Distinction between Adsorption and Absorption

It is important not to confuse adsorption with absorption.

• In adsorption the substance stays concentrated only at the surface and does not penetrate into the bulk of the material.
• In absorption the substance is taken up uniformly throughout the entire bulk of the solid.

A simple illustration

Dip a chalk stick in ink: the surface takes on the ink's colour (adsorption of the coloured molecules), while the solvent of the ink soaks deep into the stick (absorption). Snap the chalk and it is white inside — proof the colour never went below the surface. …