Amphoteric Oxide Definition and Examples

An amphoteric oxide is an oxide that can act as either an acid or base in a reaction to produce a salt and water. Amphoterism depends on the oxidation states available to a chemical species. Because metals have multiple oxidation states, they form amphoteric oxides and hydroxides.

Amphoteric Oxide Examples

Metals that display amphoterism include copper, zinc, lead, tin, beryllium, and aluminum.

Al₂O₃ is an amphoteric oxide. When reacted with HCl, it acts as a base to form the salt AlCl₃. When reacted with NaOH, it acts as an acid to form NaAlO₂.

Typically, oxides of medium electronegativity are amphoteric.

Amphiprotic Molecules

Amphiprotic molecules are a type of amphoteric species that donate or accept H⁺ or a proton. Examples of amphiprotic species include water (which is self-ionizable) as well as proteins and amino acids (which have carboxylic acid and amine groups).

For example, the hydrogen carbonate ion can act as an acid:

HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

or as a base:

HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O

Keep in mind, while all amphiprotic species are amphoteric, not all amphoteric species are amphiprotic. An example is zinc oxide, ZnO, which does not contain a hydrogen atom and cannot donate a proton. The Zn atom can act as a Lewis acid to accept an electron pair from OH−.

Related Terms

The word "amphoteric" derives from the Greek word amphoteroi, which means "both". The terms amphichromatic and amphichromic are related, which apply to an acid-base indicator that yields one color when reacted with an acid and a different color when reacted with a base.

Uses of Amphoteric Species

Amphoteric molecules that have both acidic and basic groups are called ampholytes. They are primarily found as zwitterions over a certain pH range. Ampholytes may be used in isoelectric focusing to maintain a stable pH gradient.