. What is adsorption.
A molecule in the bulk of the substance. It experiences forces of attraction uniformly from all sides by neighbouring molecules. There is no net force acting on it. If we consider a molecule near its surface we find that such a molecule is not bonded to other molecules from all sides but has certain unutilised valencies, as there are no molecules above the surface. In order to satisfy the unutilised valencies, the molecules at the surface attract and retain other molecular species when the substance is brought in contact with them. The substance, which has different concentration at the surface than in the bulk of the adjoining phases, is said to the adsorbed. The phenomenon of existence of a substance in different concentrations at the surface of another substance than in the bulk phases is called adsorption.