In the process of absorption molecules of one phase interpenetrate uniformly among those of another phase to form a solution with the second phase. In adsorption, molecules of one phase are present in higher concentration at the surface of the second phase. Water vapor is absorbed by calcium chloride while it is adsorbed by silica gel. In the same manner, ammonia is absorbed by water and is adsorbed by charcoal.
Sorption, which includes both absorption and adsorption is a process in which a substance moves from one phase to be accumulated in another phase. The difference between adsorption and absorption is illustrated graphically as shown in below figure, for the case where a substance moves from a liquid phase to a solid phase.
fig 7.2 - Types of sorption separations
In the figure, m is the amount of the substance that has moved across the interface. In adsorption, the process is rapid in the beginning and slows down near equilibrium while absorption takes place at uniform rate.
