Question

One day Reeta went to a jeweller's shop with her mother. Her mother gave old gold jewellery to the goldsmith to polish. Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?

Answer 1

Hint: Aqua regia ( king's water) is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a yellow-orange (sometimes red) fuming liquid, so named by alchemists because it can dissolve the noble metals, gold and platinum, though not all metals.

Complete step by step answer:
Gold ornaments are polished in a solution of aqua regia (mixture of nitric acid and hydrochloric acid). The outer layer of the gold dissolves in the acidic solution and the inner shiny layer is visible. Because of loss of the upper layer of jewellery the weight of ornaments is reduced.
Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions ($A{{u}^{3+}}$). The hydrochloric acid provides a ready supply of chloride ions ($C{{l}^{-}}$ ), which react with the gold ions to produce tetrachloroaurate(III) anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of chloroaurate anions ($AuC{{l}_{4}}^{-}$ ).

Note: This process results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. In addition, gold may be dissolved by the chlorine present in aqua regia.