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.