$$4HCl(aq) + Au(s) → HAuCl4(aq) + 4H_2(g)$$
In this reaction:
- Gold (Au) metal reacts with hydrochloric acid (HCl) to form a complex ion called tetrachloroaurate(III) ion (HAuCl4).
- Hydrogen gas (H2) is released as a byproduct of the reaction.
During this reaction, gold dissolves in the acid, forming a yellow or brownish-yellow solution. This process is known as gold dissolution and is commonly observed when gold comes into contact with acidic environments containing chloride ions.
However, it's important to note that this reaction typically occurs slowly at room temperature. For a more rapid reaction, factors like increased temperature, higher HCl concentration, or the presence of an oxidizing agent (such as nitric acid) may enhance the dissolution process.
It's also worth noting that while gold is not readily soluble in most common acids like HCl, it can dissolve in solutions containing both HCl and oxidizing agents like nitric acid (HNO3) or hydrogen peroxide (H2O2). This mixture, known as aqua regia, is a highly corrosive and powerful solution capable of dissolving even highly resistant metals like gold and platinum.
