Question

Which among the following shows Tyndall effect ?

• A. Salt solution
• B. Sugar solution
• C. Milk
• D. Copper sulphate solution

Hint:

The Tyndall effect is the scattering of light by particles in a colloid or a fine suspension, making the light beam visible.
{True Solutions} (e.g., salt in water, sugar in water): Particles are too small \(\rightarrow\) {No} Tyndall effect.
{Colloids} (e.g., milk, fog, smoke, jelly): Particles are large enough (but not too large to settle) \(\rightarrow\) {Shows} Tyndall effect.
{Suspensions} (e.g., muddy water, chalk in water): Particles are very large, may show Tyndall effect initially but then settle down. Milk is a colloid (an emulsion of fat and a dispersion of protein micelles).

Answer 1

The Correct Option is C

Concept: The Tyndall effect is the scattering of a beam of light as it passes through a colloid or a very fine suspension. The individual suspension particles or colloidal particles scatter and reflect light, making the beam visible from the side. Step 1: Understanding the Tyndall Effect For the Tyndall effect to be observed:
The diameter of the dispersed particles must be roughly in the range of the wavelength of light (approximately 40 nm to 900 nm).
There must be a significant difference in the refractive indices of the dispersed phase and the dispersion medium. True solutions (where solute particles are individual ions or small molecules, typically<1 nm) do not show the Tyndall effect because the solute particles are too small to scatter visible light effectively. Step 2: Classifying the given options
(1) Salt solution (e.g., NaCl in water): This is a true solution. Salt dissolves to form individual ions (Na\(^+\) and Cl\(^-\)) which are too small to scatter light. It will not show the Tyndall effect.
(2) Sugar solution (e.g., sucrose in water): This is also a true solution. Sugar dissolves as individual molecules which are too small to scatter light. It will not show the Tyndall effect.
(3) Milk: Milk is a colloid, specifically an emulsion of fat globules dispersed in an aqueous solution. It also contains casein protein micelles. The fat globules and protein micelles are large enough (colloidal size range) to scatter light. Therefore, milk shows the Tyndall effect (e.g., a beam of light passing through milk will be visible).
(4) Copper sulphate solution (\(\text{CuSO}_4\) in water): This is a true solution. Copper sulphate dissolves to form Cu\(^{2+}\) and SO\(_4^{2-}\) ions, which are too small to scatter light. It will not show the Tyndall effect (it appears clear, though colored blue). Step 3: Identifying which shows the Tyndall effect Based on the classification, Milk is a colloid and will exhibit the Tyndall effect. Salt solution, sugar solution, and copper sulphate solution are true solutions and will not.