Different fractions have different boiling points because they have different chain lengths. Short chain lengths require less energy to be broken apart, whereas longer chain require more energy to be broken apart. In addition the fractions are made up of different chain lengths and each fraction is different. Therefore, a fraction made up of shorter chains will boil faster then a fraction made up of many longer chains.

In the fractional distillation, the crude separates a mixture to different parts called fractions so there are different factions in the crude. The fractions are made up by different hydrocarbon molecules. Different fractions have a different boiling point. Because the different fractions contain different hydrocarbons, and different hydrocarbons have their definite boiling points. The temperature needs to be high enough so that it has enough energy to break down the long chain, and the short chain lengths are used less energy to break down. So the long chain length of hydrocarbon have high boiling point, and the short chain length of hydrocarbon have low boiling point.

The fractions have different lengths of chains. And the lengths of chains affects the size of the intermolecular force. The longer the molecule, the greater the intermolecular force. And the greater force requires more energy to break the molecules. Thus, this results in a higher temperature to evaporate the longer molecules. That is why the fractions have different boiling points.