[Black_Forest]

In the case of  oil is that it contains hundreds of different types of hydrocarbons all mixed together  can all be separated by distillation.

To me is not clear why for a fractional distillation column when is used the differences in boiling temperature in the lower part are collected those with higher boiling point and in the upper part those with lower boiling point.

[Bakegaku]

It's not so much a direct effect of the fact of the lower boiling point that makes chemicals go to the top, but the reason why those hydrocarbons have a lower boiling point.  What characteristics do low-boiling point hydrocarbons have verses high-boiling point ones?

[Black_Forest]

With increasing the chain lenght the boiling point is increasing.