When acidified solution of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ is shaken with aqueous solution of ${\mathrm{FeSO}}_4$ then :

When an acidified solution of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ is shaken with an aqueous solution of ${\mathrm{FeSO}}_4$ then ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ the ion is reduced to ${\mathrm{Cr}}^{3+}$ions.

The dichromate(VI) ion ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$ is the oxidizing agent. As a result, ${\mathrm{Cr}}^{3+}$ chromium(III) ions are produced.