Aqua regia is used for dissolving noble metals (Au, Pt, etc.). The gas evolved in this process is :

Solution

Chemistry ma’am · 3 steps

Nitric acid is the oxidant here, so look for what nitrate turns into after it pulls electrons off the gold.

1. Two acids, two jobs. Aqua regia is HCl and HNO₃ in a 3 : 1 ratio. HNO₃ oxidises the metal. Cl⁻ then locks the metal ion into a very stable complex such as [AuCl₄]⁻ or [PtCl₆]²⁻, which pulls the reaction forward.

   Au + 4H⁺ + NO₃⁻ + 4Cl⁻ → [AuCl₄]⁻ + NO ↑ + 2H₂O

2. Track nitrogen. Gold goes from 0 to +3 and loses 3 electrons. Nitrogen gains those 3 electrons and goes from +5 in NO₃⁻ to +2 in NO. Platinum behaves the same way: 3Pt + 16H⁺ + 4NO₃⁻ + 18Cl⁻ → 3[PtCl₆]²⁻ + 4NO + 8H₂O.
3. So. The gas given off when noble metals dissolve in aqua regia is nitric oxide, NO, which is option (d).

The idea to keep. Whenever nitric acid acts as an oxidant, ask how far nitrogen falls from +5. With weak reductants like Au and Pt, it stops at +2, giving NO.

Answer from the NTA final answer key. Solution by PrepWiser.