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.
Which of the following is not an example of adsorption?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Drying of the substance using anhydrous calcium chloride.
In adsorption the substance is concentrated only at the surface of the adsorbent; in absorption it is distributed uniformly through the bulk of the solid. Anhydrous calcium chloride takes up water throughout its mass (it forms a hydrate), so drying with \( \ce{CaCl2} \) is absorption, not adsorption.
Option (1) Retaining the colour of the ink on the surface of the chalk stick — An example of adsorption. The coloured dye stays on the surface of the chalk while the solvent soaks in.
Option (2) Drying of the substance using anhydrous calcium chloride — Not an example of adsorption. Anhydrous \( \ce{CaCl2} \) takes up water through its whole mass; this is absorption.
Option (3) Decolouration of the crude sugar — An example of adsorption. The coloured impurities are adsorbed on animal charcoal.
Option (4) Drying of air in the presence of silica gel — An example of adsorption. Water vapour is held on the large surface of silica gel.
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):
Distinction between Adsorption and Absorption – “… A distinction can be made between absorption and adsorption by taking an example of water vapour. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel. In other words, in adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.”