Absorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since the molecules are taken up by the volume, not by surface. A more general term is sorption which covers adsorption, absorption, and ion exchange.

If absorption is a physical process not accompanied by any other physical or chemical process, it usually follows the Nernst partition law:

"the ratio of concentrations of some solute species in two bulk phases in contact is constant for a given solute and bulk phases";

The value of constant K_N depends on temperature and is called partition coefficient. This equation is valid if concentrations are not too large and if the species "x" does not change its form in any of the two phases "1" or "2". If such molecule undergoes association or dissociation than this equation still describes the equilibrium between "x" in both phases, but only for the same form - concentrations of all remaining forms must be calculated by taking into account all the other equlilibria.

In the case of gas absorption, one may calculate its concentration by using e.g. the Ideal gas law, c = p/RT. Alternatively, one may use partial pressures instead of concentrations.

In many technologically important processes, the chemical absorption is used in place of the physical process, e.g. absorption of carbon dioxide by sodium hydroxide - such processes do not follow the Nernst partition law.

Technical example:

• Gases and vapours are removed from the air stream by absorption into a liquid medium such as deionized water or a specially formulated solvent or reagent. The most common application of absorption uses midget impinger with a fritted bubbler. Many tiny bubbles are formed as air is forced by suction through a porous glass frit (fritted glass). The tiny bubbles increase the surface area of the air in contact with the liquid collection media, increasing the absorption efficiency. Removal of the gas from the air is a function of the surface area and the time. The impinger's liquid is than sent to lab for analysis. Acid vapour may be scrubbed from air.