2 ml each of concentrated HCl, HNO₃, and a mixture of concentrated HCl and concentrated HNO₃ in the ratio of 3: 1 were taken in test tubes labeled A, B, and C respectively. A small piece of metal was put in each test tube. No change occurred in test tubes A and B but the metal got dissolved in test tube C respectively. What is the metal?

According to the question, to test the Metals aqua regia is the given mixture in test tube C. Aqua regia is prepared when three parts of conc. HCl and one part of conc. HNO₃ is taken as a solution mixture. The corresponding ratio in aqua regia is 3 :1. Aqua regia is capable of dissolving noble metals like gold and platinum. 

Gold (Au) can only be dissolved in dilute aqua regia, any other acids are not capable of dissolving gold. Hence, it won't get dissolved in the Conc. HCl and Conc. HNO₃.

Hence, Au is the metal.