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?
The solution used by the fake goldsmith was aqua regia.
Aqua regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3:1. It is a highly corrosive, fuming liquid that can dissolve gold and platinum, even though neither of these acids can do so alone.
When the gold bangles were dipped in aqua regia, the outer layer of gold dissolved in the solution. This removed the tarnished surface and made the bangles appear sparkling and new. However, since some gold was dissolved from the surface, the weight of the bangles decreased drastically.
The goldsmith used this chemical trick to deceive people - he cleaned their ornaments but also stole some gold in the process by dissolving it in aqua regia.
Explanation
The chapter mentions aqua regia in Section 3.2.3 as a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3:1. It is described as a highly corrosive, fuming liquid and one of the few reagents able to dissolve gold and platinum. This explains how the gold bangles lost weight while appearing cleaner - the outer layer was dissolved by aqua regia.