Gold, known for its resistance to most chemicals, is a noble metal. While typically melted by applying intense heat, certain powerful chemical mixtures can react with and dissolve it, turning the solid metal into a liquid solution. The most well-known chemical mixture capable of dissolving gold is aqua regia.
Understanding Aqua Regia
Aqua regia, which translates from Latin as "royal water," was given its name by alchemists because of its remarkable ability to dissolve the "royal" metals, gold and platinum. It is a highly corrosive, fuming liquid.
Based on the provided reference, we know several key properties of aqua regia:
- It is a fuming liquid.
- Freshly prepared aqua regia is colorless, but it turns yellow, orange or red within seconds due to the formation of nitrosyl chloride and nitrogen dioxide.
- It was so named by alchemists because it can dissolve noble metals like gold and platinum.
- Importantly, it does not dissolve all metals.
How Aqua Regia Works
Aqua regia is not a single acid but a mixture, traditionally made by combining concentrated nitric acid (HNO₃) and concentrated hydrochloric acid (HCl), typically in a molar ratio of 1:3 respectively. Neither acid can dissolve gold on its own. Nitric acid is a strong oxidizer, while hydrochloric acid provides chloride ions.
In aqua regia, the nitric acid oxidizes gold atoms (Au) into gold ions (Au³⁺). The hydrochloric acid then provides chloride ions (Cl⁻) which react with the gold ions to form tetrachloroaurate(III) anions ([AuCl₄]⁻). This complex anion is stable in the solution, effectively pulling the gold atoms into the liquid phase and preventing the reaction from reversing.
The formation of gases like nitrosyl chloride (NOCl) and nitrogen dioxide (NO₂) mentioned in the reference contributes to the fuming nature and color change of the mixture, and is a result of the reaction between the acids themselves as well as their reaction with metals.
Properties of Aqua Regia
Here are some key characteristics of aqua regia, incorporating details from the reference:
- State and Appearance: It is a fuming liquid that changes color from colorless when fresh to yellow, orange, or red quickly.
- Chemical Activity: Exceptionally corrosive and a powerful oxidizing agent.
- Ability to Dissolve Gold: Uniquely capable of dissolving noble metals such as gold and platinum.
- Naming: Earned the name "royal water" from alchemists due to its ability to dissolve gold.
- Selectivity: While potent, it does not dissolve all metals.
Applications of Aqua Regia
Due to its ability to dissolve gold and platinum, aqua regia has several important applications:
- Gold Refining: It is used in the purification process of gold, separating it from other metals.
- Etching: Used in etching processes, particularly in the production of electronic components.
- Analysis: Employed in analytical chemistry to dissolve metal samples for analysis.
Safety Considerations
Aqua regia is extremely corrosive and hazardous. It produces toxic fumes (like the nitrosyl chloride and nitrogen dioxide mentioned in the reference) and reacts violently with organic materials. Handling requires significant precautions, including appropriate ventilation and personal protective equipment.
In summary, while heat melts gold, the specific chemical mixture that dissolves gold, converting it into a liquid solution, is aqua regia, a powerful and corrosive blend known since the times of alchemists.
