Is water an acid or a base? Why both, of course, and neither! According to the Bronsted-Lowry definition, water can act as both an acid and a base, because it is able to both accept a proton to become the hydronium ion, and to donate a proton, becoming the hydroxyl ion. Let’s take a look!

2H2O <=> H3O+ + OH-

This reaction is called autoprotolysis. Notice that the arrow goes both to the left and to the right. This reaction is an equilibrium reaction. Water, of course, is a liquid, and the hydronium and hydroxyl ions are dissolved in the aqueous solvent. By the law of mass action, we can calculate the equilibrium constant by:
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