Absorption and adsorption, << ab SAWRP shuhn, ad SAWRP shuhn, >> are processes by which substances take in matter or energy, or both. In absorption, the matter or energy taken in becomes distributed throughout the absorbing material. Adsorption is the gathering of matter only. The matter collects on the surface of the adsorbing material. It does not enter the interior.
Absorption.
There are many familiar examples of absorption. Heavy drapes absorb sound energy. The sound waves make the fibers in the drapes vibrate and rub together. Friction turns the sound energy into heat so the sound cannot be reflected as an echo (see Insulation (Insulation against sound)). Colored objects and filters selectively absorb light energy. White light is composed of all colors of light. When white light strikes a colored substance, some colors of light energy are absorbed. The absorbed light energy excites electrons in the atoms of the colored substance–that is, it raises them to higher energy levels. The substance will transmit or reflect the colors of light that have not been absorbed. Dry earth absorbs water by a process called capillary action or capillarity (see Capillary action). The water in a lake absorbs oxygen by dissolving it.
Adsorption.
Usually only solid material can act as an adsorbent. The adsorbed matter can be the molecules of a liquid or a gas, or tiny particles of a solid. Adsorption is often highly selective, making it useful for separating or purifying liquids and gases. A charcoal filter adsorbs molecules on the surface of each charcoal particle. Silica gel adsorbs water molecules from moist air and holds them on the surface of each grain. Adsorption releases heat called heat of adsorption.
The interaction of adsorbed molecules with the adsorbing surface may be weak or strong. Molecules are weakly held when physical attraction holds them to the adsorbing surface (see Molecule (Molecules and matter)). This process is called physiosorption. When adsorbed molecules are united chemically with the surface, they are strongly held. This process is called chemisorption. In chemisorption, the adsorbed molecules form a single layer. In physiosorption, they may form layers several molecules deep.
