: 250
Lassaigne's test converts covalent N, S, and halogens to ionic forms detectable by standard reagents. Procedure: fuse organic compound with Na metal at red heat → plunge into water → boil → filter → test the extract. Nitrogen: NaCN forms → add FeSO4, boil → add FeCl3, acidify with H2SO4 → Prussian blue Fe4[Fe(CN)6]3. If BOTH N and S present: NaCNS forms instead → FeCl3 gives blood-red Fe(SCN)3 (not Prussian blue). Fix: use excess Na during fusion to force separate NaCN + Na2S formation. Sulfur: Na2S forms → sodium nitroprusside gives purple (very sensitive) OR lead acetate gives black PbS. Halogens: NaX forms → first boil extract with conc. HNO3 (removes interfering CN- and S2-) → add AgNO3 → AgCl (white, soluble in dilute NH3), AgBr (pale yellow, partially soluble in conc. NH3), AgI (yellow, insoluble in NH3). Fluorine: AgF is soluble → cannot be detected by this method. Beilstein test (Cu wire in flame → green = halogen) is a quick screen but cannot distinguish Cl/Br/I and gives false positives with some N-compounds. Critical JEE points: N+S interference, order of boiling with HNO3 before halogen test, NH3 solubility order for distinguishing silver halides.