The Aqua Regia Mystery
Dissolving gold with mixed acids
What this lesson covers
Why it matters
Gold sits untouched in nitric acid and hydrochloric acid alone. Mix them, and the metal vanishes into a yellow solution.
The idea in plain words
Balance the finishing move: Au + 3[Cl] → AuCl₃.
Predict first
Why does the mixture of acids dissolve gold when neither acid can do it alone?
The combination of concentrated nitric and hydrochloric acids produces nascent chlorine [Cl], a powerful oxidizing agent that gold can react with.
- The mixture is simply more acidic than the individual acids.
- The reaction generates nascent chlorine, which reacts with gold. — correct
- Heat from mixing the acids melts the gold.
- Nitric acid turns gold into a liquid state directly.
What you do
Mix one part concentrated HNO₃ with three parts concentrated HCl and watch nascent chlorine appear. Then offer the mixture some gold — the one attack gold cannot shrug off.
Check yourself
Which species is generated in the reaction between HNO₃ and HCl that allows gold to dissolve?
The equation HNO₃ + 3HCl → NOCl + 2H₂O + 2[Cl] shows that nascent chlorine [Cl] is produced.
What is the final product formed when gold reacts with nascent chlorine?
Gold combines with three chlorine atoms to form gold(III) chloride, as shown in Au + 3[Cl] → AuCl₃.
- Nitrosyl chloride (NOCl)
- Nascent chlorine ([Cl]) — correct
- Hydrogen gas (H₂)
- Water (H₂O)
- AuCl
- AuCl₃ — correct
- AuCl₂
- Au(NO₃)₃