The phenomenon of concentration of molecules of a gas or
liquid at a solid surface is called adsorption.
The substance that deposits at the surface is called Adsorbate
and the solid on whose surface the deposition occurs is called
the Adsorbent.Examples of Adsorption
(1) Adsorption of a dye by a charcoal. If finely divided charcoal is stirred into a dilute solution of
methylene blue (an organic dye), the depth of colour of the solution decreases appreciably. The dye
molecules have been adsorbed by charcoal particles.
(2) Adsorption of a gas by charcoal. If a gas (SO2, Cl2, NH3) is treated with powdered charcoal in
a closed vessel, the gas pressure is found to decrease. The gas molecules concentrate on charcoal
surface and are said to be adsorbed.

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