Here's why:
* Aqua regia is a highly corrosive mixture of concentrated nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3.
* Gold is a noble metal, meaning it's highly resistant to most acids. However, it can be dissolved by aqua regia due to the following reaction:
The reaction:
1. Nitric acid acts as an oxidizer, forming gold ions ($Au^{3+}$).
2. Hydrochloric acid reacts with the gold ions, forming tetrachloroaurate(III) anions ($[AuCl_4]^−$). This reaction is an equilibrium reaction that favors the formation of the tetrachloroaurate(III) anion.
3. The formation of the tetrachloroaurate(III) anions removes gold ions from solution, driving the overall reaction forward.
The equation:
$Au + 3 HNO_3 + 4 HCl \rightleftharpoons [AuCl_4]^- + 3NO_2 + H_3O^+ + 2 H_2O$
Important notes:
* Aqua regia is a very dangerous mixture and should only be handled by trained professionals.
* The dissolution of gold in aqua regia is a complex process that is influenced by various factors, including the concentration of the acids, temperature, and the presence of other metals.
* Aqua regia is used to dissolve gold in processes like gold refining and the production of gold salts.
Let me know if you have any other questions!