Question

Gold cannot be dissolved by any acid. However, it can be dissolved by aqua regia (kingly water) which is a mixture of:
A. Mineral water and iodine
B. nitric and hydrochloric acids
C. water and sulphur
D. alcohol and alum

Answer 1

Hint: We know that gold cannot be dissolved in any single acid. This is because of the fact that the gold lies at the bottom of the metal reactivity series. So the gold is a very less reactive metal.

Complete step by step answer
We know that most powerful acids like hydrochloric acid and nitric acid alone are not able to dissolve gold. This is because gold being one of the least reactive metals is not able to replace the hydrogen from the given acids as it is also less reactive than the hydrogen. Also we know that a metal less reactive than the other is not able to replace the other one from its compound.
But gold can be dissolved in a liquid known as aqua regia. Aqua regia is a mixture prepared by mixing the two powerful acids namely hydrochloric acid and nitric acid in the ratio of 1:3. Aqua regia is also known as kingly water or royal water. Aqua regia is able to dissolve gold because of the two different functions carried out by both the acids comprising it. The hydrochloric acid provides the chloride ions that help gold ions to form coordination complexes and the nitric acid acts as a strong oxidising agent.
$
Au{\rm{(s)}}\, + 3NO_3^ - \left( {aq} \right) + 6{H^ + } \rightleftharpoons A{u^{3 + }}\left( {aq} \right) + 3N{O_2}\left( g \right) + 3{H_2}O\\
A{u^{3 + }}\left( {aq} \right) + 4C{l^ - } \rightleftharpoons AuCl_4^ - \left( {aq} \right)
$

Hence, option (B) nitric and hydrochloric acids, is the correct option.

Note:
Thus, although the strong acids are not able to dissolve gold alone due to its least reactive nature, the aqua regia which is a mixture of hydrochloric and nitric acid is able to dissolve it. This is because both the acids in aqua regia perform different functions i.e. $HCl$ provides $C{l^ - }$ ions to form complexes and $HN{O_3}$ acts as a strong oxidising agent.