Petrol is a blend of different components found in crude petroleum oil.
Crude oil is a mixture of many different substances.
Most of these substances are hydrocarbons, which are molecules composed of carbon and hydrogen atoms. Generally, the more carbon atoms a hydrocarbon contains, the higher its boiling point. A refinery uses the differences in boiling point of the hydrocarbons to separate them by distillation. Distillation products with higher boiling point ranges may undergo further heat or chemical processing to transform them into substances with lower boiling points that can be used as petrol components. Oil companies blend different petrol components together depending on the octane number, vapour pressure, and other properties required for a particular petrol product.