A solid surface attracts contacting gas or liquid species because atoms sitting right at the surface — unlike atoms buried in the bulk — have unsatisfied ('residual') valency, and this attraction happens strictly at the surface, which is what marks adsorption out as a surface phenomenon, distinct from absorption, where the absorbed substance spreads uniformly through the whole bulk of the absorbing material. The solid that does the attracting is the adsorbent; the substance attracted and held is the adsorbate; the surface of separation where the adsorbate's concentration builds up is the interface; and removing the adsorbed substance again is desorption. Where adsorption and absorption both occur together, the combined process is called sorption (M.C. Bain), and the specific case of gases taken up within a metal (e.g. hydrogen in palladium) is called occlusion (T. Graham).
Adsorption can occur at any interfacial surface — gas–solid, liquid–solid, liquid–liquid, solid–solid or gas–liquid — and is always spontaneous, so ΔG<0, reaching equilibrium exactly when ΔG=0. Because molecules moving from a free, disordered phase into an ordered adsorbed layer necessarily lose randomness, ΔS<0 for adsorption; since ΔG=ΔH−TΔS and −TΔS is therefore positive, ΔH must be sufficiently negative for ΔG to stay negative overall — so adsorption is always exothermic, a conclusion that follows directly from the entropy decrease. Adsorption is also characteristically fast, much faster than the comparatively slow, diffusion-limited process of absorption.
Based on the nature of the forces involved, adsorption splits into two kinds. Chemisorption holds the adsorbate by genuine chemical bonds, releases a large heat of adsorption (40–400 kJ/mole), is slow, highly specific to the particular adsorbent–adsorbate pair, involves real electron transfer, forms only a single monolayer, occurs only at fixed 'active centre' sites (so it depends on surface area), and needs appreciable activation energy to form an activated complex. With rising temperature it first increases (supplying the activation energy needed) and then decreases (desorption). Physisorption, by contrast, holds the adsorbate only by weak van der Waals forces, dipole–dipole interactions and dispersion forces, releases much less heat (of the order of 40 kJ/mole), is instantaneous and non-specific, involves no electron transfer, can build up a multilayer, occurs over the whole surface rather than fixed sites, needs essentially no activation energy, decreases monotonically as temperature rises, and — unlike chemisorption's amount, which pressure cannot increase much — genuinely increases in extent as pressure rises.
The extent of adsorption depends on four qualitative factors: the surface area of the adsorbent (higher area → more adsorbed, since adsorption is a surface effect); the nature of the adsorbate (easily-liquefiable gases like SO2, NH3, HCl, CO2, which have strong van der Waals attraction and high critical temperature, adsorb readily, while 'permanent' gases like H2, N2, O2, with low critical temperature, adsorb only slowly); temperature (chemisorption rises then falls, physisorption falls steadily); and pressure (chemisorption's rate rises with pressure but its total amount is capped by the fixed number of active sites, while physisorption's extent genuinely rises with pressure). These same trends show up directly in the adsorption isobar (x/m vs T at constant P): physisorption's isobar falls steadily, while chemisorption's isobar rises to a maximum (surface activation) and then falls (desorption as kinetic energy grows).