Qualitative analysis: detecting elements

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.