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Adsorption: Introduction

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Adsorption: Introduction - Lesson Summary


The branch of chemistry that deals with the study of the phenomena that occurs on the surface or interface, that is, at the boundary separating two bulk phases, is known as surface chemistry.

An interface is a surface that forms a common boundary between two different phases, such as an insoluble solid and a liquid, two immiscible liquids, or a liquid and an insoluble gas.


The phenomenon of attracting and retaining the molecules of a substance on the surface of a solid or liquid, resulting in a higher concentration of the molecules on the surface, is known as adsorption.

The substance on the surface of which adsorption takes place is called the adsorbent, while the substance thus adsorbed is called the adsorbate.

EX : Adsorption of Oxygen molecules on to the surface of charcoal

In this example, the molecules of oxygen are the adsorbate, while the solid charcoal is the adsorbent.


The removal of the adsorbed substance from the surface, is called desorption.

It can be brought about by heating or by reducing the pressure.


The adsorption of a gas on the surface of a metal is called occlusion.

EX: Hydrogen is adsorbed on the surface of nickel or palladium.

Greater the surface area of the adsorbent, greater is the extent of adsorption.

Thus, finely divided metals and substances with porous structure are good adsorbents as they provide large surface area. Charcoal, silica gel, alumina gel etc....

Adsorption and absorption are two different phenomena.

The term adsorption refers to the attraction and retention of the molecules of a substance only on the surface of a solid or liquid, while absorption refers to the passing of a substance through the surface into the bulk of a solid or liquid.

Adsorption is a surface phenomenon, whereas absorption is a bulk phenomenon


When adsorption and absorption occur simultaneously, it is called sorption.

Thermodynamic aspect of adsorption:

Adsorption is accompanied by a decrease in enthalpy and a decrease in entropy.

ΔH=-ve ΔS=-ve ΔG=-ve

   Gibb's Equation : ΔG = ΔH - TΔS
As for the process of absorption : -TΔS = Positive
When ΔH = TΔS
          ΔG = 0

Then the adsorbate and the surface reach a state of dynamic equilibrium.


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