentration but form micelles above the CMC (e.g. soaps, detergents).

Step 3 – (iii) On the basis of interaction (affinity) between dispersed phase and medium. …

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Q.How are colloids classified on the basis of

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