Sorption

Sorption is the name given to the situation where adsorption and absorption happen at the same time in the same system. The book's illustrating example is a piece of chalk dipped into ink: the ink's dye molecules are adsorbed onto the surface of the chalk, which is why the chalk's surface visibly turns coloured, while at the same time the liquid solvent of the ink is absorbed -- it soaks deeper into the porous body of the chalk rather than staying at the surface. Because both processes -- a surface effect (adsorption of the dye) and a bulk effect (absorption of the solvent) -- occur together in this one everyday demonstration, chemists use the umbrella term sorption to describe it, rather than having to specify which of the two mechanisms is responsible for which part of what is observed. Sorption is therefore not a third, separate mechanism in its own right; it is simply the desc …