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When iron (steel) is exposed to the weather it rusts. It can easily be established that neither water nor air (oxygen) alone will cause iron to rust. Therefore the process of rusting has been discovered to be the reaction of iron and oxygen, in the presence of water, forming a new substance which is a compound of iron and oxygen.
If a weighed amount of iron filings is kept moist and exposed to the air for a week or two, then dried and weighed again, the mass of rust will be greater than the original mass of iron.
For many centuries the process of rusting was poorly understood. During the eighteenth century many scientists tried and failed to understand the processes ofburning and rusting. The key element that they were missing wasoxygen.
Some metals react readily with oxygen to form a layer of oxide. In the case of aluminium and zinc the oxide layer is coherentand seals the surface of the metal protecting it from further oxidation. Rust or iron oxide, on the other hand, does not form acoherent layer and does not stick to the iron surface.
The Iron Age
Although the centre of the Earth is probably mostly molten iron, elemental iron does not occur naturally in the Earth’s crust. Many early cultures discovered how to convert iron ore (compounds ofiron and oxygen or sulfur) to metallic iron; this marked the beginning of ‘the iron age’. In the Near East this happened around 1200 BC but was as late of 600 BC in Northern Europe.
What exactly is rust?
Rust is the red-brown oxide of iron formed by the action of moisture and oxygen on the metal. It consists mainly of hydrated iron(III) oxide (Fe2O3.H2O) and iron(III) hydroxide (Fe(OH)3). Ifoxygen is in short supply the formation Fe3O4 is favoured; this can be regarded as a mixture of Fe2O3 (magnetite) and FeO.