Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Amphoteric oxides include lead oxide and zinc oxide, among many others.

One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton (H⁺). Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water.

A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:

HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O

or as an acid:

HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

Thus, it can effectively accept or donate a proton.

Water is the most common example, acting as a base when reacting with an acid such as hydrogen chloride:

H₂O + HCl → H₃O⁺ + Cl⁻,

and acting as an acid when reacting with a base such as ammonia:

H₂O + NH₃ → NH₄⁺ + OH⁻

Not all amphoteric substances are amphiprotic

Although an amphiprotic species must be amphoteric, the converse is not true. For example, the metal oxide ZnO contains no hydrogen and cannot donate a proton. Instead it is a Lewis acid whose Zn atom accepts an electron pair from the base OH⁻. The other metal oxides and hydroxides mentioned above also function as Lewis acids rather than Brønsted acids.