Question

Which of the following is used to dissolve gold?
A. Hydrochloric Acid
B. Sulphuric Acid
C. Nitric Acid
D. Aqua Regia

Answer 1

Hint: Gold is a highly inert metal and therefore special treatment is to be done to dissolve it. So, we have to use a very strong oxidizing agent so as to oxidize gold. To solve this question, we have to find the acid or a mixture of acids which is most oxidizing in nature.

Complete answer:
The reason behind the fast that gold solid is largely unreactive is because the electrons in gold fall at energies which few molecules or chemicals match (i.e., due to relativistic effects). Aqua regia is used for dissolving gold.
Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid in the ratio of c$1:3$. Either of the two acids would not be able to do this alone as their function in aqua regia are different.
$HN{{O}_{3}}$ is a powerful oxidizer which oxidizes a small quantity of gold to $A{{u}^{3+}}$ ions. $HCl$ in the solution provides $C{{l}^{-}}$ ions to $A{{u}^{3+}}$ ions to form the complex ion $\left[ AuC{{l}_{4}} \right]$. The reaction continues until it reaches an equilibrium. This is how it dissolves gold.
So, the correct answer is “Option D”.

Note: Aqua regia dissolves other inert take metals like platinum too. Aqua regia is a yellow-orange or red fuming liquid and it can dissolve the noble metals, gold and platinum, though not all metals. In aqua regia oxidizing agent present is the nascent chlorine which reacts with noble metals to form their chlorides and gets them dissolved.