The Correct Answer and Explanation is:
The correct answer is hydrochloric acid.
Aqua Regia, a name given by alchemists, translates from Latin as “royal water” or “king’s water.” This title reflects its remarkable ability to dissolve the noble metals gold and platinum, which were considered the “royal” or most incorruptible metals. These elements are famously resistant to corrosion and will not dissolve when exposed to a single acid, making Aqua Regia a unique and powerful chemical agent.
The mixture is formed by combining concentrated nitric acid with concentrated hydrochloric acid, typically in a volume ratio of one part nitric acid to three parts hydrochloric acid. When mixed, the solution is highly corrosive, fuming, and has a distinct yellow-orange color. It is important to note that Aqua Regia is unstable and loses its effectiveness quickly, so it must be prepared fresh in a laboratory setting just before use. This instability is due to its components reacting with each other to produce volatile products like nitrosyl chloride and free chlorine, which are the active agents responsible for dissolving gold.
The chemical process is a fascinating example of cooperative action between the two acids. Neither acid can dissolve gold on its own. Nitric acid is a powerful oxidizing agent that can initially form a small number of gold ions (Au³⁺) from solid gold metal. However, this reaction quickly reaches equilibrium and stops. The crucial role of hydrochloric acid is to provide a high concentration of chloride ions (Cl⁻). These chloride ions react with the gold ions to form a very stable complex ion, the tetrachloroaurate(III) anion ([AuCl₄]⁻). By sequestering the gold ions within this stable complex, the hydrochloric acid effectively removes them from the solution. This constant removal of the gold ions continuously shifts the chemical balance, allowing the nitric acid to oxidize more and more gold atoms until the metal is completely dissolved.
