Arrange the following according to their coagulating power towards a…

The correct answer is Option (1) → (E) < (C) < (D) < (B) < (A)

A colloid is coagulated by ions of charge opposite to that of the sol particles. By the Hardy-Schulze rule, the greater the charge (valence) of the coagulating ion, the greater its coagulating power. A positively charged sol is therefore coagulated by anions, and more strongly by highly charged anions.

• (E) \( \ce{NH3} \) — 1st (least). It is a neutral molecule; it carries no charge, so it cannot neutralise the positive charge of the sol particles and has practically no coagulating power.

• (C) \( \ce{Cl^{-}} \) — 2nd. A univalent anion (charge −1), the lowest charge among the ions given.

• (D) \( \ce{S^{2-}} \) — 3rd. A divalent anion (charge −2); it stands where \( \ce{SO4^{2-}} \) stands in the book’s series, between \( \ce{PO4^{3-}} \) and \( \ce{Cl^{-}} \).

• (B) \( \ce{PO4^{3-}} \) — 4th. A trivalent anion (charge −3).

• (A) \( \ce{[Fe(CN)6]^{4-}} \) — 5th (greatest). A tetravalent anion (charge −4), the highest charge, so the greatest coagulating power.

Shortcut: Options (1) and (4) both end with (A); only Option (1) puts \( \ce{S^{2-}} \) (D) before \( \ce{PO4^{3-}} \) (B).

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 – “… 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⁻”