Many chemical reactions take place at the surfaces of solids, e.g., at the surface of solid catalysts. Corrosion of metals takes place at their surfaces. Surface means the topmost layer on the very exterior of a solid. Now-a-days, there are ways of studying the top 0.5 to 2 nm, i.e., two to five of the topmost layers of atoms of the solid surface.

Adsorption. The existence of a substance at a surface in different concentration than in the adjoining bulk is called adsorption. It is selective absorption.

Absorption. It is the phenomenon in which a substance is uniformly distributed all over the surface.

Adsorbent. The substance present in bulk, on the surface of which adsorption is taking place is called adsorbent.

Adsorbate. The substance which is being adsorbed on the surface of another substance is called adsorbate.

Desorption. The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Physical adsorption or physisorption. If the adsorbate is held on a surface of adsorbent by weak Vander Waal's forces, the adsorption is called physical adsorption or physisorption.

Chemical adsorption or chemisorption. If the forces holding the adsorbate are as strong as in chemical bonds, the absorption process is known as chemical adsorption or chemisorption.

Enthalpy or heat of adsorption. Adsorption generally occurs with release in energy, i.e., it is exothermic in nature.