Pure hydrochloric acid (HCl) does not react with gold. Even concentrated solutions of HCl will not dissolve the metal under standard conditions. To dissolve gold, you need aqua regia, which combines hydrochloric acid with nitric acid.
Why gold resists hydrochloric acid
Gold is a noble metal, meaning it is highly resistant to corrosion and oxidation. This stability comes from its electronic structure and relativistic effects that make its outer electrons tightly bound. For a metal to dissolve in an acid, the acid must act as an oxidizing agent, stripping electrons from the metal atoms to form ions.
Hydrochloric acid provides hydrogen ions (H+) and chloride ions (Cl-). The reduction potential of hydrogen is zero volts by definition. In contrast, the reduction potential for gold(III) ions is approximately +1.52 volts. Because gold’s potential is significantly higher, hydrogen ions lack the chemical strength to oxidize gold atoms. Therefore, no reaction occurs, and the gold remains solid.
The role of chloride ions
Although HCl cannot oxidize gold on its own, the chloride ions play a crucial supporting role when another oxidizing agent is present. Chloride ions are excellent ligands, meaning they bind strongly to metal ions in solution.
When gold is oxidized to Au3+ ions, these ions are unstable and would normally precipitate out or redeposit onto the metal surface. However, if plenty of chloride ions are available, they surround the gold ions to form tetrachloroaurate complexes ([AuCl4]-). This complexation keeps the gold dissolved in the liquid, driving the reaction forward.
How aqua regia dissolves gold
Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, typically in a 1:3 volume ratio. This combination creates a chemical environment capable of breaking down gold.
- Nitric acid (HNO3) acts as the oxidizer. It is strong enough to strip electrons from gold atoms, converting them into gold ions.
- Hydrochloric acid (HCl) supplies the chloride ions. These ions immediately complex with the gold ions, keeping them in solution and preventing them from reforming solid gold.
The synergy between the two acids is key. Nitric acid alone can oxidize gold slightly, but the resulting oxide is passivated. Hydrochloric acid alone cannot oxidize gold. Together, they achieve what neither can do independently.
Practical observations
If you place a pure gold coin in a jar of concentrated hydrochloric acid, nothing visible will happen. The acid will not bubble, change color, or erode the metal. The gold will remain unchanged regardless of how long it stays submerged, provided the temperature remains within standard laboratory ranges.
This inertness is why gold has been used for jewelry and currency for millennia. It does not rust, tarnish, or corrode in air or water. Only specific chemical mixtures or extreme conditions can alter its physical state.
Safety and handling
While pure HCl is not dangerous to gold, aqua regia is a highly reactive and corrosive mixture. It produces toxic nitrogen dioxide fumes and generates significant heat during the reaction. Handling aqua regia requires proper ventilation, protective gear, and careful neutralization after use. It should never be attempted by individuals without proper chemical training and safety equipment.