Question

2 ml each of concentrated $HCl$, $HN{O_3}$ and a mixture of concentrated $HCl$ and $HN{O_3}$ in the ratio of 3:1 were taken in test tubes labeled 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 1

Mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid is known as aqua regia. Now, by using this we can easily find the correct option from the given ones.

Complete step by step answer:
It is known to you that aqua regia hydrochloric acid (which is a reducing agent) is oxidised by nitric acid (which is an oxidising agent) to form nascent chlorine.

Now, coming to the question we can easily find out the metal which do not dissolved in test tube A and B but got dissolved in test tube C by inspecting each of the above given option:
Option A: Al is the chemical symbol of aluminium metal. Aluminium will get dissolved in all the test tubes as it reacts with hydrochloric acid, nitric acid and aqua regia. So, this option is incorrect.
Option B: Au is the chemical symbol of gold metal. Gold does not react with $HCl$ and $HN{O_3}$ due to its noble nature but it got dissolved in aqua regia. So, this option is correct.
Option C: Cu is the chemical symbol of copper. It does not react with hydrochloric acid but it does react with nitric acid and aqua regia. So, this option is also incorrect.
Option D: Pt is the chemical symbol of platinum and like gold it also does not react with hydrochloric acid and nitric acid but it does react with aqua regia. So, this option is also correct.

Therefore, from above we can conclude that option B and D both are correct options to the given question.

Note: a) It should be remembered to you that noble metals like gold, silver and platinum can be dissolved only by aqua regia and cannot be dissolved in any other single acid.
b) Also, remember that aqua regia is also used for cleaning glassware and in the gold refining process.