The phenomenon of existence of a substance in different concentrations at the surface of another substance than in the bulk phases is called adsorption. 

The substance whose surface adsorbs molecular species of another substance is called adsorbent and the substance, which gets adsorbed, is called adsorbate. 

Difference between Adsorption and Absorption 

When a substance is uniformly distributed throughout the bulk of another substance, this shows that absorption has taken place. It occurs at the uniform rate throughout the body of the material. One the contrary, adsorption involves unequal distribution of the molecular species in the bulk and the surface. It is a surface phenomenon. It is rapid in the beginning and gradually slows down at the equilibrium. The forces involves in adsorption are either weak Vander Waal’s forces or strong chemical bond forces. 

In certain cases, both the adsorption and the absorption take place simultaneously and it is not easy to distinguish them. Such substance is said to be sorbed and the phenomenon is known as sorption. 

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