Amphoterism

Amphoterism is the tendency of a chemical compound to behave either as an acid or as a base. For example, aluminium is amphoteric, as it can form aluminate salts (reacting as an acid with a base) or react as a base with an acid to form an aluminium salt.

Elements

Many transition metals have amphoteric properties, such as chromium, copper and zinc. The amphoterism of these elements tends to be easily demonstrated.

Acids and bases

Nitric acid tends to act as a base in relatively pure acids, such as hydrofluoric acid and sulfuric acid.

Sodium bicarbonate, commonly known as baking soda, typically acts as a base, neutralizing acids. However, it can react as an acid with strongly alkaline solutions such as those containing sodium hydroxide, forming sodium carbonate in the process.

Ammonia will be deprotonated in the presence of strong bases to form amides.