The adsorption on a given surface generally increases with increase in pressure (for gases) and concentration (for solutions) at a constant temperature.
The extent of adsorption of a gas per unit mass of adsorbent depends upon the pressure of the gas. The relation between the amount of substance adsorbed by the adsorbent and the equilibrium gas pressure (or concentration for solutions) at constant temperature is called an adsorption isotherm.
The extent of adsorption is usually expressed as x/m where x is the mass of adsorbate and m is the mass of the adsorbent.
The extent of adsorption (x/m) increases with pressure and becomes maximum corresponding to pressure ps called equilibrium pressure. Since adsorption is a reversible process, the desorption also takes place simultaneously. At this pressure (ps) the amount of gas adsorbed becomes equal to the amount of gas desorbed so that the extent of adsorption becomes constant even though the pressure is increased. This state is also called saturation state and ps is called saturation pressure.