The substance that acts as a reductant in the following reaction3Ni+Cr2O72−+14H+⟶3Ni2++2Cr+4+7H2O, is :

Let oxidation state of Cr in Cr2O72−=x∴ 2x+7(−2)=−2;2x=12;x=+6From the given equation, we have:3Ni∘+Cr26+O7−2+14H+⟶3Ni2++2Cr3++7H2OSince oxidation state of Ni has increased from zero to +2, Ni will act as a reductant.