While gold typically requires extremely high temperatures (over 1000°C) to melt from a solid into a liquid state, there is a specific, highly corrosive liquid mixture known as aqua regia that can react with and dissolve gold, effectively turning it into a liquid form through a chemical process rather than melting by heat.
What is Aqua Regia?
Based on chemical properties, aqua regia is a distinct and powerful solution. It is described as a yellow-orange or red fuming liquid. This name, Latin for "royal water," refers to its ability to dissolve the noble metals like gold, which do not react with most single acids.
How Aqua Regia Dissolves Gold
Unlike melting which is a physical change caused by heat, dissolving is a chemical process where a substance breaks down and disperses within a solvent. Aqua regia achieves this with gold through a powerful oxidation-reduction reaction.
- The mixture is potent because it combines two strong acids: nitric acid and hydrochloric acid, typically in a 1:3 ratio.
- As stated in chemical descriptions, the key to its ability to attack gold lies in the oxidizing agent present, which is the nascent chlorine.
- This nascent chlorine reacts with noble metals to form their chlorides and gets them dissolved. For gold (Au), the nascent chlorine (Cl) and other components in aqua regia react to form soluble gold ions (AuCl₄⁻) in solution.
This process is not true melting, but rather the conversion of solid gold into a soluble form dispersed within the liquid mixture.
Dissolving vs. Melting Gold
It's important to understand the difference between these two processes:
- Melting: This is a physical change where solid gold turns into liquid gold solely due to sufficient heat. The chemical composition of the gold remains unchanged (Au atoms). Gold's melting point is approximately 1064 °C (1948 °F).
- Dissolving: This is a chemical process where gold reacts with a substance (like aqua regia) to form new chemical compounds (gold ions/complexes) that disperse within the liquid. The gold is no longer in its elemental solid form.
Therefore, while no liquid melts gold at typical temperatures, aqua regia dissolves it through a chemical reaction.
Significance of Aqua Regia
The ability of aqua regia to dissolve gold is historically and practically significant.
- Gold Refining: It is widely used in the refining process to purify gold and recover it from scrap materials or electronic waste.
- Analytical Chemistry: Aqua regia can prepare gold samples for chemical analysis.
In summary, when discussing liquids that interact with gold, aqua regia is the notable one because it can dissolve this noble metal through a powerful chemical reaction involving nascent chlorine, rather than melting it via heat.
