In ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}\to \mathrm{C}{\mathrm{O}}_2$ oxidation number of carbon change from $+3$ $\mathrm{t}\mathrm{o}\mathrm{ }+4$ .

In $\mathrm{S}{\mathrm{O}}_4^{2-}\to \mathrm{S}{\mathrm{O}}_3^{2-}$ oxidation number of sulphur change from $+\mathrm{6 }\mathrm{t}\mathrm{o}+4$ .

In $\mathrm{M}\mathrm{n}{\mathrm{O}}_4^{2-}\to \mathrm{M}\mathrm{n}{\mathrm{O}}_4^{-}$ oxidation number of Mn change from $+\mathrm{6 }\mathrm{t}\mathrm{o}+7$ .

In $\mathrm{F}{\mathrm{e}}^{3+}\to \mathrm{F}{\mathrm{e}}^{2+}$ oxidation number of carbon change from +3 to +2.