Coagulating power of an ion for a colloidal solution depends on:

The correct answer is Option (4) → Its size and magnitude of charge both

A sol is coagulated by the ion that carries charge opposite to that of the colloidal particles; this coagulating ion neutralises the charge on the particles. By the Hardy-Schulze rule, the greater the valence (charge) of the coagulating ion, the greater its coagulating power: \( \ce{Al^{3+}} \) > \( \ce{Ba^{2+}} \) > \( \ce{Na^{+}} \) for a negative sol, and \( \ce{[Fe(CN)6]^{4-}} \) > \( \ce{PO4^{3-}} \) > \( \ce{SO4^{2-}} \) > \( \ce{Cl^{-}} \) for a positive sol.

Among ions of the same charge, the size of the ion also matters: an ion that approaches the particle surface more closely (smaller hydrated size) is adsorbed more readily and neutralises the charge more effectively. So the coagulating power depends on both the magnitude of charge and the size of the ion; the book states only the charge part (Hardy-Schulze rule).

Why the other options are incorrect

• Option (1) Its magnitude of charge only — Incorrect. Charge is the main factor, but ions of equal charge (for example \( \ce{Li^{+}} \), \( \ce{Na^{+}} \), \( \ce{K^{+}} \)) still differ in coagulating power because of their size.

• Option (2) Its size only — Incorrect. Size alone cannot explain why \( \ce{Al^{3+}} \) coagulates a negative sol far better than \( \ce{Na^{+}} \); that is a charge effect.

• Option (3) Its tendency to hydrate — Incorrect. Hydration affects coagulation only through the effective size of the ion; it is not the deciding property on its own.

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):
By Addition of Electrolytes