Aqua regia is a concentrated mixture of nitric and hydrochloric acids. Aqua regia can dissolve gold and platinum. These metals do not react even with hot and concentrated nitric acid. The high reactivity of Aqua regia is due to the formation of nascent chlorine by the oxidation of HCl by nitric acid. The two acids react to generate soluble nitrosyl chloride, water and nascent chlorine:
HNO3 + 3HCl ⟶ NOCl + 2H2O + 2Cl
Nascent chlorine is the chlorine atom alone without any bonding which is also considered as chlorine-free radical and is represented as [Cl]. In the case of aqua regia, the product is more powerful than the sum of its parts, which is what makes it so useful.
