When a mixture of potassium chlorate, oxalic acid and sulphuric acid is heated, the following reaction occurs : KClO3+5+H2C2O4+H2SO4+6→K2SO4+6+KCl−1+CO2+H2O i.e. maximum change in oxidation number is observed in Cl(+5 to-1). Thus, Cl is the element which undergoes maximum change in the oxidation state.