Fractional distillation: GCSE Chemistry Definition

What is fractional distillation?

Crude oil is a mixture of different substances and isn’t very useful on its own. However, the individual substances it contains, called fractions, are incredibly valuable and have many uses.

• Fractions are groups of hydrocarbons with similar chain lengths.

• Each fraction has molecules with similar properties, such as boiling point, which depend on the size of the hydrocarbon chain.

• The larger the hydrocarbon molecule, the more carbon and hydrogen atoms it contains.

Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.

Fractional distillation is the process used to separate the different fractions in crude oil. This takes place in a fractional distillation / fractionating column:

The fractional distillation column

How does fractional distillation work?

Fractional distillation is carried out in a fractionating column. It works by:

1. Heating crude oil

   • Crude oil is heated until it turns into a mixture of vapours.

2. Entering the column

   • The vapours are introduced into the fractionating column, which is hottest at the bottom and cooler towards the top.

3. Condensing hydrocarbons

   • Hydrocarbons with high boiling points condense (turn back to liquid) near the bottom, where it is hottest.

   • Hydrocarbons with lower boiling points rise up the column and condense near the top, where it is cooler.

This separation process ensures that hydrocarbons are grouped into fractions based on their boiling points, allowing them to be used for specific applications.

Fractional distillation Revision Resources to Ace Your Exams

To explore fractional distillation further, you should check out the revision notes on fractional distillation & petrochemicals and practise our exam questions to ensure success. Flashcards on hydrocarbons, fuel & feedstock are also a great way to revise the wider topic of hydrocarbons before an exam.  

Explore our GCSE Chemistry Revision Resources.