Question
How are colloids classified based on the physical state of the dispersed phase and dispersion medium? What are sols, gels, emulsions, foams, and aerosols with everyday examples?
(JEE Main, NEET, CBSE 12 — colloid classification table is a direct 1-mark question that appears almost every year)
Solution — Step by Step
A colloid (or colloidal solution) has particles sized 1 nm to 1000 nm — larger than true solution particles but smaller than suspension particles. They pass through filter paper but not through parchment membranes (dialysis principle).
Two components: dispersed phase (the scattered particles) and dispersion medium (the continuous phase they are scattered in). Both can be solid, liquid, or gas — giving us 8 types (gas in gas is not a colloid, it is always a true solution).
| Dispersed Phase | Dispersion Medium | Name | Example |
|---|---|---|---|
| Solid | Liquid | Sol | Gold sol, muddy water, starch solution |
| Liquid | Liquid | Emulsion | Milk (fat in water), mayonnaise |
| Gas | Liquid | Foam | Whipped cream, shaving cream |
| Solid | Gas | Aerosol (solid) | Smoke, dust storm |
| Liquid | Gas | Aerosol (liquid) | Fog, mist, cloud |
| Solid | Solid | Solid sol | Ruby glass (gold in glass), alloys |
| Liquid | Solid | Gel | Jelly, cheese, butter |
| Gas | Solid | Solid foam | Pumice stone, foam rubber, bread |
Gas + Gas is NOT a colloid — gases always form true (homogeneous) solutions.
Emulsions deserve extra focus because they come in two sub-types:
- Oil-in-water (O/W): Oil droplets dispersed in water. Examples: milk, vanishing cream. Stabilised by water-soluble emulsifiers (like soap with the polar head in water).
- Water-in-oil (W/O): Water droplets dispersed in oil. Examples: butter, cold cream. Stabilised by oil-soluble emulsifiers.
How to distinguish: Dilution test — O/W emulsions can be diluted with water; W/O emulsions can be diluted with oil. Dye test — water-soluble dye colours O/W uniformly.
flowchart TD
A["Colloid Classification"] --> B["By Dispersed Phase"]
B --> C["Solid dispersed"]
B --> D["Liquid dispersed"]
B --> E["Gas dispersed"]
C --> C1["In liquid → Sol"]
C --> C2["In gas → Aerosol (smoke)"]
C --> C3["In solid → Solid sol (ruby glass)"]
D --> D1["In liquid → Emulsion (milk)"]
D --> D2["In gas → Aerosol (fog)"]
D --> D3["In solid → Gel (jelly)"]
E --> E1["In liquid → Foam (whipped cream)"]
E --> E2["In solid → Solid foam (pumice)"]Why This Works
The classification is purely physical — based on the state of matter of each component. What makes colloids special is the particle size range (1-1000 nm), which gives them unique properties: Tyndall effect (scattering of light), Brownian motion (random zigzag movement), electrophoresis (movement under electric field), and coagulation (clumping by addition of electrolytes).
The reason gas-in-gas is excluded is simple: gas molecules are always in molecular-level mixing, giving particles well below 1 nm. You cannot have colloidal-sized gas clusters dispersed in another gas under normal conditions.
Common Mistake
Students often confuse gels and sols. A sol has solid particles dispersed in a liquid. A gel is the reverse — liquid trapped in a solid network. Jelly is a gel (liquid trapped in a protein network), NOT a sol. Butter is also a gel (water trapped in fat). If the question asks “dispersed phase of jelly,” the answer is liquid, not solid.
Quick memory trick for aerosols: if you can SEE through it barely, it is fog (liquid aerosol). If you CANNOT see through it at all, it is smoke (solid aerosol). Both are dispersed in gas (air).