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.
The process of adsorption is accompanied by
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Decrease in entropy
During adsorption, molecules of the gas (or solute) that were moving freely become held on the surface of the adsorbent. Their freedom of movement is reduced, so the entropy decreases (ΔS is negative). Attractive forces are set up between adsorbate and adsorbent, so heat is released: adsorption is exothermic (ΔH is negative).
For the process to be spontaneous, ΔG must be negative:
\( \Delta G = \Delta H - T\Delta S \)
Since −TΔS is positive, ΔG stays negative only while ΔH is negative enough to outweigh it. As adsorption goes on, ΔH becomes less negative, and when ΔH = TΔS, ΔG = 0 and adsorption equilibrium is reached.
Why the other options are incorrect
Option (1) Increase in entropy — Incorrect. The adsorbed molecules are more ordered than the free molecules, so entropy falls, not rises.
Option (3) Increase in enthalpy — Incorrect. Adsorption releases heat (enthalpy of adsorption is negative); enthalpy decreases.
Option (4) Increase in Gibbs free energy — Incorrect. Adsorption is spontaneous, so ΔG is negative; Gibbs energy decreases.
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):
Mechanism of Adsorption – “When a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., ΔS is negative. Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system.”