During halogen test, sodium fusion extract is boiled with concentrated $$HNO_3$$ to

In the Lassaigne's test (sodium fusion extract test) for halogens, the sodium fusion extract is boiled with concentrated $$HNO_3$$ before adding $$AgNO_3$$.

Purpose:

During sodium fusion, if nitrogen or sulphur is present in the organic compound, sodium cyanide ($$NaCN$$) or sodium sulphide ($$Na_2S$$) may form. These interfere with the silver nitrate test for halogens because:

$$NaCN + AgNO_3 \to AgCN \downarrow$$ (white precipitate)

$$Na_2S + 2AgNO_3 \to Ag_2S \downarrow$$ (black precipitate)

These precipitates would give false positive results for halogens. Boiling the extract with concentrated $$HNO_3$$ decomposes these interfering compounds:

$$NaCN + HNO_3 \to NaNO_3 + HCN \uparrow$$

$$Na_2S + 2HNO_3 \to 2NaNO_3 + H_2S \uparrow$$

The volatile $$HCN$$ and $$H_2S$$ escape, leaving only the halide ions in solution for accurate detection with $$AgNO_3$$.

The correct answer is Option B: to decompose cyanide or sulphide of sodium.