5. Water: Acid or Base?

When an acid is dissolved in water, the following protolysis reaction will occur:

HA(aq) + H₂O(l) H₃O⁺(aq) + A^-(aq)

In this reaction water behaves like a base: it accepts the hydrogen ions donated by the acid HA.

When a base is dissolved in water, the following protolysis reaction will occur:

A^-(aq) + H₂O(l) HA(aq) + OH^-(aq)

In this reaction water behaves like an acid: it donates the hydrogen ions accepted by the base A^-.

So water can react as an acid (donate protons) and as a base (accept protons). That is why we call it an ampholyte.

The autoionization reaction of water:

H₂O(l) + H₂O(l) H₃O⁺(aq) + OH^-(aq)

can be seen as the protolysis reaction of the acid H₂O, as well as the protolysis reaction of the base H₂O.

Hence, the equilibrium constant of this reaction can be seen as the acid dissociation constant of water as well as the base dissociation constant of water:

These equilibrium constants are very small. So the equilibrium state is positioned completely at the left. Only theoretically water is an acid/base, in practice, we consider water as a neutral particle.

Summary

Water is an Ampholyte H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
Water is a very weak acid Ka = 10-14
Water is a very weak base Kb = 10-14