The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. There are two types of adsorption. In physisorption, attractive forces are mainly Van der Waals while in chemical adsorption adsorbate is held to the adsorbent by chemical bonds. Adsorption increases with increase in pressure and decreases as temperature is increased.
Adsorption is spontaneous because:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(\Delta H\) is \(-ve\) and is more than \(T\Delta S\)
A process is spontaneous when ΔG is negative:
\( \Delta G = \Delta H - T\Delta S \)
On adsorption ΔS is negative, so −TΔS is positive. ΔG can be negative only if ΔH is negative and larger in magnitude than TΔS.
Why the other options are incorrect
Option (1) \(\Delta S\) is \(-ve\) and is more than \(\Delta H\) — Incorrect. A negative ΔS works against spontaneity; it is ΔH that must outweigh TΔS, and ΔS and ΔH cannot be compared directly (different units).
Option (2) \(\Delta S\) is \(+ve\) and is more than \(\Delta H\) — Incorrect. The entropy of the gas decreases on adsorption; ΔS is not positive.
Option (4) \(\Delta H = \Delta S\) — Incorrect. This has no meaning (J mol−1 against J K−1 mol−1); the condition ΔH = TΔS describes equilibrium, where adsorption stops.
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 – …
