Why is fractional distillation may be used to separate crude oil into its constituents?

Answer 1

Fractional distillation separates things by boiling point. The problem is that the boiling points of many of the petroleum constituents are so close together that separation is often incomplete. Thus gasoline or motor oil will contain many different molecules with similar boiling points. Complete separation is possible but not practical unless you want $60/gallon gasoline.

Answer 2

Because crude oil consists of many different hydrocarbons, each having its own boiling point.
If you know that a mixture consists of compounds having different boiling point, the best way to separate them is by distillation.

Answer 3

Why Fractional Distillation is use because the substances in the crude oil has different boiling point, thus when a substance has reach its boiling point, it will evapourate.