Question

Question 21.
Which of the following is used for dissolution of gold ?
(a) Hydrochloric acid
(b) Sulphuric acid
(c) Nitric acid
(d) Aqua regia

Answer 1

Aqua-regia is used for the dissolution of gold. It is a mixture of conc. ${\text{HNO}}_3$ and conc. HCl in the ratio 1:3. Nitric acid is a strong oxidising agent which reacts with gold to produce gold ions. These ions react with the chloride ions from the hydrochloric acid to form $({\text{AuCl}}_4{)}^{-}$.

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

Hence, the correct option is (d).