@A00155@ with @D01801@ into two or more fragments, both or all of which are bound to the surface of the @A00153@ The process may be homolytic, as in the chemisorption of hydrogen: \[\text{H}_{2}+2\text{*}\rightarrow 2\text{H}^{\text{*}}\] where * represents a surface site, or heterolytic, as in: \[\text{H}_{2}+\text{M}^{n+}+\text{O}^{2-}\rightarrow \text{H}^{-}\text{M}^{n+}+\text{HO}^{-}\] where \(\text{M}^{n+}\) and \(\text{O}^{2- }\) are surface sites in which the ions are of lower @C01329@ than the ions in the bulk phase. Associative desorption is the reverse of dissociative adsorption.