• The phenomenon of adsorption was introduced by Scheele during the discovery of uptake of gases by charcoal.

• Lowitz found that charcoal could take up colouring matter from solutions as well.
• The term adsorption was given by Kayser
• The concept of adsorption was developed by Kayser and Raymonds.

Definition:

"The phenomenon of higher concentration at a surface brought about by the action of surface forces is known as adsorption".

Or

It is the change in concentration in the interfacial layers between two phases of the system .

• When the concentration in the inter-facial layer between two phases greater than in the bulk of other phase, the adsorption of the substance is said to have taken place at interface. It is termed as positive adsorption. On the other hand decrease in concentration is known as negative adsorption.

Term related to absorption

Adsorbent: It is the substance on which another substance is adsorbed. It is also called interfac, e.g., solid alumina, charcoal, silica gel.

(i) It is very specific and selective in nature

(ii) It is spontaneous process, $\Delta G, \Delta H , \Delta S$ are –ve

(iii) It is always exothermic $(\Delta H = -ve)$

In positive adsorption the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk.

In negative adsorption the concentration of the adsorbate is less relative to its concentration in the bulk.

• Occlusion: The adsorption of gases on the surface of metal is known as occlusion. e.g. $H_2$ gas on palladium surface.
• Desorption: It is reverse of adsorption, i.e., removal of adsorbed substance.

(1) Inert gases, $CO_2, NH_3, Cl_2, SO_2$ are adsorbed on charcoal.

(2) Air becomes moisture-free on silica gel.

(3) Application of dyes on animal charcoal.

It arises at the surface of solids due to presence of un-balanced forces at the surface . These forces result during crystallisation of solids or due to presence of unpaired d-orbital electrons.

It is a surface phenomenon. Here substance is uniformly distributed

. throughout the Substance.

Fast process Slow process

Exothermic process Endothermic process

E.g. silica gel adsorbs water Anhydrous $CaCl_2$ absorbs water

• Sorption: When adsorption and absorption occur simultaneously it is called sorption. Factors Effecting Adsorption

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