Cracking

Crude oil contains a mixture of long and short alkane molecules, but the short ones are the most useful. The long chains are broken down into shorter chains by cracking.

The long molecules are not very useful, cracking breaks the molecules by heating them. The cracking produces shorter chain alkanes, which are useful as fuels. Cracking also produces alkenes, which are used to make polymers.

Cracking parrafin
Parradin is an alkane. Liquid parrafin can be cracked in the laboratory using the apparatus.
 * The porous pot is heated strongly
 * Parrafin_cracking.gif liquid paraffin is heated and evaporates
 * The paraffin vapour passes over the hot porous pot and the hydrocarbon molecules break down.
 * One of the products is ethene, which is a gas and collects in the other tube