Lassaigne's test is used for detection of:

Step 1: What Does Lassaigne's Test Detect?

Lassaigne's test (sodium fusion test) is a qualitative test used to detect heteroatoms in organic compounds. The compound is fused with sodium metal, converting covalently bonded elements into their sodium ionic compounds:

| Element | Product in Na Fusion | Detection Reagent | Result | |---|---|---|---| | N | $\ce{NaCN}$ | $\ce{FeSO4}$ → Prussian blue | Blue precipitate | | S | $\ce{Na2S}$ | Sodium nitroprusside | Purple/violet colour | | Cl | $\ce{NaCl}$ | $\ce{AgNO3}$ | White precipitate | | Br | $\ce{NaBr}$ | $\ce{AgNO3}$ | Pale yellow precipitate | | I | $\ce{NaI}$ | $\ce{AgNO3}$ | Yellow precipitate | | P | $\ce{Na3PO4}$ | $\ce{HNO3 + (NH4)2MoO4}$ | Yellow precipitate |

Step 2: Evaluate Each Option

(a) Nitrogen and Sulphur only — Lassaigne's test detects N, S, halogens AND phosphorus. This option is incomplete. ❌

(b) Nitrogen, Sulphur and Phosphorus only — Still missing halogens. ❌

(c) Phosphorus and halogens only — Missing N and S. ❌

(d) Nitrogen, Sulphur, Phosphorus and Halogens — This is the complete and correct answer. ✅

Answer: (d)

Key Points to Remember:

• Lassaigne's test (standard scope): N, S, halogens
• NCERT specifically mentions: nitrogen (Prussian blue), sulphur (violet with nitroprusside), halogens (AgNO₃ precipitates)
• Phosphorus detection is a separate test (not part of standard Lassaigne's in NCERT)
• The compound must be fused with Na metal (NOT K, NOT Mg)