Lassaigne’s test is used to detect nitrogen, sulphur, halogens, and phosphorus in an organic compound. The elements are converted from their covalent form into the ionic form by fusing the compound with sodium metal.

During sodium fusion, the following reactions take place:

$$\text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN}$$ $$2\text{Na} + \text{S} \xrightarrow{\Delta} \text{Na}_2\text{S}$$

$\text{Na} + \text{X} \xrightarrow{\Delta} \text{NaX}$ (where X = Cl, Br, or I) If both nitrogen and sulphur are present, the reaction is: $\text{NaSCN} + 2\text{Na} \rightarrow \text{NaCN} + \text{Na}_2\text{S}$. The fused mass is boiled with distilled water to obtain the sodium fusion extract.

For the test for nitrogen, the sodium fusion extract is boiled with iron(II) sulphate and then acidified.

For the test for halogens, the extract is acidified with nitric acid and treated with silver nitrate ($\text{X}^- + \text{Ag}^+ \rightarrow \text{AgX}$). A white precipitate soluble in ammonium hydroxide indicates chlorine, a yellowish precipitate sparingly soluble indicates bromine, and a yellow precipitate insoluble in ammonium hydroxide indicates iodine.

If nitrogen or sulphur is also present, the extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide, as they would otherwise interfere with the silver nitrate test. Phosphorus is also detected using this extract.