Recognising oxidation and reduction
Oxidation and reduction are opposite chemical changes. They can be recognised by considering the transfer of oxygen or hydrogen, or by following the oxidation state of an element.
| Change considered | Oxidation | Reduction |
|---|---|---|
| Oxygen | Gain of oxygen | Loss of oxygen |
| Hydrogen | Loss of hydrogen | Gain of hydrogen |
| Oxidation state | Increases | Decreases |
Changes involving oxygen
Magnesium is oxidised when it gains oxygen:
In the reaction below, carbon gains oxygen and is oxidised, while copper(II) oxide loses oxygen and is reduced:
Changes involving hydrogen
Loss of hydrogen can indicate oxidation, while gain of hydrogen can indicate reduction.
Ammonia loses hydrogen and is oxidised. Bromine gains hydrogen and is reduced.
Changes in oxidation state
An element is oxidised when its oxidation state increases and reduced when its oxidation state decreases. Oxidation-state changes therefore provide another way to identify the two processes in a reaction.
In a redox reaction, oxidation and reduction occur together: one species is oxidised while another is reduced.