Depending upon the strength of the interaction, all adsorption processes can be divided into the two categories of chemical and physical adsorption. The former, also called irreversible adsorption or chemisorption, is characterized mainly by large interaction potentials, which lead to high heats of adsorption often approaching the value of chemical bonds. This fact, coupled with other spectroscopic, electro spin resonance, and magnetic susceptibility measurements confirms that chemisorption involves true chemical bonding of the gas or vapor with the surface. Because chemisorption occurs through chemical bonding it is often found to occur at temperatures above the critical temperature of the adsorbate. Strong bonding to the surface is necessary in the presence of higher thermal energies, if adsorption is to occur at all. In addition, chemisorption is usually associated with an activation energy, as is true for most chemical reactions. Furthermore, chemisorption is necessarily restricted to, at most, a single layer of chemically bound adsorbate on the surface. Another important feature of chemisorption is that the adsorbed molecules are more localized on the surface when compared to physical adsorption. because of the formation of a chemical bond between an adsorbate molecule and a specific site on the surface the adsorbate is less free to migrate about the surface. This fact often enables the number of active sites on catalysts to be determined by simply measuring the quantity of chemisorbed gas.

The second category, reversible or physical adsorption, is a general phenomenon, which occurs whenever an absorbable gas (the adsorptive) is brought in contact with the surface of the solid adsorbent. Physisorption exhibits characteristics that make it most suitable for surface area determinations as indicated by the following:

1.Physical adsorption is accompanied by low heats of adsorption with no violent or disruptive structural changes occurring to the surface during the adsorption measurement.

2.Unlike chemisorption, physical adsorption may lead to surface coverage by more than one layer of adsorbate.

3.Pores can be filled completely by the adsorptive for more volume measurements. Such pore condensation phenomena can be used also to calculate the pore size and its distribution.

4.Physical adsorption equilibrium is achieved rapidly since no activation energy is required as is generally true in chemisorption. An exception here is adsorption in small pores where diffusion can limit the adsorption rate.

5.Physical adsorption is fully reversible, enabling both the adsorption and desorption processes to be studied.

6.Physically adsorbed molecules are not restricted to specific sites and are free to cover the entire surface. For this reason, surface areas rather than number of sites can be calculated.