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Science

Obtaining and using metals

Oxidation and reduction

Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide:

magnesium + oxygen → magnesium oxide

2Mg + O2→ 2MgO

Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen:

copper oxide + hydrogen → copper + water

CuO + H2→ Cu + H2O

Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is:

  • very reactive metals – electrolysis
  • less reactive metals - reduction

Rusting

Iron and steel rust when they come into contact with water and oxygen: this is a form of corrosion. Both water and oxygen are needed for rusting to occur. Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron(III) oxide, which we see as rust. Here is the word equation for the reaction:

iron + water + oxygen → hydrated iron(III) oxide

In the experiment below, the nail does not rust when air - containing oxygen - or water is not present:

Calcium chloride absorbs water in the sealed right-hand test tube, so there is air but no water, and so no rusting. In the central test tube a nail is inside boiled water which is sealed by oil, so there is no oxygen, and so no rusting. In the left test tube there is water and oxgen with the nail, and it rusts.

Calcium chloride absorbs water in the right-hand test tube

Salt dissolved in water does not cause rusting, but it does speed it up, as does acid rain [acid rain: Rain that contains a high level of acids. It is believed to be caused by pollution and, maybe, natural activity such as volcanic eruptions. ].

Aluminium does not rust (corrode) because its surface is protected by a natural layer of aluminium oxide which prevents the metal below from coming into contact with air and oxygen. Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminium oxide does not flake off.

More reactive elements are more likely to oxidise.

Back to Obtaining and using metals index

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