Appearance of blood red colour, on treatment of the sodium fusion extract of an organic compound with ${\mathrm{FeSO}}_4$ in presence of concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ indicates the presence of element/s

The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound.

In Lassaigne's test, if both N and S are present in the organic compound, they are converted to $\mathrm{NaSCN}$.

If ${\mathrm{SCN}}^{-}$ is added to a solution with iron(III) ions, a blood-red solution forms mainly due to the formation of $\left[\mathrm{Fe}\right(\mathrm{SCN}\left)\right({\mathrm{H}}_2\mathrm{O}{)}_5{]}^{2+}$$$