Although they may look and sound very similar, the meanings of these words are very different.
Absorption refers to the process by which a material takes in the atoms, molecules and ions of another substance. Through the process of absorption one substance takes up another substance through minute pores or spaces between its molecules.
Adsorption, on the other hand, refers to the process by which molecules of a substance collect on the surface of another substance. The molecules are attracted to the surface but do not enter the solid’s minute spaces, as in absorption. Rather, they just gather on the surface of the material.
These two very different occurrences are similar only in that both processes involve the transfer of a volume of mass or energy. Absorption refers to the transfer of a volume into another volume and adsorption refers to the transfer of a volume on to the surface of another mass.