Large hydrocarbon molecules can be broken down into smaller, more useful molecules. This is cracking. There is a high demand for fuels with small molecules, so cracking the larger fractions gives products that are useful as fuels.
There are several methods. In catalytic cracking, the hydrocarbon vapour is passed over a hot catalyst (about 550 °C). In steam cracking, the hydrocarbon is mixed with steam and heated to a very high temperature (about 850 °C). The products include alkanes and another type of hydrocarbon called alkenes. Cracking is balanced like any equation: for example C10H22 → C8H18 + C2H4. The ten carbon atoms split into eight and two, so the alkene has two carbon atoms and four hydrogen atoms.
Alkenes are more reactive than alkanes. They react with bromine water, which is used as a test for alkenes. Bromine water is orange. When it is shaken with an alkene it turns colourless, and when it is shaken with an alkane it stays orange.
Alkenes are used to make polymers and as starting materials for the production of many other chemicals. Modern life depends on hydrocarbons: they are our fuels, and the feedstock for the polymers and other products of the petrochemical industry.
