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project on adsorption

                    Adsorption


Introduction
There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it 
comes into contact. Thesemolecules remain only at the surface and do
not go deeper into the bulk.

Adsorption
The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed 
adsorption.
 
Adsorbate
The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate .

Adsorbent
The material on the surface  of which the adsorption takes place is called adsorbent.

Adsorption in action
(i) If a gas like O2 , H2 , CO, Cl2 , NH3 or SO2  is taken in a closed vessel containing powdered charcoal, it is observed that the pressure of the gas in the enclosed vessel decreases. The gas molecules concentrate at the surface of the charcoal, i.e., gases are adsorbed at the surface. 
(ii) In a solution of an organic dye, say methylene blue, when animal
charcoal is added and the solution is well shaken, it is observed that the filtrate turns colourless. The molecules of the dye, thus, 
accumulate on the surface of charcoal, i.e., are adsorbed.
(iii) Aqueous solution of raw sugar, when passed over beds of animal
charcoal, becomes colourless as the colouring substances are
adsorbed by the charcoal.
(iv) The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.

Desorption
The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Types of  Adsorption

Physical  adsorption or physisorption:
If accumulation of gas on the surface of a solid occurs on account of weak van der Waals’ forces, the adsorption is termed as physical 
adsorption or physisorption.

Chemical adsorption or chemisorption:
When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption.



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Chemisorption involves a high energy of activation and is, therefore, often referred to as activated adsorption.

A physical adsorption at low temperature may pass into  chemisorption as the temperature is 
increased.

For example:dihydrogen is first adsorbed on nickel by van
der Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption. 
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