Question

2 mL each of concentrated HCl, HNO₃ and a mixture of concentrated HCl and concentrated HNO₃ in the ratio of 3: 1 were taken in test tubes labelled as A, B and C. A small piece of metal was put in each test tube. No change occurred in test tubes A and B but the metal got dissolved in test tube C respectively. The metal could be

• A. $\mathrm{Al}$
• B. $\mathrm{Au}$
• C. $\mathrm{Cu}$
• D. $\mathrm{Pt}$

Answer 1

The correct option is B

$\mathrm{Au}$

Explanation for Correct option

Option (B) $\mathrm{Au}$

Aqua regia

• Solution C is a mixture of concentrated $\mathrm{HCl}$and concentrated ${\mathrm{HNO}}_3$which is called aqua regia. Aqua Regia dissolves gold.
• Aqua regia is a yellow-orange or sometimes red color fuming liquid. Aqua regia is a mixture of nitric acid and hydrochloric acid, at a molar ratio of $1:3$optimally.

Use of Aqua regia

• It can dissolve noble metals such as gold and platinum but not all metals.
• Aqua regia is mainly used for the manufacture of chloroauric acid.
• It is used as an electrolyte in the Wohlwill process to refine gold of the highest quality $(99.999\%)$.

Explanation for incorrect option

Option (A) $\mathrm{Al}$

• Since aluminum is weakly oxidized to its particular ionic form.
• Thus it will not behave suitably in aqua regia.
• This option is incorrect.

Option (C) $\mathrm{Cu}$

• Copper is a weak metal.
• Thus it is oxidized with difficulty.
• The option is incorrect.

Option (D) $\mathrm{Pt}$

• Platinum doesn't oxidize easily.
• Thus this option is incorrect.

Hence the metal is Aurum (Gold).