Amphoterism

In chemistry, an amphoteric substance is one that can react as either an acid or base.

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