Amphoterism

Amphoteric chemical species can act as an acid or a base, if another acid or base is present.

Water is an amphoteric species. If a base is mixed with water, the water molecules donate protons, as an acid would. If an acid is mixed with water, then the water behaves as a base.

Other amphoteric substances include ammonia and the hydroxides of zinc, aluminum, chromium, lead, and tin.