Gold's Chemical Inertness: Gold is a noble metal, meaning it's highly resistant to chemical attack. Nitric acid (HNO₃) is a strong oxidizing acid that can dissolve many metals, but it does not react with gold.
The Process of Gold Extraction:
To extract gold from an alloy, a process called aqua regia is used. This involves a mixture of concentrated nitric acid (HNO₃) and hydrochloric acid (HCl) in a molar ratio of 1:3. Here's how it works:
1. Nitric Acid's Role: The nitric acid acts as an oxidizer, converting the gold (Au) to gold ions (Au³⁺). This reaction is slow but essential.
2. Hydrochloric Acid's Role: The hydrochloric acid then reacts with the gold ions, forming tetrachloroaurate(III) anions ([AuCl₄]⁻). This reaction removes gold ions from the solution and drives the overall reaction forward.
3. Gold Extraction: The tetrachloroaurate(III) anions are soluble in the aqua regia solution, effectively separating the gold from the alloy.
Important Notes:
* Safety: Aqua regia is extremely corrosive and dangerous. Handling it requires specialized equipment and safety precautions.
* Alternative Methods: There are other methods for extracting gold from alloys, including cyanide leaching and electrolysis.
* Recovery: After the gold is extracted, it needs to be further processed to recover it in its pure form.
In summary, nitric acid alone cannot dissolve gold. The process of aqua regia, a mixture of nitric acid and hydrochloric acid, is necessary to extract gold from alloys.
