Only the surface atoms of an adsorbent play an active role in adsorption. These atoms posses unbalanced forces of various types such as, Vander Waal’s forces and chemical bond forces.
Thus, the residual force-field on a free surface which is responsible for adsorption is produced. For example, when a solid substance is broken into two pieces, two new surfaces are formed and therefore, the number of unbalanced forces becomes more. As a result the tendency for adsorption become large.
Reversible and Irreversible adsorption: The adsorption is reversible, if the adsorbate can be easily removed from the surface of the adsorbent by physical methods.If the adsorbate can not be easily removed from the surface of the adsorbent is called irreversible adsorption.
Example for reversible adsorption: A gas adsorbed on a solid surface can be completely removed in vacuum.
Example for irreversible adsorption: Adsorption of O2 on tungusten adsorbent.