The process of accumulation of molecular species at the surface of solid or liquid is called adsorption. It may be physisorption or chemisorption. Low temperature is favourable for physisorption. Amount of gas adsorbed by a solid depends upon nature of gas. Easily liquefiable gases are readily adsorbed. Catalyst increases the rate of reaction. Catalysis may be homogeneous or heterogeneous. Enzymes are biological catalysts. They work at optimum pH and temperature. Solutions are of three types i.e. true solutions, colloids and suspensions. Depending on the interaction between dispersed phase and dispersion medium colloids may be Lyophilic and Lyophobic. Depending on type of particles there may be multimolecular, macro-molecular and associated colloids. Sol, gel, emulsions are also various types of colloids.
Physical adsorption may become chemisorption as:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Temperature is increased
Physisorption is held by weak van der Waals forces, needs no activation energy and is favoured by low temperature. Chemisorption involves the formation of chemical bonds between the adsorbate and the surface; like a chemical reaction, it needs activation energy, so it increases with temperature up to a point.
A gas physically adsorbed at low temperature therefore may change into chemisorption at a high temperature: on heating, the adsorbed molecules gain the activation energy needed to form chemical bonds with the surface. For example, dihydrogen is first physisorbed on nickel at low temperature and is chemisorbed (as H atoms) when the temperature is raised. High pressure also favours chemisorption.
Why the other options are incorrect
Option (1) Surface area is increased — Incorrect. A larger surface area increases the extent of both kinds of adsorption; it does not change one kind into the other.
Option (3) Activation energy is increased — Incorrect. Activation energy is a property of the process; a higher activation energy would make chemisorption harder, not bring it about. What is needed is enough thermal energy to cross it.
Option (4) Amount of adsorbate is increased — Incorrect. More adsorbate (higher pressure) increases the amount adsorbed, but the nature of the forces (van der Waals or chemical bond) depends on the temperature.
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):
Irreversibility – “As chemisorption involves compound formation, it is usually irreversible in nature. Chemisorption is also an exothermic process but the process is very slow at low temperatures on account of high energy of activation. Like most chemical changes, adsorption often increases with rise of temperature. Physisorption of a gas adsorbed at low temperature may change into chemisorption at a high temperature. Usually high pressure is also favourable for chemisorption.”