Physical adsorption of a gaseous species may change to chemical adsorption with
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Increase in temperature
Physisorption is held by weak van der Waals forces and needs almost no activation energy, while chemisorption forms chemical bonds and needs activation energy. At low temperature a gas such as \( \ce{H2} \) on nickel is only physically adsorbed; when the temperature is raised, the molecules get enough energy to cross the activation barrier and the physisorption changes into chemisorption.
Why the other options are incorrect
Option (1) Decrease in temperature — Incorrect. Lowering the temperature removes the energy needed for chemical bond formation; it favours physisorption.
Option (3) Increase in surface area of adsorbent — Incorrect. A larger surface area increases the extent of adsorption but does not change its nature.
Option (4) Decrease in surface area of adsorbent — Incorrect. It only reduces the amount adsorbed; the type of adsorption is unchanged.
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 – “… Chemisorption involves a high energy of activation and is, therefore, often referred to as activated adsorption. Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, dihydrogen is first adsorbed on nickel by van der Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption.”