Friday, October 26, 2012


When a solid surface is exposed to a gas or a liquid, molecules of the gas or the solution phase accumulate or concentrate at the surface.
                        “The phenomenon of higher concentration of molecular species (gases or liquid) at the surface of a solid than in bulk is called Adsorption”. The solid substance on the surface of which adsorption occurs is called Adsorbentwhile the substance which gets adsorbed on the solid surface is called Adsorbate. The adsorbent may be a solid or a liquid and the adsorbate may be gas or a solute in some solutions. For example, activated charcoal (Finely divided charcoal) adsorbs a number of gases like ammonia, phosgene etc. Here, charcoal is adsorbent and the gases are adsorbate. Water vapours are adsorbed by silica gel. In this case silica gel is adsorbent and water vapours are adsorbate.
                        The term adsorption must be carefully distinguished from another like sounding term absorption. Adsorption implies concentration at surface only i.e it is surface phenomenon whereas in Absorption, the substance assimilated is uniformly distributed throughout the body of solid or liquidFor example, a chalk crayon dipped in ink is adsorbed and on breaking a chalk it is found white from inside. On the other hand water is absorbed by a sponge and is distributed throughout the sponge uniformly.
                        Some times adsorption and absorption takes place simultaneously and is difficult to distinguish the two, such process is termed as Sorption(Introduced by MC Bain).

(The adsorption of gases on the surfaces of metals is called Occulsion)

Explanation for Adsorption

Molecules in the interior of a liquid are completely surrounded by other molecules on all sides and hence the intermolecular forces of attractions are exerted equally in all directions. However, a molecule at the surface of a liquid is surrounded by large number of molecules in the liquid phase and fewer molecules in the vapour phase i.e., in the space above the liquid phase. As a result, these molecules lying at the surface, experience some net inward forces of attraction which cause surface tension. Similar inward forces of attraction exist at the surface of solids. Whereas in case of certain solids such as transition metal like Ni, there are unutilized free valencies at the surface.
Explanation For Adsorption
                        Because of unbalanced inward forces of attraction or free valencies at the surface, liquids and solids have property to attract and retain the molecular species which comes in contact to these surfaces.

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