Adsorption is the adhesion of molecules (or ions and atoms) to the surface of a solid or liquid. The molecules get accumulated only at the surface and do not enter the bulk of the adsorbing material.
- The substance whose molecules get adsorbed at the surface is called the adsorbate.
- The substance on whose surface the process takes place is called the adsorbent.
- It is a surface phenomenon.
Alumina gel, silica gel, zeolites, activated carbon, graphite, finely divided metals, etc. are some examples of good adsorbents.
Desorption is the reverse process in which the adsorbed substance is removed from the surface of the adsorbent.
Absorption is a physical or chemical phenomenon or a process in which atoms, molecules or ions enter some bulk phase – solid or liquid material. This is a different process from adsorption since molecules undergoing absorption are taken up by the volume, not by the surface. Adsorption is based on the surface where a film of adsorbate is developed on the surface, and absorption includes the complete volume of the absorbing agent.
Differences between Absorption and Adsorption
|Definition||Assimilation of the molecular system throughout the bulk of the solid or liquid medium.||Accumulation of molecular species at the bottom instead of the liquid or solid.|
|Phenomenon||A bulk phenomenon.||A surface phenomenon.|
|Heat exchange||Endothermic process||Exothermic process|
|Temperature||The temperature has no effect.||Influenced by low temperature|
|Rate of reaction||Occurs at a uniform rate.||Increases steadily and reaches equilibrium.|
|Concentration||It is constant throughout the medium.||The Concentration at the bottom of adsorbent is different from that in bulk.|
Mechanism of adsorption
The process of adsorption arises due to the fact that the forces acting on the surface particles of a substance are not the same as that acting on the bulk of the material. Unlike the particles inside the bulk, on the exposed surface, the particles are not surrounded by atoms on all sides. Consequently, the forces on the inside balance each other, whereas those on the surface are unbalanced. The unbalanced residual forces on the surface have the tendency to attract the adsorbate particles. This leads to the process of adsorption. It is to be noted here that at a given temperature and pressure, the greater the surface area of adsorbent, higher is the extent of adsorption.
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