( I ) The noble metals like gold, platinum dissolve in aqua regia
( II ) 3: 1 ratio of Conc HCl and Conc HNO3 is called aqua regia
Aqua-regia is a mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid. This mixture is used to dissolve noble metals like gold and platinum.
Au + NO–3 + 4H+ + 4Cl–→AuCl4– + NO + 2H2O
3Pt + 4NO3– + 16H+ + 18Cl–→3PtCl62– + 4NO + 8H2O