Question
A beam of light is passed through a colloidal solution and the path of light becomes visible. Name this phenomenon and explain why it occurs in colloids but not in true solutions.
Also classify the following into sol, gel, emulsion, or foam: (i) milk (ii) butter (iii) shaving cream (iv) paint.
Solution — Step by Step
The visible light path in a colloid is the Tyndall effect, first observed by John Tyndall. It happens because colloidal particles (1–1000 nm) are large enough to scatter light sideways — this scattered light makes the beam visible.
True solutions have particles smaller than 1 nm. They’re too tiny to scatter visible light, so the beam passes through without any visible path.
Light scattering requires particles comparable in size to the wavelength of visible light (~400–700 nm). Colloidal particles fall right in this range — they scatter light efficiently via Rayleigh scattering.
Ions and molecules in a true solution are far too small (sub-nanometer). They don’t scatter visible light — the solution looks perfectly clear.
We identify each by asking: what is the dispersed phase and what is the dispersion medium?
| Substance | Dispersed Phase | Dispersion Medium | Type |
|---|---|---|---|
| Milk | Liquid (fat) | Liquid (water) | Emulsion |
| Butter | Liquid (water droplets) | Solid (fat) | Emulsion (solid) |
| Shaving cream | Gas | Liquid | Foam |
| Paint | Solid (pigment) | Liquid | Sol |
- Phenomenon: Tyndall effect
- Milk → emulsion (liquid-in-liquid)
- Butter → solid emulsion (often just called emulsion in NCERT context)
- Shaving cream → foam (gas dispersed in liquid)
- Paint → sol (solid dispersed in liquid)
Why This Works
The key to colloid classification is always the two-phase framework: dispersed phase + dispersion medium. Once you identify both phases and their states (solid/liquid/gas), the type follows automatically from the table.
The Tyndall effect is essentially a diagnostic test for colloids. If you shine a torch through a liquid and see a cone of light, it’s a colloid. If the path is invisible, it’s a true solution. This is why the headlights of a car are clearly visible in fog (a colloid of water droplets in air) but not on a clear night.
NCERT lists 8 types of colloids based on dispersed phase and medium combinations. In NEET, the most frequently tested are: sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid), and aerosol (liquid in gas — like clouds and fog). Learn these four cold.
Alternative Method
For classification questions, use this quick mental filter:
- Gas in liquid → Foam (shaving cream, whipped cream)
- Liquid in liquid → Emulsion (milk, mayonnaise)
- Solid in liquid → Sol (paint, starch solution, gold sol)
- Liquid in solid → Gel (cheese, jellies, boot polish)
Paint is commonly confused with an emulsion because it looks creamy. But the pigment particles are solid, not liquid — so it’s a sol.
Common Mistake
Students write that the Tyndall effect is due to reflection of light. It is not — it is scattering (diffraction from particles comparable to light’s wavelength). Reflection requires a flat surface; scattering occurs in all directions from individual particles. CBSE and NEET both test this distinction. Write “scattering” in your answer, not “reflection”.
Another slip: classifying butter as a gel. Butter is a solid emulsion — water droplets are dispersed in a solid fat matrix. A gel has liquid as the dispersed phase in a solid medium, which fits, but NCERT specifically calls butter an emulsion. Stick with the NCERT classification to be safe in board exams.