Which substance serves as reducing agent in the following reaction ? $14{\mathrm{H}}^{+}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+3\mathrm{Ni}⟶2{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O}+3{\mathrm{Ni}}^{2+}$

The compound undergo oxidation itself and reduces others is known as reducing agent. In this reaction O. N. of Ni changes from 0 to + 2 and hence Ni acts as a reducing agent.