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

Step 1: Understand the Problem

In Lassaigne's test, before testing for halogens with $\ce{AgNO3}$, the sodium extract is boiled with dilute $\ce{HNO3}$. Why?

The sodium extract may contain:

• $\ce{NaCN}$ (from nitrogen)
• $\ce{Na2S}$ (from sulphur)
• $\ce{NaX}$ (from halogens)

Step 2: The Problem with $\ce{AgNO3}$ Test Without Pre-treatment

If we add $\ce{AgNO3}$ directly to the extract without acidification:

• $\ce{NaCN + AgNO3 -> AgCN(v)}$ — white precipitate (mimics AgCl)
• $\ce{Na2S + 2AgNO3 -> Ag2S(v)}$ — black precipitate (masks halide test)

Both $\ce{AgCN}$ and $\ce{Ag2S}$ would interfere with and confuse the halide test.

Step 3: Why Boil with Dilute $\ce{HNO3}$?

Boiling with dilute $\ce{HNO3}$ decomposes $\ce{NaCN}$ and $\ce{Na2S}$:

• $\ce{NaCN + HNO3 -> HCN (gas, volatile) + NaNO3}$ — $\ce{CN^-}$ is destroyed
• $\ce{Na2S + 2HNO3 -> H2S (gas, expelled) + 2NaNO3}$ — $\ce{S^{2-}}$ is removed

Now only $\ce{NaX}$ (halide) remains, which then reacts with $\ce{AgNO3}$ to give an unambiguous precipitate.

Step 4: Evaluate Each Option

(a) AgCN is soluble in HNO₃ — AgCN is actually NOT soluble in dilute HNO₃; this is not the reason.

(b) Silver halides are soluble in HNO₃ — Actually, silver halides are INSOLUBLE in HNO₃. This statement is incorrect and cannot be the reason.

(c) Ag₂S is soluble in HNO₃ — $\ce{Ag2S}$ dissolves in dilute HNO₃, but this is not the primary reason.

(d) Na₂S and NaCN are decomposed by HNO₃ ✅ — This is the correct reason. HNO₃ decomposes Na₂S and NaCN, eliminating interfering ions before the halide test.

Answer: (d)

Key Points to Remember:

• Boiling with dilute HNO₃ before halide test removes interfering CN⁻ and S²⁻
• Without this step, AgCN and Ag₂S would form and confuse results
• Silver halides: AgCl (white), AgBr (pale yellow), AgI (yellow) — all insoluble in dilute HNO₃
• AgCl and AgBr dissolve in $\ce{NH3(aq)}$; AgI does not