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.
Which of the following is wrong about chemisorption?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It is reversible in nature
In chemisorption the adsorbate is held to the surface by chemical bonds, as in compound formation. So it is highly specific, needs activation energy, has a high enthalpy of adsorption (80–240 kJ mol−1) and is irreversible. Physisorption (van der Waals forces) is the reversible type.
Option (1) It involves the formation of the compound on the surface of a solid adsorbent. — True of chemisorption. Oxygen is chemisorbed on metals as an oxide layer, and hydrogen on transition metals as a hydride.
Option (2) It is specific in nature — True of chemisorption. Only a gas that can bond with the surface is chemisorbed; hydrogen is chemisorbed on nickel but not on every solid.
Option (3) It first increases with increase in temperature then decreases — True of chemisorption. It needs activation energy, so it rises at first with temperature; being exothermic, it falls at still higher temperatures.
Option (4) It is reversible in nature — Wrong about chemisorption. A compound is formed on the surface, so lowering the pressure does not release the gas: chemisorption is irreversible.
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 Chemisorption
