Crude oil is a complex mixture of hydrocarbons, and each component has its own boiling point. When crude oil is heated, the components with lower boiling points vaporize and leave the liquid behind. These vapors are then condensed and collected as separate fractions, such as gasoline, diesel, and heating oil.

The process of distillation is used to separate the components of crude oil because it is a very efficient and cost-effective method. It is also a very versatile process, and it can be used to separate a wide variety of different mixtures.