The extent of adsorption of a gas on the surface of a solid depends on the following factors:
(a) Nature of gas
(b) Nature of solid
(c) Specific area of solid
(d) Pressure of gas
(f) Activation of solid
(i) Nature of gas:
Since physical adsorption is non-specific in nature, any gas will be adsorbed on the surface of a solid to some extent or other. However, under any given conditions of temperature and pressure, easily liquefiable gases such as NH3, CH4HCI, CI2, SO2, CO etc. are adsorbed more than permanent gases like H2, O2, N2 etc. Chemisorption is specific in nature. Therefore, only those gases will be adsorbed which form chemical bonds with it.
(ii) Nature of solid:
Activated charcoal is the most common adsorbent for easily liquefiable gases. Poisonous gases such as CH4 and CO fall in this group. Therefore, it is used in gas masks. Other gases such as O2, H2 and N2 adsorb more on metals such as Ni, Pt and Pd.
(iii) Specific area of solid:
Specific area of an adsorbent is the surface area available for adsorption per gm of adsorbent. Greater the specific area of an adsorbent, greater will be the adsorption. The specific area of an adsorbent can be increased by making the surface rough. The pores must be large enough to allow penetrations of gas molecules.