2 mL each of concentrated $HCl, {HNO}_{3}$ and a mixture of concentrated $HCl$ and concentrated ${HNO}_{3}$ in the ratio of 3 : 1 were taken in test tubes labelled as A, B and C. A small piece of gold 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. Give reason why this observation was made.

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Solution

A 3:1 mixture of concentrated $H C l a n d {HNO}_{3}$ is known as aqua regia.
Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task and will actually dissolve a virtually undetectable amount of gold, forming gold ions.

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A 3:1 mixture of concentrated HCl and HNO3 is known as aqua regia. Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task and will actually dissolve a virtually undetectable amount of gold, forming gold ions.