Lassaigne's Test - Detection of Nitrogen

Boil sodium extract with $$\text{FeSO}_4$$ and acidify with conc. $$\text{H}_2\text{SO}_4$$.

Formation of a Prussian Blue precipitate/coloration confirms Nitrogen:

$$6\text{NaCN} + \text{FeSO}_4 \to \text{Na}_4[\text{Fe(CN)}_6] + \text{Na}_2\text{SO}_4$$

$$3\text{Na}_4[\text{Fe(CN)}_6] + 4\text{Fe}^{3+} \to \text{Fe}_4[\text{Fe(CN)}_6]_3 \downarrow \text{ (Prussian Blue)} + 12\text{Na}^+$$