Question
Classify colloids based on the physical state of the dispersed phase and dispersion medium. Give examples for each type and explain the Tyndall effect.
(JEE Main, NEET, CBSE 12 — colloid classification is a guaranteed 1-mark question)
Solution — Step by Step
| Dispersed Phase | Dispersion Medium | Type | Example |
|---|---|---|---|
| Solid | Liquid | Sol | Paints, gold sol, starch sol |
| Solid | Gas | Aerosol (solid) | Smoke, dust |
| Solid | Solid | Solid sol | Gemstones (ruby), alloys |
| Liquid | Liquid | Emulsion | Milk, face cream |
| Liquid | Gas | Aerosol (liquid) | Fog, mist, cloud |
| Liquid | Solid | Gel | Jelly, cheese, butter |
| Gas | Liquid | Foam | Shaving cream, whipped cream |
| Gas | Solid | Solid foam | Pumice, foam rubber |
Note: gas-in-gas is NOT a colloid — gases are completely miscible and form true solutions.
Lyophilic (solvent-loving): The dispersed phase has strong affinity for the medium. Self-forming, reversible, more stable. Examples: starch, gelatin, gum arabic.
Lyophobic (solvent-fearing): Weak affinity for the medium. Need special methods to prepare, irreversible, less stable. Examples: gold sol, silver sol, Fe(OH)3 sol.
The Tyndall effect is the scattering of light by colloidal particles. When a beam of light passes through a colloid, it becomes visible (like sunlight through a dusty room). True solutions do not show this effect because their particles are too small.
Other important properties:
- Brownian motion: Zig-zag movement of colloidal particles due to collision with medium molecules
- Electrophoresis: Movement of charged colloidal particles under electric field
- Coagulation: Colloid particles aggregate and settle when an electrolyte is added
graph TD
A[Colloid Types] --> B["Sol: Solid in Liquid"]
A --> C["Emulsion: Liquid in Liquid"]
A --> D["Foam: Gas in Liquid"]
A --> E["Gel: Liquid in Solid"]
A --> F["Aerosol: Solid/Liquid in Gas"]
A --> G["Solid Sol: Solid in Solid"]
H[Properties] --> I["Tyndall Effect"]
H --> J["Brownian Motion"]
H --> K["Electrophoresis"]
H --> L["Coagulation"]Why This Works
Colloids occupy the middle ground between true solutions and suspensions — particle size between 1-1000 nm. At this size, particles are small enough to remain dispersed (no settling under gravity) but large enough to scatter light (Tyndall effect) and carry surface charges.
The stability of colloids comes from two factors: surface charge (like charges repel, preventing aggregation) and solvation layer (in lyophilic colloids, a layer of solvent molecules shields particles). Adding an electrolyte neutralises the surface charge, causing coagulation — this is the Hardy-Schulze rule.
Alternative Method
For JEE and NEET, the most tested examples:
- Milk = emulsion (liquid fat in liquid water) — an oil-in-water emulsion
- Butter = gel (liquid water in solid fat) — a water-in-oil system
- Fog = aerosol (liquid water in gas air)
- Smoke = aerosol (solid carbon in gas air)
- Blood = sol (solid particles in liquid plasma)
If the question gives an everyday example, match it to the phase combination.
Common Mistake
The most common error: saying “fog is a solution.” Fog is a colloid (liquid water droplets dispersed in air). A true solution would be water vapour in air (humidity) — which is invisible. Fog is visible precisely because the water droplets are large enough to scatter light (Tyndall effect).
Also, students confuse emulsion and sol. Both have a liquid dispersion medium, but in a sol the dispersed phase is solid (gold sol), while in an emulsion both phases are liquids (oil in water). Check the dispersed phase, not just the medium.