It is the adsorption in which molecules of the adsorbate are held to the surface of the adsorbent by chemical bond which may be covalent or ionic. Chemisorption is thus highly selective since only certain types of molecules will be adsorbed by a particular solid. This depends on the chemical properties of gas and the adsorbent. The chemical bond produces strong forces hence chemical adsorption has high heat of adsorption from 10 kcals to 100 kcals per mole. Unlike physical adsorption, chemical adsorption is not reversible. Hydrogen is strongly adsorbed by the metals;nickel, iron and platinum.
In many cases, it has been observed that adsorption is neither physical nor chemical but a combination of the two. Some systems show physical adsorption at low temperatures but as the temperature is raised physical adsorption changes into chemical adsorption.