An amphoteric substance is an oxide of an element that can act as both an acid or a base.
Common amphoteric substances are the oxides of the elements and beryllium, aluminum, zinc, tin and lead.
Aluminum oxide reacting with an acid:
Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)
Aluminum oxide reacting with a base:
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4](aq) or sodium tetrahydroxoaluminate
The oxides of metals are basic in nature. For example, the oxides of the alkali metals (Group I) form alkali or basic solutions.
Sodium oxide + water → Sodium hydroxide solution
Na2O(s) + H2O(l) → NaOH(aq)
The soluble oxides of non-metals are acidic in nature. Examples include, carbon dioixde, sulfur dioxide and nitrogen dioxide.
Sulfur dioxide + water → Sulfurous acid
SO2(g) + H2O(l) → H2SO3(aq)
Insoluble non-metallic oxides like carbon monoxide do not form acidic solutions.
As you move across a row in the Periodic table the oxides of the elements change from a basic nature to an acidic nature.
|Period or row 3||Na||Mg||Al||Si||P||S||Cl||Ar|
The amphoteric oxides occur as the transition is made from metallic oxide to non-metallic oxide in each row of the Periodic Table
|Across row||Metallic oxides → non-metallic oxides|
|Across row||Basic oxides → Amphoteric oxides → Acidic Oxides|
Note: Amphiprotic susbtances are a smaller chemical subset of amphoteric substances.