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.
 

Example: 

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

 

Background Theory:

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
Oxides Na2O MgO Al2O3 SiO2 P4O10 SO2 Cl2O No oxides
Nature Basic Basic Amphoteric Insoluble Acidic Acidic Acidic No oxides

 

The amphoteric oxides occur as the transition is made from metallic oxide to non-metallic oxide in each row of the Periodic Table

Row Trend
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.

Molar Mass
 

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