Context: Lassaigne’s test for halogens

In sodium fusion, the organic compound is fused with sodium so that covalently bonded elements are converted into ionic salts:

• Halogens $\to \text{NaX}$
• Nitrogen $\to \text{NaCN}$
• Sulphur $\to \text{Na}_2\text{S}$

The sodium fusion extract is then tested for halides using $\text{AgNO}_3$.

Why is concentrated $HNO_3$ added before the halogen test?

If the extract contains:

• $\text{NaCN}$, it can give $\text{AgCN}$ with $\text{AgNO}_3$
• $\text{Na}_2\text{S}$, it can give $\text{Ag}_2\text{S}$ with $\text{AgNO}_3$

These precipitates interfere with the halogen test because they may be mistaken for silver halides.

Role of boiling with concentrated $HNO_3$

Boiling the sodium fusion extract with concentrated $HNO_3$ decomposes/removes cyanide and sulphide ions:

• cyanide and sulphide are destroyed
• then only halide ions remain to react with $\text{AgNO}_3$

Check the options

• A: remove unreacted sodium — Incorrect. Unreacted sodium is already consumed during preparation of the extract.
• B: decompose cyanide or sulphide of sodium — Correct.
• C: extract halogen from organic compound — Incorrect. Halogen is already converted into ionic form during sodium fusion.
• D: maintain the pH of extract — Incorrect. This is not the purpose here.

Final answer

The correct option is B.