Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk. The substance adsorbed is known as adsorbate and the substance on which adsorption takes place is called adsorbent. In physisorption adsorbate is held to the adsorbent by weak van der Waals forces and in chemisorption by strong chemical bond.
In coagulation of a positive sol, the flocculating power will be in the order
- \(PO_4^{3-}\)
- \(Cl^-\)
- \([Fe(CN)_6]^{4-}\)
- \(SO_4^{2-}\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → C > A > D > B
A sol is coagulated by the ion that carries a charge opposite to that of the colloidal particles. By the Hardy–Schulze rule, the greater the valence of the flocculating ion, the greater its power to cause coagulation.
A positive sol (such as \( \ce{Fe(OH)3} \) sol) is coagulated by anions. Comparing their charges:
C. [Fe(CN)6]4– — charge −4
A. PO43– — charge −3
D. SO42– — charge −2
B. Cl– — charge −1
Shortcut: Options (1) and (2) both begin with C > A; the doubly charged SO42– (D) must come before Cl– (B), which leaves Option (1).
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 – …
