any of a class of chemical reactions that occur in a single phase (gaseous, liquid, or solid), one of two broad classes of reactions—homogeneous and heterogeneous—based on the physical state of the substances present. The most important of homogeneous reactions are the reactions between gases (gas) (e.g., the combination of common household gas and oxygen to produce a flame) and the reactions between liquids (liquid) or substances dissolved in liquids (e.g., the reactions between aqueous solutions of acids and bases). From the theoretical standpoint, homogeneous reactions are the simpler of the two classes of reactions because the chemical changes that take place are solely dependent on the nature of the interactions of the reacting substances. Compare heterogeneous reaction.