The effect of a substance that, without itself undergoing change, aids a chemical change in other substances. The main forms of catalysis are: homogeneous catalysis, in which the catalyst is in the same phase as the reactants and products; and heterogeneous catalysis, in which the catalyst is in a different phase to the reacting system. A third, mainly biological form is enzyme catalysis, in which enzymes catalyze reactions. These are large complex organic molecules and do not form true solutions.