${\mathrm{Na}}_2{\mathrm{SO}}_4$ and ${\mathrm{Na}}_2\mathrm{S}$ can be distinguished from each other by using :

${\mathrm{Na}}_2{\mathrm{SO}}_4$ and ${\mathrm{Na}}_2\mathrm{S}$ can be distinguished from each other by using dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$, acidified ${\mathrm{KMnO}}_4$ solution, sodium nitroprusside solution and cadmium acetate solution.

${\mathrm{Na}}_2\mathrm{S}⟶{\mathrm{H}}_2\mathrm{S}$

${\mathrm{S}}^{2-}⟶\mathrm{S}\downarrow$

${\mathrm{S}}^{2-}⟶{\mathrm{Na}}_4\left[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NOS}\right]$

${\mathrm{SO}}^{2-}⟶{\mathrm{CaSO}}_4\downarrow {\text{ while S }}^{2-}⟶\mathrm{CaS}\text{ (soluble) }$.