Amphoteric Oxide Definition and Examples

What You Need to Know About Amphoterism

Copper oxide is an example of an amphoteric oxide.
Copper oxide is an example of an amphoteric oxide. DEA/A.RIZZI / Getty Images

Amphoteric Oxide Definition

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 acid-base indicator that yield 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.