Question
When a beam of light is passed through three beakers containing (A) salt solution, (B) starch solution, and (C) muddy water, in which beaker(s) will the path of light become visible? Use the Tyndall effect to distinguish between true solutions, colloids, and suspensions.
(NCERT Class 12, Surface Chemistry)
Solution — Step by Step
- Beaker A (salt solution): True solution — particle size < 1 nm
- Beaker B (starch solution): Colloid — particle size 1-1000 nm
- Beaker C (muddy water): Suspension — particle size > 1000 nm
The Tyndall effect occurs when particles are large enough to scatter light but small enough to remain dispersed. This happens when particle size is comparable to the wavelength of visible light (400-700 nm).
- Beaker A: No scattering. Particles too small. Light passes straight through. No visible path.
- Beaker B: Tyndall effect observed. Colloidal particles scatter light, making the beam path visible as a cone of light.
- Beaker C: Particles are too large — they block/absorb light rather than scattering it uniformly. The mixture is opaque. Path not clearly visible as a defined beam.
| Property | True Solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | 1-1000 nm | > 1000 nm |
| Tyndall effect | No | Yes | No (opaque) |
| Filterability | Passes through filter paper | Passes through filter paper | Retained by filter paper |
| Settles on standing | No | No | Yes |
| Visibility | Transparent | Translucent | Opaque |
Answer: The path of light is visible only in Beaker B (starch solution — a colloid).
Why This Works
The Tyndall effect is a consequence of light scattering by particles whose dimensions are in the right range. When light hits a very small particle (like Na⁺ or Cl⁻ ions in salt solution), it barely interacts — the light wave simply passes around it (Rayleigh scattering is negligible for such tiny particles at visible wavelengths).
Colloidal particles (1-1000 nm) are just the right size. They scatter visible light effectively, especially shorter wavelengths (blue). This is the same physics behind why the sky appears blue — air molecules scatter blue light more than red.
Suspension particles are so large that they simply block light or reflect it diffusely. You don’t see a clean beam path; you see an opaque or turbid mixture.
Alternative Method
A quick lab test without a light beam: try filtering. True solutions pass through filter paper and even ultra-filters. Colloids pass through ordinary filter paper but are stopped by ultra-filters (semi-permeable membranes). Suspensions are caught by ordinary filter paper.
NEET frequently asks matching-type questions: match the property with the type of mixture. The three key distinguishing tests are Tyndall effect, filterability, and settling behaviour. Memorise the comparison table above — it covers 90% of what’s asked.
Common Mistake
Students sometimes say suspensions also show the Tyndall effect because “their particles are bigger so they scatter more light.” This is wrong. Suspension particles are so large that they cause absorption and diffuse reflection, not the focused scattering needed for the Tyndall effect. The Tyndall effect requires particles in a specific size range — not just “big particles.”