adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion). Certain solids have the power to adsorb great quantities of gases. Charcoal, for example, which has a great surface area because of its porous nature, adsorbs large volumes of gases, including most of the poisonous ones, and is therefore used in gas masks. Certain finely divided solids have great adsorptive properties; for example, minute particles of platinum attract and hold multitudes of hydrogen molecules on their surfaces. Its ability to adsorb other gases makes platinum very useful in the production of sulfuric acid by the contact process and in the preparation of ammonia. Adsorption occurs also in solutions; colloidal particles suspended in a solution may adsorb much of the solvent (see colloid). Bone black and charcoal are used in industry to remove colors from solutions, since they adsorb many coloring materials and carry these with them when separated from the solution. Liquid dye held to the surface of cloth by adsorption permeates the fibers so that when the liquid has evaporated the dye still remains. Adsorption is employed in the hydrogenation of oils, in gas analysis, and in chromatography, a method used in the chemical analysis of closely related substances.
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