Answer the question on the basis of passage given below:
Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk. The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature is expressed as isotherm. A catalyst is a substance, when adsorbed enhances the rate of a chemical reaction without itself getting used up in the reaction. Depending on the type of the particles of the dispersed phase, colloids are classified as multimolecular, macromolecular and associated colloids and soap is such an example of associated colloids. Colloids show various properties such as Tyndall effect, colour, Brownian movement and charge development. There are positively charged sols and negatively charged sols. Due to this, electrophoresis and electroosmosis are shown by colloids.
Colloidal solutions illuminate the path of beam of light due to:
A. their range of diameter of particles between \(10^{-9}\) to \(10^{-6}\) m.
B. scattering of light
C. the formation of tyndall cone by a concave lens.
D. their clear solution.
E. large magnitude difference in the refractive indices of the dispersed phase and the dispersion medium.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, B and E only
When a beam of light passes through a colloid, the particles scatter the light and its path becomes visible as a bright cone when seen from the side (the Tyndall effect). A true solution shows no such path. The effect needs two conditions: the particles must not be much smaller than the wavelength of light, and the refractive indices of the dispersed phase and the dispersion medium must differ greatly.
(A) their range of diameter of particles between 10−9 to 10−6 m. — Correct. Colloidal particles are 1–1000 nm across, large enough to scatter visible light; particles of a true solution are too small to do so.
(B) scattering of light — Correct. The illuminated path is light scattered by the particles.
(C) the formation of tyndall cone by a concave lens. — Incorrect. The Tyndall cone is the bright path formed by scattering from the particles themselves, not by a lens.
(D) their clear solution. — Incorrect. A clear true solution shows no Tyndall effect; it is the colloid’s particles, not clarity, that light up the path.
(E) large magnitude difference in the refractive indices of the dispersed phase and the dispersion medium. — Correct. This is one of the two conditions for the Tyndall effect.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Optical Properties – “… Tyndall effect is observed only when the following two conditions are satisfied. (i) The diameter of the dispersed particles is not much smaller than the wavelength of the light used; and (ii) The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.”