Answer the question on the basis of the passage given below:
Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance which absorbs. In physisorption, the attractive forces are mainly van der Waals forces while in chemisorption actual bonding occurs between the particles of adsorbent and adsorbate. Gases which are easily liquefiable are adsorbed more easily on the surface of a solid as compared to the gases which are liquefied with difficulty. Adsorption increases with increase in pressure and decreases as the temperature is increased.
The effect of pressure on adsorption is high if
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → temperature is low
Physical adsorption of a gas on a solid is an exothermic and reversible equilibrium:
\( \ce{Solid + Gas <=> Gas/Solid} + \text{Heat} \)
An increase in pressure pushes this equilibrium to the right (fewer gas molecules), so more gas is adsorbed. Because the forward process gives out heat, it is favoured at low temperature.
At a low temperature, therefore, the amount adsorbed is large and rises steeply as the pressure is raised: the effect of pressure is high. At a high temperature very little gas stays adsorbed, and raising the pressure changes the amount only a little (the isotherms at higher temperatures lie lower and are flatter).
Why the other options are incorrect
Option (2) temperature is high — Incorrect. At a high temperature the adsorbed gas leaves the surface, so the extent of adsorption, and its rise with pressure, is small.
Option (3) temperature is neither low nor high — Incorrect. There is no special middle temperature; the lower the temperature, the larger the effect of pressure.
Option (4) charcoal powder is replaced by charcoal pieces — Incorrect. Pieces have a much smaller surface area than powder, so less gas is adsorbed at every pressure and the effect of pressure is smaller.
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):
Characteristics of Physisorption – …
