A mixture of concentrated nitric acid and concentrated hydrochloric acid in the ratio 1:3 respectively. It is a very powerful oxidizing mixture and will dissolve almost all metals, including such noble metals as gold and platinum, hence its name (‘royal water’). Aqua regia was much prized by the alchemists for its ability to dissolve gold—a property that the constituent acids do not possess individually. The effect of the nitric acid is to dissolve a small amount of gold to form Au³⁺ ions. The hydrochloric acid then reacts with these ions to form AuCl₄⁻, thus removing them. This disturbs the equilibrium of the reaction between gold and nitric acid, resulting in more gold being dissolved by the nitric acid (an example of Le Chatelier’s principle).