Besides their usefulness as solvents, acids and bases are important as catalysts. Because of their small size, mobility, and charge, H⁺ and OH^- ions from acids and bases can attack compounds in such a way as to make reactions occur more easily and faster. This is the key to their catalytic effectiveness.

If a substance provides a faster pathway for reaction but is regenerated again at the end of the process, it is a true catalyst. If catalysts are ions or molecules dissolved in the same solution as the reactants and products, they are known as homogeneous catalysts. This is the type of catalysis discussed in the postscript to this chapter.

In Chapter 15 we saw examples of heterogeneous catalysts, in which the catalyst is a separate phase - a surface to which the gaseous or dissolved reactants diffuse and from which the products separate.

In either type of catalysis the principle is the same: A catalyst is a substance that accelerates a thermodynamically spontaneous reaction by providing an alternate mechanism, without itself being consumed by the overall reaction. It can participate in several steps of the process, as long as it is regenerated at the end.

Acids and bases are widely used homogeneous catalysts.