21. Explain how to identify the reducing agent in a reaction of magnesium with oxygen.

The balanced chemical equation between magnesium and oxygen to form the magnesium oxide is,

Mg + O2 - - - > MgO

The oxidation state of the reactants are both zero and that of the product is Mg2+, so 2 and that of oxygen is - 2. The substances oxidized is magnesium because its oxidation number increased. This is also the reducing agent because it reduced the oxidation.