Adsorption, the process by which a substance attracts and holds particles of another substance to its surface. The adsorbed particles can be molecules or atoms of a gas, liquid, or solid, and the adsorbing substance can be either a solid or a liquid. Adsorption is sometimes confused with absorption, a process by which one substance is taken up internally by another.

Both solids and liquids adsorb most effectively when they are in a form in which a relatively small amount of matter presents a large amount of surface area. Finely divided solids, such as clay, and porous solids, such as charcoal, are thus good adsorbents. Fine liquid droplets, such as those that occur in sprays, are also good adsorbents.

A number of materials are used in industry as adsorbents. The most important industrial adsorbent is activated charcoal, or activated carbon, which will adsorb many substances in amounts nearly equal to its own weight. Activated charcoal is produced by treating charcoal with superheated steam, carbon dioxide, or air, and is usually made into granules or powder. It is used in air conditioner filters; to remove impurities from sugar during refining; to withdraw impurities and unwanted moisture from air and other gases; in the treatment of municipal water supplies; and in a number of other ways.

Other important industrial adsorbents include silica gel, a coarse, porous sand; activated alumina, a granular, highly porous form of aluminum oxide; and fuller's earth, composed mainly of clay minerals.