Amphoterism

In chemistry, an amphoteric substance is one that can react as either an acid or base. The word is derived from the Greek prefix ampho- (αμφί-) which means both and the suffix -ic (-ικός) which means the attribute that the given substance has to react either as acid or as base.

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Examples

Examples include amino acids, proteins, and water. Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides.

For example, zinc oxide (ZnO) reacts differently depending on the pH of the solution:

In acids: ZnO + 2H⁺ → Zn²⁺ + H₂O

In bases: ZnO + H₂O + 2OH^- → [Zn(OH)₄]^2-

This effect can be used to separate different cations, such as zinc from manganese.

There are many other examples of chemical compounds which are also amphoteric, for the simplest example water:

Base (Proton Acceptor): H₂O + HCl → H₃O⁺ + Cl⁻

Acid (Proton Donor): H₂O + NH₃ → NH₄⁺ + OH⁻

(Indeed, it can do both at once: 2H₂O → H₃O⁺ + OH⁻)

Aluminium hydroxide is as well:

Base (neutralizing an acid): Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Acid (neutralizing a base): Al(OH)₃ + NaOH → NaAl(OH)₄

Beryllium hydroxide is also amphoteric:

Base: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O

Acid: Be(OH)₂ + 2NaOH → Na₂Be(OH)₄

See also

• Amphiprotic