Why is fractional distillation used on crude oil?

Because crude oil is actually a mixture of lots of different hydrocarbons. Each of these hydrocarbons has a different boiling point because they will be made up of different length of hydrocarbon chain. The longer the chain, the higher the boiling point.

At the bottom of the fractionating column the crude oil is heated so the majority of the hydrocarbons evaporate and turn into a gas. These gases rise up the tower which is temperature controlled so they cool down at different levels. Once each hydrocarbon cools down enough it will condense back to a liquid and be collected. The lower the boiling point the nearer the top of the tower this will happen as this is where the coolest temperatures are. The very smallest hydrocarbons remain as a gas and are collected at the top