The various components of crude oil are separated in a refinery based on differences in

Differences in boiling points.

When changing states, the temperature of a substances increases because when going from a liquid to a gas, it heats up (use water as an example). water when heated up turns to water vapor, and when heated further it turns to a gas. when going from gas, to liquid, back to solid, it is cooling down- condensing the molecules back together.