Adsorption

This section sets up the physical picture behind adsorption before splitting it into its two types (Section 11.3). At the surface of any liquid or solid, the molecules or ions do not have their attractive forces fully satisfied the way bulk molecules do, because they lack neighbours on one side. A bulk molecule inside a liquid or solid is pulled equally in all directions by surrounding molecules, so the net force on it is zero, but a surface molecule only has neighbours below and to the sides -- there is nothing pulling from above -- so it experiences a net inward, unbalanced pull. Because of this unsaturation, solid and liquid surfaces have a natural tendency to attract and hold onto gas molecules or dissolved substances that come into close contact with them, and the substance being held accumulates specifically at the surface rather than spreading through the bulk. The following subsections name the two substances involved (adsorbent and …