Which of the following is incorrect about chemical adsorption?

The correct answer is Option (4) → It’s not specific

Chemical adsorption (chemisorption) takes place when the adsorbate forms chemical bonds with the surface of the adsorbent. A bond forms only when the gas can react with that particular surface, so chemisorption is highly specific.

• Option (1) It’s monolayered — A correct statement about chemical adsorption. Once the surface is covered by one layer of chemically bonded molecules, no more bonds to the surface can form, so a unimolecular layer results.

• Option (2) ΔH° > 40 kJ/mol — A correct statement about chemical adsorption. Its enthalpy of adsorption is high, 80–240 kJ mol⁻¹, of the order of the enthalpy of a chemical reaction.

• Option (3) It’s irreversible — A correct statement about chemical adsorption. The chemical bonds formed are strong, so the adsorbed gas cannot be removed by just lowering the pressure.

• Option (4) It’s not specific — Not a correct statement about chemical adsorption. Chemisorption occurs only between a gas and a surface that can react with each other, so it is highly specific. Lack of specificity belongs to physisorption, which arises from weak van der Waals’ forces.

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):
Types of Adsorption – …