During halogen test, sodium fusion extract is boiled with concentrated HNO3 to

Step 1: Understand the Purpose of Boiling with HNO₃

In the Lassaigne test, the sodium extract (sodium fusion extract) may contain:

• $\ce{NaCN}$ (if N was present)
• $\ce{Na2S}$ (if S was present)
• $\ce{NaX}$ (if halogen was present)

Before testing for halogens with $\ce{AgNO3}$, the extract is boiled with dilute/concentrated $\ce{HNO3}$.

Step 2: Why Boil with $\ce{HNO3}$?

If $\ce{NaCN}$ and $\ce{Na2S}$ are present alongside $\ce{NaX}$, adding $\ce{AgNO3}$ directly would produce:

• $\ce{AgCN}$ (white ppt) — looks like AgCl and interferes with the halide test
• $\ce{Ag2S}$ (black ppt) — masks the halide precipitate

Boiling with dilute/conc. $\ce{HNO3}$ decomposes $\ce{NaCN}$ and $\ce{Na2S}$: $\ce{NaCN + HNO3 -> HCN(g) + NaNO3}$ $\ce{Na2S + 2HNO3 -> H2S(g) + 2NaNO3}$

Both $\ce{HCN}$ and $\ce{H2S}$ are gases and escape — the interfering $\ce{CN^-}$ and $\ce{S^{2-}}$ are removed.

Now the extract only contains $\ce{NaX}$ → $\ce{AgNO3}$ gives an unambiguous halide precipitate.

Step 3: Evaluate the Options

| Option | Assessment | |---|---| | (a) Remove unreacted sodium | Sodium would have already been used up; not the reason | | (b) Decompose cyanide or sulphide of sodium ✅ | Correct — removes $\ce{CN^-}$ and $\ce{S^{2-}}$ interference | | (c) Extract halogen from organic compound | Extraction is done during sodium fusion, not here | | (d) Maintain pH of extract | pH adjustment is not the purpose |

Answer: (b)

Key Points to Remember:

• Always boil with HNO₃ before the AgNO₃ halide test — to eliminate CN⁻ and S²⁻
• AgCN (white) and Ag₂S (black) would give false results without this step
• HCN and H₂S gases escape when acidified with HNO₃
• This is a mandatory pre-treatment step in Lassaigne's halogen test