NEET 2021 Chemistry Question 74 Solution

When a converging beam of light is passed through a colloidal solution in a dark room, the path of the beam becomes illuminated by a blueish light when viewed at right angles. This phenomenon is known as the Tyndall effect, caused by the scattering of light by colloidal particles.

According to NCERT Surface Chemistry, the Tyndall effect is observed only when two conditions are satisfied: 1. The diameter of the dispersed particles is not much smaller than the wavelength of the light used ($1\text{ nm} - 1000\text{ nm}$). 2. The refractive indices of the dispersed phase and the dispersion medium differ greatly.

- NaCl, Glucose, and Urea solutions: Solute molecules/ions dissolve completely at the molecular level with particle diameters $< 1\text{ nm}$, forming true homogeneous solutions. True solutions DO NOT scatter visible light. - Starch solution: Starch is a high-molecular-weight polysaccharide that forms a macromolecular colloidal sol in water. The large macromolecular aggregates (size $1-100\text{ nm}$) scatter light effectively, producing a visible Tyndall cone.