The fundamental principle: organic compounds contain N, S, and halogens in covalent form (undetectable by ionic reagents). Sodium fusion converts them to ionic form.

Fusion procedure: Heat small piece of Na metal in ignition tube to redness → add organic compound → heat strongly → plunge red-hot tube into distilled water (tube shatters) → boil → filter. The filtrate (Lassaigne's extract) contains NaCN, Na2S, NaX in solution.

Why sodium? Na is strongly electropositive — it rips apart covalent bonds and forms ionic compounds. The high temperature ensures complete decomposition. Excess Na is important — it ensures separate NaCN and Na2S form (rather than NaCNS when both N and S are present).

Caution: The test cannot detect N in diazo compounds (R-N=N-R) or hydrazines easily because N2 may escape as gas during fusion. Also fails for some aromatic nitro compounds — sometimes need extra Na or use copper turnings to ensure complete reduction.