Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in chemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquefying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquefied with difficulty. Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.
Rate of physisorption increases with
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → decrease in temperature
Physisorption is exothermic (adsorbate + adsorbent ⇌ adsorbate on surface + heat). By Le Chatelier’s principle, lowering the temperature shifts this equilibrium towards adsorption, so physical adsorption increases as the temperature decreases. It also increases with higher pressure and larger surface area.
Why the other options are incorrect
Option (1) decrease in pressure — Incorrect. Adsorption of a gas increases with increase in pressure; lowering the pressure causes desorption.
Option (2) decrease in surface area — Incorrect. A smaller surface area offers fewer sites, so less gas is adsorbed.
Option (3) increase in temperature — Incorrect. Since physisorption is exothermic, raising the temperature decreases it.
Note 1: 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):
Reversible Nature – “… Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature (Le-Chatelier’s principle).”
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 10-06 S1 30-05 S1 | – |