A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance called dispersion medium. Colloids are classified on the basis of physical state of dispersed phase and dispersion medium, nature of interaction between dispersed phase and dispersion medium and types of particles of dispersed phase. There are various methods for the preparation of colloidal solutions such as chemical methods, electrical disintegration and peptization. Colloidal solutions when prepared, generally contain excessive amount of electrolytes, and some other soluble impurities. The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. Purification of colloidal solution is carried out by various methods such as Dialysis, Electrodialysis, Ultrafiltration etc. Colligative properties, Tyndall effect, colour, Brownian movement, charge on colloidal particles, electrophoresis, coagulation are various properties exhibited by colloidal solution.
Identify correct increasing order of coagulation power of electrolytes:
A. NaCl < MgCl2 < AlCl3
B. AlCl3 < MgCl2 < NaCl
C. Na4[Fe(CN)6] < Na3PO4 < NaCl < Na2SO4
D. KCl < K3PO4 < K2SO4 < K4[Fe(CN)6]
E. KCl < K2SO4 < K3PO4 < K4[Fe(CN)6]
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, E only
By the Hardy-Schulze rule, the coagulating power of an ion increases with its charge (valence). The ion that coagulates a sol has the charge opposite to that of the sol: cations coagulate a negative sol, anions a positive sol.
(A) NaCl < \( \ce{MgCl2} \) < \( \ce{AlCl3} \) — Correct. For a negative sol the active ions are \( \ce{Na^{+}} \) < \( \ce{Mg^{2+}} \) < \( \ce{Al^{3+}} \) (charge 1, 2, 3).
(B) \( \ce{AlCl3} \) < \( \ce{MgCl2} \) < NaCl — Incorrect. This is the reverse order.
(C) \( \ce{Na4[Fe(CN)6]} \) < \( \ce{Na3PO4} \) < NaCl < \( \ce{Na2SO4} \) — Incorrect. The anion charges −4, −3, −1, −2 are not in increasing order.
(D) KCl < \( \ce{K3PO4} \) < \( \ce{K2SO4} \) < \( \ce{K4[Fe(CN)6]} \) — Incorrect. \( \ce{PO4^{3-}} \) (−3) is placed below \( \ce{SO4^{2-}} \) (−2).
(E) KCl < \( \ce{K2SO4} \) < \( \ce{K3PO4} \) < \( \ce{K4[Fe(CN)6]} \) — Correct. For a positive sol: \( \ce{Cl^{-}} \) < \( \ce{SO4^{2-}} \) < \( \ce{PO4^{3-}} \) < \( \ce{[Fe(CN)6]^{4-}} \).
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):
Coagulation or Precipitation – …
