Crude oil is often a dark, sticky liquid that cannot be used without changing it. The first part of refining crude oil is to heat it until it boils. The boiling liquid is separated into different liquids and gases in a distillation column. These liquids are used to make petrol, paraffin, diesel fuel etc.

Crude oil is a mixture of different chemical called hydrocarbons. The boiling oil turns into a mixture of gases in the column. The gases flow up the column which is hottest at the bottom and cooler at the top. The gases cool down as they go up the column until they condense (turn back into liquid again). The separated liquids and gases, after cleaning and further processing, are used to make many products.

Liquids from refining oil still have to be changed to make them more useful. Sometimes it’s to make them clean enough to be used. Sometimes it’s to turn some of the unwanted liquids into things people want to buy.

The heavier liquids are in less demand from customers so are turned into lighter products that are in demand. One of the processes is called catalytic cracking. It breaks down some of the heavy liquids from the distillation column.

The heavy liquids are changed into simple and more useful liquids and gases. Cracking is just one of many chemical changes in an oil refinery.