Question

Question 21
2 mL each of concentrated $\text{HCl},{\text{HNO}}_3$ and a mixture of concentrated $\text{HCl}$ and concentrated ${\text{HNO}}_3$ in the ratio of 3 : 1 were taken in test tubes labelled as A, B and C. 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. The metal could be
(a) $\text{Al}$
(b) $\text{Au}$
(c) $\text{Cu}$
(d) $\text{Pt}$

Answer 1

Option (b) is correct.

Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task and will actually dissolve a virtually undetectable amount of gold, forming gold ions $\left({\text{Au}}^{3+}\right)$. The reaction is:

$\text{Au}+3{\text{HNO}}_3+4\text{HCl}\rightleftharpoons [{\text{AuCl}}_4{]}^{-}+3[{\text{NO}}_2]+[{\text{H}}_3\text{O}{]}^{+}+2{\text{H}}_2\text{O}$