JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Practical and Analytical Chemistry

Lassaigne's test shows nitrogen only when carbon is also present, because the fused sodium must weld the two into cyanide, and it shows halogen when an ionisable halogen is turned into sodium halide. Hydrazine hydrochloride, ammonium chloride and hydroxylamine hydrochloride all lack carbon, so although they hold nitrogen and chlorine no sodium cyanide can form and the nitrogen test fails. Methylamine hydrochloride holds carbon together with its nitrogen and also chlorine, so fusion with sodium yields both $\mathrm{NaCN}$ and $\mathrm{NaCl}$; the compound giving a positive test for both nitrogen and halogen is methylamine hydrochloride.