In chemistry, an amphoteric substance is a chemical species that can act as both an acid and a base, depending on reaction conditions. The phenomenon is called amphoterism.
Amphoterism applies to Bronsted-Lowry and Lewis acids and bases. The word comes from Greek word amphoteros or amphoteroi, which means “each or both of two” and means a substance is “either acidic or alkaline.”
An amphoteric substance has a transferable hydrogen (proton) and an atom with lone electron pairs. Examples of amphoteric substances include water, amino acids, proteins, and many metal oxides and hydroxides.
Oxides and hydroxides of aluminum, antimony, arsenic, antimony, bismuth, beryllium, chromium, cobalt, copper, gallium, germanium, gold, iron, lead, silver, tellurium, tin, and zinc are amphoteric. However, oxides of the alkali metals and alkaline earths only form basic oxides and are not amphoteric. The most amphoteric metal oxides and hydroxides come from metals found near the center of the periodic table.
Some substances normally considered either an acid or a base are actually amphoteric. For example, sulfuric acid (H2SO4) acts as an acid in water, but it acts as a base or is amphoteric in superacids. In general, acids able to donate multiple hydrogen ions are amphoteric. Similarly, bases with highly charged metal cations are amphoteric.
Examples of Amphoterism
For example, consider the amphoterism of water (H2O):
Water accepts a proton when reacted with an acid, such as hydrochloric acid (HCl).
H2O + HCl ⇌ H3O+ + Cl–
Water donates a proton when it reacts with a base, such as ammonia (NH3).
H2O + NH3 ⇌ NH4+ + OH–
Similarly, amphoteric metal oxides, such as aluminum oxide (Al2O3) act as a base when reacted with acid and as an acid when reacted with a base.
Al2O3(s) + 6H30+(aq) + 3H2O(l) → 2[Al(OH2)6]3+(aq)
Al2O3(s) + 2OH–(aq) + 3H2O(l) → 2[Al(OH)4]–(aq)
How to Identify Amphoteric Substances
To identify an amphoteric substance, look for the capacity to both add and remove hydrogen ions from other molecules. The amphoteric substance might contain multiple hydrogen ions, both acidic and basic components on the same molecule, or multiple charges on its cation or anion.
It may help to see a species that is not amphoteric. For example, nitrous acid (HNO2) is an acid, not it’s not amphoteric because it can’t serve as a base. It dissociates to release a hydrogen cation and the NO2– anion. Nitrous acid only donates one hydrogen ion or proton. It can’t accept any more protons, while the NO2– anion (the conjugate base) isn’t able to donate any.
HNO2 → H+ + NO2−
Amphiprotic: An amphiprotic species is a type of amphoteric substances that either accepts or donates a proton (H+), depending on conditions. Under the Lewis theory of acids and bases, amphiprotic species are amphoteric, but not all amphoteric substances are amphiprotic. Under the Bronsted-Lowry theory of acids and bases, amphoteric and amphiprotic mean the same thing.
Ampholytes: Ampholytes are amphoteric compounds containing both acidic and basic group. For example, an amino acid has an amino group capable of donating hydrogen and a carboxyl group able to accept hydrogen.
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