Answer the questions based on the passage given below:
Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance that absorbs. In physisorption the attractive forces are mainly van der Waals forces while in chemisorption ionic / covalent bonds are formed between particles of adsorbent and adsorbate. The catalytic activity of finely divided iron in Haber's process of ammonia manufacture can be explained by adsorption theory. Adsorption being an exothermic process, the heat of adsorption is utilized in enhancing the rate of the reaction. Adsorption has many applications being used in gas masks, control of humidity, chromatograph separation, curing diseases etc.
Which of the following is not correct regarding the adsorption of a gas on the surface of solid?
A. On increasing temperature, adsorption increases continuously
B. Enthalpy and entropy changes are negative
C. Chemical adsorption is more specific than physical adsorption
D. High temperature favours physical adsorption.
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and D only
(A) On increasing temperature, adsorption increases continuously — Not correct. Adsorption is exothermic, so physisorption decreases as the temperature rises; chemisorption first rises (it needs activation energy) and then falls. Neither increases without limit.
(B) Enthalpy and entropy changes are negative — Correct. Heat is released (ΔH < 0) and the adsorbed gas loses freedom of movement (ΔS < 0).
(C) Chemical adsorption is more specific than physical adsorption — Correct. Chemisorption needs a chemical bond between the adsorbate and the surface, so it occurs only for particular pairs; van der Waals forces are universal.
(D) High temperature favours physical adsorption — Not correct. Physisorption is favoured by low temperature, because it is exothermic and the weak van der Waals forces are overcome on heating.
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 – “… Adsorption, therefore, is invariably an exothermic process. In other words, ΔH of adsorption is always negative. 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.”
Types of Adsorption – Table 5.1: Comparison of Physisorption and Chemisorption
