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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

Al

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B

Au

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C

Cu

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D

Pt

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Solution

The correct option is B

Au


Explanation for Correct option

Option (B) Au

Aqua regia

  • Solution C is a mixture of concentrated HCland concentrated HNO3which 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 1:3optimally.

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) 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) Cu

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

Option (D) Pt

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

Hence the metal is Aurum (Gold).


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