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.
The ease with which the gases \(H_2,\, \ CH_4,\, \ CO_2\) and \(NH_3\) are adsorbed on the surface of charcoal is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(NH_3 > CO_2 > CH_4 > H_2\)
In physical adsorption the gas is held by van der Waals forces. Easily liquefiable gases, that is gases with a higher critical temperature, have stronger van der Waals forces and are adsorbed more readily. So the ease of adsorption follows the critical temperatures:
\( \ce{NH3} \) (about 406 K) > \( \ce{CO2} \) (about 304 K) > \( \ce{CH4} \) (190 K) > \( \ce{H2} \) (33 K)
Ammonia, a polar molecule that liquefies easily, is adsorbed most; hydrogen, the hardest of the four to liquefy, is adsorbed least.
Shortcut: only Option (4) starts with \( \ce{NH3} \) (easiest to liquefy) and ends with \( \ce{H2} \) (hardest).
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 Physisorption – …
