Question

A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument, the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?

Answer 1

The goldsmith used the solution called Aqua regia.

Aqua regia:

• Aqua regia is a combination of strong acids that has the ability to dissolve some noble metals.
• Aqua regia is a mixture of Hydrochloric acid $\left(\mathrm{HCl}\right)$ and concentrated nitric acid $\left({\mathrm{HNO}}_3\right)$ which is present in $3:1$ ratio.
• When two very strong acids are combined, they react together to form a very acidic solution.
• Aqua regia releases nascent chlorine which makes it highly reactive and this can be done by the oxidation of $\mathrm{HCl}$ by nitric acid.
• The chemical reaction can be depicted as:

${\mathrm{HNO}}_3\left(\mathrm{aq}\right)+3\mathrm{HCl}\left(\mathrm{aq}\right)⟶\mathrm{NOCl}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2\mathrm{Cl}\bullet$

The noble metals such as Gold$\left(\mathrm{Au}\right)$ as well as Platinum$\left(\mathrm{Pt}\right)$ can be dissolved by aqua regia. When the upper surface of a dull gold ornament dissolves, it loses its weight.