In chemisorption, or chemical adsorption, the pollutants attach themselves to the grain surfaces as a result of the formation of a chemical (usually covalent) bond. In this case, the energy of attachment is very much greater than in physical adsorption. Thus, a molecule undergoing chemisorption arising from bond formation with the surface atoms. Although it is very difficult to differentiate between physical and chemical adsorption, one can generally say that the amount of physically adsorbed material decreases with increasing temperature, while this relation for chemically adsorbed material is reversed.