The reduction can be considered as the removal of oxygen, the addition of hydrogen, or the gain of electrons.
- The H+ ions, with an oxidation number of +1, are reduced to H2, with an oxidation number of 0, in the reaction:
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
- Another example is the reaction that extracts iron from iron ore:
Fe2O3 + 3CO → 2Fe + 3 CO2
The iron oxide undergoes reduction (loses oxygen) to form iron while the carbon monoxide is oxidized (gains oxygen) to form carbon dioxide.