Answer the question on the basis of passage given below:
A colloid is a heterogeneous system in which a substance is dispersed (dispersed phase) as fine particles in another substance called dispersion medium. Their particle size is intermediate between that of pure solutions (<1 nm) and suspension (>1000 nm).
They are known for interesting properties such as enormous surface area, Tyndall effect, Brownian Motion etc. They are widely used in industries and day to day life. Some prominent uses of colloids are those in smoke precipitators, purification of water, medicines, cleansers etc.
Which of the following electrolyte exhibits maximum flocculating power in the coagulation of a positive metal hydroxide sol?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → K4[Fe(CN)6]
A sol is coagulated by the ion of opposite charge to its particles, so a positive metal hydroxide sol is coagulated by the anions of the electrolyte. By the Hardy-Schulze rule, the greater the valence of the flocculating ion, the greater its coagulating power:
\( \ce{[Fe(CN)6]^{4-}} \) > \( \ce{PO4^{3-}} \) > \( \ce{SO4^{2-}} \) > \( \ce{Cl^{-}} \)
All four electrolytes give the same cation, \( \ce{K^{+}} \), which plays no part here. The tetravalent ferrocyanide ion of \( \ce{K4[Fe(CN)6]} \) has the highest charge, so it has the maximum flocculating power.
Why the other options are incorrect
Option (1) KCl — Incorrect. \( \ce{Cl^{-}} \) carries only one negative charge, so it has the least flocculating power of the four.
Option (2) K2SO4 — Incorrect. \( \ce{SO4^{2-}} \) is divalent, stronger than \( \ce{Cl^{-}} \) but weaker than \( \ce{PO4^{3-}} \) and \( \ce{[Fe(CN)6]^{4-}} \).
Option (3) K3PO4 — Incorrect. \( \ce{PO4^{3-}} \) is trivalent; it is a strong coagulant but one charge short of the ferrocyanide ion.
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 – “… The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa. It has been observed that, generally, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule. In the coagulation of a negative sol, the flocculating power is in the order: Al3+ > Ba2+ > Na+. Similarly, in the coagulation of a positive sol, the flocculating power is in the order: [Fe(CN)6]4– > PO43– > SO42– > Cl–”