Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in chemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquefying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquefied with difficulty. Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.
Identify the correct order of ease of adsorption of the following gases on the surface of charcoal:
A. \(CH_4\)
B. \(H_2\)
C. \(NH_3\)
D. \(CO_2\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C > D > A > B
Charcoal adsorbs gases by physical adsorption (van der Waals forces). Easily liquefiable gases, i.e. gases with a higher critical temperature, have stronger van der Waals forces and are adsorbed more readily.
C. NH3 — critical temperature about 405 K, the highest, so it is adsorbed most.
D. CO2 — critical temperature about 304 K.
A. CH4 — critical temperature 190 K.
B. H2 — critical temperature 33 K, the lowest, so it is adsorbed least.
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):
Nature of Adsorbate – “The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases (i.e., with higher critical temperatures) are readily adsorbed as van der Waals’ forces are stronger near the critical temperatures.”