Oxidation and reduction by oxygen transfer

A substance can change when it gains or loses oxygen. In this model, oxidation is the gain of oxygen, while reduction is the loss of oxygen.

Oxidation: gaining oxygen

When a metal reacts with oxygen to form a metal oxide, the metal gains oxygen and is therefore oxidised. For example, magnesium reacts with oxygen to form magnesium oxide:

Magnesium has gained oxygen in forming magnesium oxide, so magnesium has been oxidised.

Reduction: losing oxygen

Reduction is the opposite: a substance is reduced when oxygen is removed from it. For example, copper oxide reacts with hydrogen to form copper and water:

Copper oxide loses oxygen to form copper, so copper oxide has been reduced. The hydrogen gains that oxygen to form water, so hydrogen has been oxidised.

To identify oxidation or reduction, follow the particular substance through the reaction: if it gains oxygen, it is oxidised; if it loses oxygen, it is reduced.

Formation of metal oxides

When a metal reacts with oxygen, the two substances combine chemically to form a new compound called a metal oxide. The oxygen can come from the air.

This reaction follows a general pattern:

The name of the product identifies the metal and shows that it has combined with oxygen. For example, magnesium forms magnesium oxide, while copper forms copper oxide.

This is not just a mixture of metal and oxygen: a chemical reaction has produced a new compound. The metal that reacts has been changed into an oxide.