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 men beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?

Answer 1

Hint: As the shine of the bangles is increased in the process along with loss of the mass, some gold atoms from the surface of the bangles should have reacted with the solution. Gold generally does not react with other compounds.

Complete step by step solution:
We will observe the process and give the scientific reason behind that.
- When the lady gave her gold bangles, the man put those into a solution. After that treatment, the bangles shine like they were new. Another observation given to us is that the bangles were lighter than they were before.
- As the bangles were lighter, there must be some loss in the mass of the bangles. So, the solution is something that reacts with gold and so that the bangles become lighter in weight. There might be a case in which some amount of the metal may have dissolved in a solution.
- Gold is a precious metal and its reactivity is very low. So, the gold must have dissolved in the solution.
- We know that gold is not soluble in normal solvents. It is not even soluble in inorganic concentrated acids. The only solution which can dissolve gold is aqua regia.
- Aqua regia is a mixture of concentrated nitric acid ($HN{O_3}$) and concentrated hydrochloric acid ($HCl$) in a ratio of 1:3. That means that there is 1 part of nitric acid and 3 parts of hydrochloric acid in the mixture.
- So, when the man dipped the bangles in this solution, the gold on the surface of the bangles dissolved in the solution. That gave a new shine to the bangles. But because the gold on the surface got dissolved in the solution, a notable amount of loss of mass was there.
- Thus, we can say that that solution was aqua regia and its nature was acidic.

Note: Note that gold is a noble metal. Noble metals are resistant to corrosion. Alongside gold, silver ($Ag$), Platinum ($Pt$), Ruthenium ($Ru$), Osmium ($Os$) are also noble metals. Note that aqua regia can dissolve any metal.