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Science

Making cars

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Iron and steel rust when they come into contact with water and oxygen. They rust faster in salty water or acid rain. Aluminium, on the other hand, does not corrode easily, because its surface is protected by a layer of aluminium oxide.

Steel and aluminium have advantages and disadvantages when used to make cars, which are recycled to re-use valuable materials and cut down on waste.

Rusting

Iron and steel rust when they come into contact with water and oxygen. Both water and oxygen are needed for rusting to occur. In the experiment below, the nail does not rust when air or water is not present. Remember that 21 per cent of the air is oxygen.

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

Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron(III) oxide, which we see as rust. This is the word equation for the reaction:

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

Salt dissolved in water does not cause rusting, but it does speed it up, as does acid rain.

Aluminium

Unlike iron and steel, aluminium does not rust or corrode in moist conditions. Its surface is protected by a natural layer of aluminium oxide. This prevents the metal below from coming into contact with air and oxygen.

Block of aluminium metal - image does not show the transparent oxide layer

The layer of aluminium oxide does not flake off, unlike rust which can flake off the surface of iron and steel objects.

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