In a liquid a solid substance a molecule present within the bulk of the substance is being attracted infirmly from all sides by the neighbouring molecules. Hence there is no bet force acting on the molecule or there are no unbalanced forces of the molecule. On the other hand, a molecule present at the surface is not attracted to other molecules form all sides because there are no neighbouring molecules above the surface. Hence, it possesses some unbalanced or residual forces. As a result of these unbalanced forces the molecules present at the liquid surfaces tend to satisfy their residual forces by attracting the molecules of other species when brought in contact with them and retaining them on the surface. This causes increase in concentration of the molecular species near the surfaces as compared to its bulk phases. This accumulation of molecular species at the surface rather than in bulk of a solid or liquid is referred to as adsorption. The molecular species or a substance which concentrates or accumulates at the surface is called adsorbate. The material on the surface of which adsorption is called adsorbent.
The adsorbate and adsorbent process of removal of an adsorbed substances from surfaces is called desorption. It is reverse of adsorption and can be brought about by reducing the pressure or by heating.
Some examples of adsorption
(i) When gases like O2, H2, CO, Cl2, NH3 or SO2, are taken in a closed vessel containing powdered charcoal. It is observed that the pressure of the enclosed vessel decreases. The gas molecules get adsorbed on the surface of charcoal.
(ii) Aqueous solution of raw sugar when passed over beds of animal charcoal becomes colourless because the colouring matter of sugar is adsorbed by the animal charcoal surface.
(iii) The air becomes dry in the presence of silica gel because water molecules get adsorbed on the surface of silica gel.
(iv) When animal charcoal is added to a solution of some organic dye (say methylene blue) and the contents are filtered after thorough shaking, it is observed that the filtrate is almost colourless. The molecules of the dye are adsorbed on the surface of animal charcoal.
The above examples clarify that the solid surfaces can hold the gas or liquid molecules because of adsorption.