What is crude oil and how does fractional distillation work?

Crude oil is formed from detritus (dead living matter) of animal, plants and microscopic organisms under intense pressure and temperature. It is trapped under impervious rocks which stop the liquid and gas from escaping. The crude oil is extracted by drilling and separated into fractions for further use. 

The mixture of compounds in crude oil is separated into more useful fractions by fractional distillation. The molecules with a lower molecular weight, such as butane, is separated first as a gas. This is because it requires less heat energy to evaporate. Heavier molecular weight compounds are separated at later stages as liquids, finally, leaving a tar-like substance called bitumen used in road surfacing.