In short
Oxidation is the loss of electrons and reduction is the gain of electrons. They always happen together in a redox reaction.
A helpful memory is OIL RIG: Oxidation Is Loss, Reduction Is Gain. The substance that loses electrons is oxidised and is the reducing agent. The substance that gains them is reduced and is the oxidising agent. The oxidation number rises in oxidation and falls in reduction. Oxidation can also mean gaining oxygen or losing hydrogen.
In , zinc goes from to , so it is oxidised. Copper goes from to , so it is reduced.
Also searched as: redox, oxidation and reduction.
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