Difference between Adsorption and Absorption

Key Difference: Absorption is a bulk phenomenon in which an absorbate completely penetrates into the body of a solid or liquid to form a compound or a solution. On the other hand, adsorption is a surface phenomenon in which molecules of an adsorbate concentrates only on the surface of an adsorbent.

Many people consider absorption and adsorption as the same. However, both are different from each other. Absorption refers to a process in which molecules of a substance penetrate into the bulk of a solid, or liquid materials to form solutions or compounds. In simple language, it depicts the penetration of one substance into another in a complete manner. The molecules or atoms of a substance gets totally penetrated into the volume of the other molecules. It is sucked that much that it literally becomes a part of the other substance. It can either be a chemical reaction or a physical process. Generally, absorptions occur by the process of diffusion.

Chemical Reaction – carbon dioxide absorbed by a solution of potassium carbon

Physical Process – Air absorbed in water by dissolving

On the other hand, adsorption deals with only the adhesion on the surface. It is referred to a process by which the molecules of a gas or liquid solute stick only at the surface a solid or a liquid (adsorbent). It results into higher concentration of adsorbate molecules on the surface. Unlike, absorption, they are not totally consumed by the other substance. It can be simply referred as the process by which molecules collect at the surface of the substance.

There are two types of adsorption modes – chemical and physical. In chemical adsorption the molecules and surface are bonded by weak Vander Walls forces. On the other hand, in chemical adsorption, a chemical bond is formed between molecules and surface.

Therefore, the primary difference between adsorption and absorption is that absorption is a bulk phenomenon which means that it happens throughout the body of the material, whereas adsorption remains to be a surface phenomenon. Adsorption is always exothermic, whereas absorption is endothermic. Sorption includes both processes of absorption and adsorption.

Comparison between Adsorption and Absorption:

	Adsorption	Absorption
Defintion	Accumulation of a gas or liquid solute on the surface of a solid or a liquid	Diffusion of a substance into a liquid or solid to form a solution or compound
Example	Inert gases are adsorbed on charcoal.	A dry sponge absorbs water
Heat exchange	Exothermic with exception of adsorption of H2 on glass	Endothermic
Attaining Equilibrium	Comparatively faster	Comparatively slowly
Concentration	Concentration on the surface of the adsorbent is different from than in the bulk	Concentration remains the same throughout the material
Rate of occurrence	It is rapid initially but later its rate starts to decline	It takes place at uniform rate