surface chemistry, study of chemical reactions in which the reactants are first adsorbed onto a surface medium (see adsorption) that then acts as a catalyst for the reaction; after the reaction the products are desorbed and the surface is left unchanged. Since the entire reaction takes place on the surface, the amount of surface area of catalyst per unit weight determines the effectiveness of the surface in the reaction. Some silica surfaces have over 200 square meters of surface area per gram. An example of a surface reaction is the reaction of an unsaturated organic molecule with hydrogen on finely divided platinum or with bromine on finely divided silica. Enzyme reactions can, in principle, also be considered surface reactions, since the reaction takes place on the enzyme surface after the enzyme has bound the reactants; however, usually only heterogeneous (two-phase) reactions are considered true surface reactions, while enzyme reactions are homogeneous (one-phase) systems.