An amphoteric, or amphiprotic, species is one that can act either as an acid or a base, depending on its chemical environment. In the Brønsted–Lowry sense, an amphoteric species can either gain or lose a proton. Water is the most common example. When water reacts with a base, it behaves as an acid:
When water reacts with an acid, it behaves as a base:
The partially dissociated conjugate base of a polyprotic acid is usually amphoteric (e.g., HSO4− can either gain an H+ to form H2SO4 or lose an H+ to form SO42−). The hydroxides of certain metals (e.g., Al, Zn, Pb, and Cr) are also amphoteric. Furthermore, species that can act as either oxidizing or reducing agents (see Chapter 29, Reaction Types) are considered to be amphoteric as well, since by accepting or donating electron pairs, they act as Lewis acids or bases, respectively.