Zinc pieces are added to acidified solution of ${\mathrm{SO}}_3^{2-}$.

Gas liberated can:

Hydrogen sulphide gas is produced when zinc reacts with an acidified sulphite solution.
Lead acetate paper turns black when hydrogen sulphide gas is present.

$3\mathrm{Zn}+{\mathrm{SO}}_3^{2+}+2{\mathrm{H}}^{+}\to 3\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{S}\uparrow$

It also decolourize acidified ${\mathrm{KMnO}}_4$ solution.