Lassaigne's test for N, S and halogens.

Sodium-fuse a compound, then run the extract through FeSO4, lead acetate, nitroprusside and AgNO3 tests; colours and precipitates appear only for elements present, and skipping the HNO3 boil gives a false halogen result.

Show which of the six compounds gives no Prussian blue despite containing N (hydrazine, no C), and get the blood-red colour for N + S.

Forgetting to boil off CN⁻/S²⁻ before AgNO3; any N compound gives Prussian blue.

• To detect carbon and hydrogen, heat the compound with copper(II) oxide: its carbon ends up as CO₂ (lime water turns milky) and its hydrogen as water (white anhydrous copper sulphate turns blue).
• In Lassaigne's test the compound is fused with sodium metal, turning N, S and halogens into NaCN, Na₂S and NaX, which are then extracted with water as the sodium fusion extract.
• Nitrogen: iron(II) sulphate is added to the extract, which is boiled and then acidified with sulphuric acid; a Prussian blue colour, from iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃·xH₂O, confirms nitrogen.
• Sulphur: acetic acid and lead acetate give a black precipitate of PbS; sodium nitroprusside gives a violet colour.
• When both N and S are present, NaSCN forms and gives a blood-red colour with Fe³⁺ instead of Prussian blue.
• Halogens: acidify the extract with nitric acid, boil it to destroy any cyanide or sulphide, then add silver nitrate; chlorine gives a white precipitate that dissolves in ammonia, bromine a pale yellow one sparingly soluble in ammonia, and iodine a yellow one insoluble in ammonia.
• Phosphorus: heating the compound with sodium peroxide, an oxidising agent, turns its phosphorus into phosphate; after boiling with nitric acid, this gives a yellow precipitate with ammonium molybdate.

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