6.1.2—Changes in oxidation numbers to help balance

Oxidation is an increase in oxidation number; reduction is a decrease. The two changes occur together because electrons lost by one species are gained by another.

Assign oxidation numbers before and after the reaction, then identify which element changes. This works even when no free electrons appear in the equation.

In Zn + Cu²⁺ → Zn²⁺ + Cu, zinc changes 0→+2 and is oxidised; copper changes +2→0 and is reduced.

Oxidation does not always mean adding oxygen, and reduction does not always mean removing oxygen. Use the oxidation-number change as the general test.