Catalysis helps to speed up a chemical reaction. An agent added to the chemical reaction to promote catalysis is called a catalyst. There are two forms of catalysis and they are characterized based on the catalyst involved in the process. A form of catalysis in which the catalyst is in the same physical state as that of the reactants in the chemical reaction is referred to as homogeneous catalysis. An example of a homogeneous catalysis is one wherein the catalyst and the reactants are in the gaseous phase. An example is acid catalysis. The acid dissolved in water produces a proton that speeds up chemical reaction, such as in the hydrolysis of esters. Without the acid, the hydrolysis of ester in aqueous solution does not hydrolyze at a faster rate. Another example is organocatalysis. Soluble organometallic compounds act as catalyst in certain chemical reactions, particularly in the hydroformylation and transfer hydrogenation.
In contrast, heterogeneous catalysis is a form of catalysis wherein the reactants and the catalysts are in different physical state, such as that one involving solid catalysts acting on substrates in a liquid or gaseous phase.