The Lassiagne's extract is boiled with dil HNO₃ before testing for halogens because,

Solution

Chemistry ma’am · 4 steps

Get rid of cyanide and sulphide before the silver goes in, or they will fake a halogen result.

1. What the extract holds. Fusing with sodium turns N into NaCN, S into Na₂S and halogens into NaX. All three end up together in the same extract.
2. Why they interfere. The halogen test adds AgNO₃ and looks for a precipitate. But CN⁻ gives white AgCN and S²⁻ gives black Ag₂S, so you could see a precipitate even when no halogen is present.
3. What the acid does. Boiling with dilute HNO₃ converts them into the gases HCN and H₂S, which boil off and leave only the halide ions behind.

   NaCN + HNO₃ → NaNO₃ + HCN ↑ ; Na₂S + 2HNO₃ → 2NaNO₃ + H₂S ↑

4. So. The acid is there to decompose Na₂S and NaCN before AgNO₃ is added, which is option (c).

The idea to keep. In Lassaigne's test, always ask which ions could fool your reagent. Remove them first, and the precipitate you see then belongs to the element you are actually testing for.

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