Question

Gold is soluble in
a.) \[{\text{Conc}}{\text{. HCl}}\]
b.) \[{\text{Conc}}{\text{. }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\]
c.) \[{\text{Conc}}{\text{. HN}}{{\text{O}}_3}\]
d.) Aqua-regia

Answer 1

Hint: .Nitric acid acts as a powerful oxidizing agent which can help in dissolving a significant amount of gold. Hydrochloric acid provides constant supply of chlorine ions which help in establishing equilibrium.

Complete answer:
Aqua regia is a mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3. It is a red-yellow coloured fuming liquid which has the capability to dissolve noble elements such as gold and platinum.
While dissolving gold, each acid used in aqua-regia plays a different role. High oxidising power of nitric acid oxidizes gold atoms to \[{\text{A}}{{\text{u}}^{ + 3}}\] state and hydrochloric acid provides a ready supply of chloride ions [\[{\text{C}}{{\text{l}}^ - }\]], which react with the gold ions to produce tetrachloroaurate (III). Hydrochloric acid also helps in establishing the equilibrium in the reaction and favours formation of \[{\text{AuCl}}_4^ - \] i.e. chloroaurate ions.

So, the correct answer is “Option D”.

Additional Information: Aqua regia is mainly used in production of chloroauric acid and is also used as an electrolyte in the process of extraction of pure gold. It also has uses in some analytical procedures and in etching. Apart from gold it also dissolves platinum.
Even being a noble element, gold shows formation of few compounds. Compounds in which gold is in +1 oxidation state are called as Aurous compounds whereas the ones in which it is in +3 oxidation state, are called as Auric compounds. Gold ions can be readily precipitated by adding any other metal as a reducing agent.

Note: Hydrochloric acid, sulphuric acid and nitric acid are all very strong acids and have the capacity to dissolve most of the chemical compounds. Due to its powerful dissolving capacity, aqua regia is also called as royal water. Aqua regia is also used in salt analysis for dissolving NiS. Certain metals like Ti, Ag and Ir do not dissolve in aqua regia.