Question

Noble metals like gold is dissolved in which of the following?
A. Hydrochloric acid
B. Sulphuric acid
C. Nitric acid
D. Aqua regia

Answer 1

Hint:Gold is highly precious and costly metal available in nature and it is highly inert metal. So, there is a need of special treatment to make it dissolve. There is a need for a strong oxidizing agent to make it dissolve.

Complete step-by-step answer:- Gold atoms have high nuclear charge due to relativistic effect.
- Gold is highly inert in nature due to this the separation of gold from its ore is very difficult.
- The strong oxidizing agent used to dissolve gold is aqua regia.
- Aqua regia is a solution containing a mixture of nitric acid and hydrochloric acid in 1:3 ratio.
- The two strong acids can’t dissolve the gold alone.
- Both the acids react together and form nascent chlorine, the nascent chlorine reacts with gold and forms a complex containing dissolved gold.
- The chemical reaction of aqua regia with gold is as follows.
\[\begin{align}
  & 3HCl+HN{{O}_{3}}\to NOCl+2{{H}_{2}}O+2\overset{\bullet }{\mathop{Cl}}\, \\
 & 3\overset{\bullet }{\mathop{Cl}}\,+Au\to [AuC{{l}_{4}}] \\
\end{align}\]
- This is how gold dissolves in aqua regia.
- Therefore the acid which dissolves gold is aqua regia.

So, the correct option is D.

Note: Aqua regia has a property to dissolve platinum metal too. Aqua regia is a red fuming liquid and it makes noble metals to dissolve like gold and platinum but not all noble metals. The strong oxidizing agent in aqua regia is the nascent chlorine which makes gold and platinum to form respective chlorides and makes them to dissolve.