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Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

• the original starting hydrocarbonsare alkanes
• the products of cracking include alkanes and alkenes, members of a different homologous series
• For example, hexane can be cracked to form butane and ethene:

hexane → butane + ethene

C₆H₁₄ → C₄H₁₀ + C₂H₄

The starting compound will always fit the rule for an alkane, C_nH_2n+2. The first product will also follow this rule. The second product will contain all the other C and H atoms. The second product is an alkene, so it will follow the rule C_nH_2n.