Aqua regia is a mixture of nitric acid and which other acid?

Aqua regia is a mixture of nitric acid and hydrochloric acid.

The name aqua regia means “royal water,” a reference to the mixture’s ability to dissolve noble metals such as gold and platinum. The usual preparation combines concentrated nitric acid and hydrochloric acid in an approximate 1:3 molar ratio. Freshly mixed aqua regia may turn yellow, orange, or red as reactive chlorine and nitrosyl chloride form.

Neither acid alone dissolves gold effectively in the same way. Nitric acid oxidizes gold atoms, while chloride ions from hydrochloric acid bind the resulting gold ions into soluble chloroaurate complexes. Removing the dissolved gold ions allows the oxidation process to continue, giving the mixture its unusual metal-dissolving power.

Aqua regia is not a stable storage solution: its reactive products escape as gases, so it loses strength over time and is generally prepared when needed. It is highly corrosive and produces hazardous fumes. The mixture should not be confused with sulfuric acid mixtures or with ordinary hydrochloric acid, which is also known as muriatic acid in some contexts.