In a colloidal state the particle size of the dispersed phase range between 103 to 106 pm and colloidal sol is heterogeneous in nature. Based on the nature of interaction between the dispersed phase and dispersion medium colloidal sol are classified as lyophilic and lyophobic. Stability of sol is due to presence of charge on the sol particles and the neutralisation of the same is known as coagulation or precipitation.
Based on this answer the following.
The Arseneous sulphide As2S3 sol has negative (-) charge. The maximum power to precipitate it is of :
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → AlCl3
A negative sol such as \( \ce{As2S3} \) is coagulated by the positive ions (cations) of the added electrolyte. By the Hardy–Schulze rule, the greater the valence of the flocculating ion, the greater its power to cause precipitation, so \( \ce{AlCl3} \), which supplies the trivalent Al3+, has the maximum coagulating power.
Why the other options are incorrect
Option (1) H2SO4 — Incorrect. Its cation, \( \ce{H^{+}} \), is univalent; the divalent \( \ce{SO4^{2-}} \) carries the same charge as the sol and does not coagulate it.
Option (2) Na3PO4 — Incorrect. The trivalent \( \ce{PO4^{3-}} \) is an anion and acts on positive sols; its cation \( \ce{Na^{+}} \) is univalent.
Option (3) CaCl2 — Incorrect. \( \ce{Ca^{2+}} \) is divalent, weaker than \( \ce{Al^{3+}} \).
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 – …
