Aqua regia, a mixture of concentrated HNO₃ and HCl, was developed by alchemists as a means to "dissolve" gold. The process is a redox reaction with this simplified skeleton reaction:\mathrm{Au}(s)+\mathrm{NO}_{3}^{-}(a q)+\mathrm{Cl}^{-}(a q) \longrightarrow \mathrm{AuCl}_{4}^{-}(a q)+\mathrm{NO}_{2}(g) (a) Balance the reaction by the half-reaction method.