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.
Entropy during adsorption is negative because
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the disorderness in the system decreases.
In adsorption, gas molecules that were moving freely are held on the surface of the adsorbent. Their freedom of movement is reduced, so the randomness (disorder) of the system decreases and the entropy change ΔS is negative. Adsorption is still spontaneous because it is exothermic (ΔH negative), so ΔG = ΔH − TΔS is negative.
Why the other options are incorrect
Option (1) the disorderness in the system increases — Incorrect. An increase in disorder would make ΔS positive, the opposite of what happens.
Option (3) bond formation results in more disorderness — Incorrect. Binding molecules to a surface restricts their movement, so it lowers the disorder.
Option (4) disorderness remains unaffected — Incorrect. Then ΔS would be zero, but the adsorbed molecules lose freedom of movement.
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.”