Aqua Regia

Aqua Regia (Latin, “royal water”), mixture of concentrated hydrochloric and nitric acids, containing one part by volume of nitric acid (HNO ₃) to three parts of hydrochloric acid (HCl). Aqua regia was often used in alchemy and its name is derived from its ability to dissolve the so-called noble metals, particularly gold, which are inert to either of the acids used separately. It is a powerful oxidizing agent, and is still occasionally used in the laboratory for dissolving gold and platinum. It can dissolve all metals except for silver, as an insoluble chloride barrier forms around the silver when it is in contact with this mixture of acids. Aqua regia is a powerful solvent because of the combined effects of the H⁺, NO ₃^-, and Cl^- ions in solution. The three ions react with gold atoms, for example, to form water, nitric oxide (NO), and the stable ion AuCl^- ₄, which remains in solution.