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 graph represents an adsorption isotherm?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → graph (3)
An adsorption isotherm is the curve that shows how the extent of adsorption, x/m, changes with the pressure of the gas at constant temperature. Its shape is the one in graph (3): x/m rises steeply at first and then levels off, because the surface approaches saturation (no free sites are left) at high pressure. This is the shape of the Freundlich isotherm.
Why the other options are incorrect
Option (1) — Incorrect. A curve that falls steadily is how physisorption changes as the temperature is raised (an adsorption isobar), not an isotherm.
Option (2) — Incorrect. A curve that rises and then falls is the chemisorption isobar: adsorption first increases because activation energy is supplied, then decreases at high temperature.
Option (4) — Incorrect. A curve that keeps rising more and more steeply never reaches saturation, which is not how adsorption on a surface behaves.
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):
Adsorption Isotherms – …
