Many metals react with oxygen to make metal oxides. For example, magnesium burns rapidly in air:
Magnesium + oxygen → magnesium oxide
2Mg(s) + O2(g) → 2MgO(s)
The reactions are oxidation reactions because the metal gains oxygen.
Oxygen can be removed from metal oxides in chemical reactions. For example:
Zinc oxide + carbon → zinc + carbon dioxide
2ZnO(s) + C → 2Zn(s) + CO2(g)
In this reaction, carbon is oxidised because it gains oxygen. At the same time, copper oxide is reduced because oxygen is removed from it.
Unreactive metals such as gold are found in the Earth's crust as the uncombined elements. However, most metals are found combined with other elements to form compounds. Chemical reactions are needed to extract metals from their compounds.
Most metals are extracted from ores found in the Earth's crust. An ore is a rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile.
The extraction method used depends upon the metal's position in the reactivity series.
If a metal is less reactive than carbon, it can be extracted from its compounds by heating with carbon. Copper is an example of this. Molten copper can be produced from copper oxide by heating with carbon:
Copper oxide + carbon → copper + carbon dioxide
2CuO(s) + C(s) → 2Cu(l) + CO2(g)
Metal oxide + carbon → metal + carbon dioxide
Copper oxide is reduced as carbon is oxidised, so this is an example of a redox reaction.
The table summarises the extraction methods used for different metals.
Although an unreactive metal is found as the metal itself, chemical reactions are often needed to remove other elements that might contaminate it.
Iron(III) oxide is reduced to molten iron when it reacts with carbon. One of the products is carbon monoxide:
iron(III) oxide + carbon → iron + carbon monoxide
Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
In the reaction of iron(III) oxide with carbon, state which substance is reduced and which substance is oxidised.
Carbon gains oxygen, so it is oxidised. Iron(III) oxide loses oxygen, so it is reduced.