Gold Mining, Congo

Aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid, dissolves gold. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reaction. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl4− ions, or chloroauric acid, thereby enabling further oxidation.
Some free halogens react with gold. Gold also reacts in alkaline solutions of potassium cyanide. With mercury, it forms an amalgam.
Less common oxidation states
Less common oxidation states of gold include −1, +2, and +5.