Aqua regia is the substance that dissolves gold.
Understanding Aqua Regia
Aqua regia, which translates to "royal water," is a highly corrosive mixture of concentrated nitric acid and hydrochloric acid. Neither of these acids alone can dissolve gold, but in combination, they are capable of doing so.
Here's a breakdown of the process:
-
Nitric Acid's Role: Nitric acid (HNO₃) acts as an oxidizer. It enables the formation of gold ions (Au³⁺).
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Hydrochloric Acid's Role: Hydrochloric acid (HCl) reacts with the gold ions (Au³⁺) to form tetrachloroaurate(III) anions ([AuCl₄]⁻).
The overall reaction can be summarized as follows:
Au + 3 HNO₃ + 4 HCl → [AuCl₄]⁻ + 3 NO₂ + H₃O⁺ + 2 H₂OWhy a Mixture is Necessary
- Nitric acid alone can't dissolve gold because it forms a passive layer of gold oxide which stops further reaction.
- Hydrochloric acid alone can't oxidize gold, which is required to put the metal into solution.
- Aqua regia, with both nitric and hydrochloric acids, allows for oxidation of gold by nitric acid, which is necessary for gold to react, and then hydrochloric acid forms stable ions, pulling the gold into solution by the formation of tetrachloroaurate(III) anions.
Practical Insights
- Safety Precautions: Handling aqua regia requires extreme care due to its highly corrosive nature and toxic fumes. It should always be used with proper safety equipment, in a well-ventilated area.
- Industrial Applications: The ability of aqua regia to dissolve gold is crucial in gold refining and chemical analysis.
- Laboratory Use: It is a valuable tool in analytical chemistry for dissolving trace amounts of metals.
Key Takeaways
| Component | Role in Dissolving Gold |
|---|---|
| Nitric Acid | Acts as an oxidizer, forming gold ions (Au³⁺). |
| Hydrochloric Acid | Reacts with gold ions to form tetrachloroaurate(III) anions ([AuCl₄]⁻), and thus pulls the gold into solution. |
| Aqua Regia | A necessary mixture of nitric and hydrochloric acid used to dissolve gold. |
