Why do we boil the extract with conc. HNO3 in Lassaigne’s test for halogens?

The correct option is "for the decomposition of Na₂S and NaCN formed".

Step-by-Step Explanation:

1. In Lassaigne's test, organic compounds are fused with sodium metal. If nitrogen or sulfur is present along with halogens in the organic compound, sodium cyanide (NaCN) and sodium sulfide (Na₂S) are formed during the sodium fusion reaction:
Na + C + N → NaCN
2Na + S → Na₂S

2. If these impurities (NaCN and Na₂S) are not removed, they will react with silver nitrate (AgNO₃), which is added in the next step to detect halogens. The reactions would produce interfering precipitates:
NaCN + AgNO₃ → AgCN↓ (white precipitate) + NaNO₃
Na₂S + 2AgNO₃ → Ag₂S↓ (black precipitate) + 2NaNO₃
These white and black precipitates interfere with the identification of halogen precipitates (like AgCl, AgBr, and AgI).

3. To prevent this interference, the Lassaigne's extract (sodium fusion extract) is boiled with concentrated nitric acid (HNO₃) before adding silver nitrate. Concentrated HNO₃ decomposes NaCN and Na₂S into volatile gases (HCN and H₂S, respectively), which escape from the solution:
NaCN + HNO₃ → NaNO₃ + HCN↑
Na₂S + 2HNO₃ → 2NaNO₃ + H₂S↑

4. By boiling off these gases, the cyanide and sulfide ions are completely removed from the solution, allowing the silver nitrate to react exclusively with the halide ions present in the extract to give clear, non-interfered halogen tests.