In chemistry, an amphoteric substance is one that can react as either an acid or base. [GoldBookRef | title = amphoteric | file = A00306] The word is derived from the Greek prefix ampho- (αμφί-) meaning "both".Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides. Other examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water and ammonia.


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

:(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 → Na [Al(OH)4]

Some other examples include:

*Beryllium hydroxide
**with Acid: Be(OH)2 + 2HCl → BeCl2 + 2H2O
**with Base: Be(OH)2 + 2NaOH → Na2Be(OH)4

*Lead oxide
**with acid: PbO + 2HCl → PbCl2 + H2O
**with base: PbO + Ca(OH)2 +H2O → Ca2+ [Pb(OH)4] 2-

*Zinc oxide
**with acid: ZnO + 2HCl → ZnCl2 + H2O
**with base: ZnO + 2NaOH + H2O → Na22+ [Zn(OH)4] 2-

Some elements not mentioned that are able to form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au [ [ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes] (program download required)]

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