Amphoteric: Definition and Examples in Chemistry

Amphoteric substances can act as either an acid or a base

Illustration of molecule in water
Self-ionizing compounds, such as water, are examples of amphoteric molecules which are also amphiprotic. Yuji Sakai/Getty Images

An amphoteric substance is one that can act as either an acid or a base, depending on the medium. The word comes from the Greek amphoteros or amphoteroi, meaning "each or both of two" and, essentially, "either acid or alkaline."

Amphiprotic molecules are a type of amphoteric species that either donates or accepts a proton (H+), depending on the conditions. Not all amphoteric molecules are amphiprotic. For example, ZnO acts as a Lewis acid, which can accept an electron pair from OH but cannot donate a proton.

Ampholytes are amphoteric molecules that exist primarily as zwitterions over a given pH range and have both acidic groups and basic groups.

Here are some examples of amphoterism:

  • Metal oxides or hydroxides are amphoteric. Whether a metal compound acts as an acid or a base depends on the oxide oxidation state.
  • Sulfuric acid (H2SO4) is an acid in water but is amphoteric in superacids.
  • Amphiprotic molecules, such as amino acids and proteins, are amphoteric.