Chemistry For IGCSE 0620 Cambridge: Chapter 14

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Basic concept each fraction rises up the column because the temperature at the bottom is high so it evaporates as gas , once it reaches a temperature less than its boiling point it condenses and gets separated

Crude oil is a mixture of hydrocarbon molecules .The molecules are chemically bonded with strong covalent bonds. but contain different numbers of carbon atoms.

The weak attractive forces between molecules have to be broken for the hydrocarbon to boil.The longer a hydrocarbon molecule is the stronger the intermolecular forces are between the molecules . The stronger forces means it needs more energy to overcome the large forces so higher boiling point is required.