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.

Note that acidity and basicity of compounds are independent of each other. A weak acid does not necessarily be a stronger base than a strong acid, and vice-versa. While most amphoteric compounds are both weak acids and weak bases, this is not always the case: the hypothetical compound hydrogen astatide is super-amphoteric, behaving both as a superacid (hydroastatic acid) and a superbase (astatine hydride). On the other hand, there are compounds that are weaker than water in both acid and base respects: these compounds can be protonated or deprotonated, but the resulting conjugate acids and bases will be super-strength and unstable, reacting with anything to return to their molecular state. Hydrocarbons are examples of such compounds: both carbide anions and hydrocarbonium cations are super-strength and react with water to completion.