Aqua regia is a mixture of concentrated nitric acid (HNO₃) and hydrochloric acid (HCl) in a 1:3 ratio. It is a highly corrosive, fuming liquid known for its ability to dissolve noble metals like gold and platinum, which are otherwise inert to most acids.

The nitric acid acts as a strong oxidizing agent, while the HCl provides chloride ions that form stable complex ions with the oxidized metal cations, driving the reaction forward.

For gold, the reaction is: $\mathrm{Au}+{\mathrm{HNO}}_3+4\mathrm{HCl}\to \left[\mathrm{Au}{\mathrm{Cl}}_4\right]{}^{-}+{\mathrm{NO}}_2+H_{2}O+H^{+}$

Final Answer: Au to [AuCl₄]^–