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) Al (b) Au (c) Cu (d) Pt

Answer: (b) Au

Solution C is a mixture of concentrated HCl and concentrated HNO3 which is called aqua regia. Aqua Regia dissolves gold.

Aqua regia

Aqua regia is a yellow-orange or sometimes red colour 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

Some uses of aqua regia are listed below.

  • 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%).

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