2 ml each of concentrated $H C l$ , $H N O_{3}$ and a mixture of concentrated $H C l$ and concentrated $H N O_{3}$ 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 l

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A u

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C u

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P t

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Solution

The correct option is B $A u$
As per the given question, the Aqua regia is the given mixture to test the metals in test tubes A, B and C. A mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid(Aqua regia - 3:1 ratio) is capable of dissolving gold.
Gold is a metal that cannot be dissolved in dilute acids and dissolves only in aqua regia.
Hence, the metal in test tube C must be gold (Au). The correct option is (B).

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