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There is a well-known, experimentally verified and reversible reaction in which an electron-positron pair annihilates into two photons.

This is a reversible reaction, however, and hydrolysis can therefore be prevented by first adding a considerable amount of the soluble product of the reaction, namely, hydrochloric acid.

Since hydrolysis is a reversible reaction, the relative masses of the reacting substances influence the point at which equilibrium will be reached.

Such a reaction is called a reversible reaction.

A reversible reaction can therefore be completed in either direction when one of the products of the reaction is removed as fast as it is formed.