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When a solid is brought in contact with a gas or a solution its surface tends to attract and retain the molecule of the gas or the solute.
For Example :- When finally divided active charcol or clay is stirred with a dilute solution of a dye , the intensity of the color of the solution is decreases .
(II) :- When Pd is added to a flask containing H2 gas , the pressure of the gas in flask decreases considerable .
The phenomenon of attracting and retaining the molecules of a substance by a solid ( or liquid ) on its surface resulting into a higher concentration of the molecule on the surface is known as Adsorption.
The substance adsorbed is called Adsorbate, and the substance which is adsorbed is called Adsorbent .
Exp_ - When NH3 is adsorbed on charcol . NH3 is adsorbate and charcol is adsorbent .
The process of removing and and adsorbed substance from a surface on which it is absorbed is called Desorption .
#_ Absorption is different from Adsorption .In absorption the substance is uniformly distributed throughout the body of solid or liquid.
Adsorption can be classified as Physical Adsorption (Physisorption) and Chemical Adsorption (Chemisorption) .
When te Adsorbate is held on the surface by weak vanderwall forces. The process is called Physical Adsorption. This type of adsorption can be reversed by heating .
When the force is holding the adsorbate on the surface are of magnitude of Chemical bond forces , The process is called Chemical Adsorption this type of adsorption can be irreversible .
Adsorption is generally accomprate by evolution of heat i.e., it is Exothermic.
NOTE:- The enthalpy change when one gram mole of the Adsorbate is Adsorbed on the surface of yhw adsorbent is called Enthalpy of Adsorption .
[ Usually - For physical adsorption - 3 KJ]
[For chemical adsorotion - 30 KJ]
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