Explain the process of fractional distillation of crude oil

Crude oil is a mixture of several different organic (carbon containing) molecules with different chain length. The different chain lengths mean that theses molecules have different boiling points; the longer molecules have stronger inter-molecular forces than the shorter molecules so the longer molecules have higher boiling points (lower volatility). This property enables the molecules to be separated into fractions with similar boiling points, this process is performed in a fractional distillation column. The column is hottest at the bottom and coolest at the top, so the short chain fractions such as petrol will condense near the top whereas long chain molecules such as those in bitumen will remain a liquid and separate at the bottom.More volatile and less viscous fractions will separate near the top, less volatile and more viscous fractions will separate near the bottom.