Crude oil (AQA GCSE Chemistry Combined Science): Revision Notes
Crude oil
What is crude oil?
Crude oil is a dark, thick liquid that formed millions of years ago. When tiny sea creatures like plankton died, they got buried under thick layers of mud and rock. Over a very long time, heat and pressure changed these dead creatures into crude oil.
This formation process took millions of years under specific conditions of heat and pressure deep underground. The organic matter from ancient sea life was slowly transformed through geological processes into the crude oil we extract today.
Crude oil is actually a mixture containing thousands of different compounds. Most of these are hydrocarbons - molecules made of hydrogen and carbon atoms joined together.
How we separate crude oil - fractional distillation
Since crude oil is a mixture, we need to separate it into useful parts called fractions. We do this using a process called fractional distillation.
How fractional distillation works
The process happens in a tall tower called a fractionating column:
- Crude oil gets heated until it turns into vapour (gas)
- The hot vapour rises up the column
- Different parts of the vapour cool down and turn back to liquid at different heights
- This happens because different molecules have different boiling points
- Smaller molecules have lower boiling points, so they rise higher before condensing
- Larger molecules have higher boiling points, so they condense lower down
The key principle here is that smaller molecules rise higher in the column because they have lower boiling points. This separation by boiling point is what makes fractional distillation so effective for separating crude oil.
What comes out where
From top to bottom of the column, we get:
- Petroleum gases (like propane) - very small molecules, used for camping gas
- Petrol - small molecules, used in cars
- Kerosene - medium molecules, used in jet fuel
- Diesel oil - larger molecules, used in trucks and buses
- Heavy fuel oil - very large molecules, used to heat buildings
Two main uses for crude oil products
1. As fuels
Many fractions burn well and release lots of energy. We use them to:
- Power cars (petrol)
- Fly planes (kerosene)
- Heat homes (fuel oil)
- Cook food (petroleum gas)
2. As feedstock
Feedstock means starting materials. The petrochemical industry uses some fractions to make:
- Plastics and polymers
- Solvents for cleaning
- Lubricants for machines
- Detergents for washing
The petrochemical industry transforms crude oil fractions into countless everyday products. From the plastic in your phone case to the synthetic fibres in your clothes, many modern materials start as crude oil fractions.
Why crude oil matters
Crude oil is a finite resource. This means there's only a limited amount on Earth. Once we use it all up, it's gone forever. We can't make more because it takes millions of years to form naturally.
Crude oil is irreplaceable on human timescales. While new oil may form naturally, the process takes millions of years - far longer than human civilisation has existed. This makes conservation and finding alternatives critically important for our future.
This is why it's important to:
- Use crude oil wisely
- Find alternative energy sources
- Recycle plastic products made from oil
Key Points to Remember:
- Crude oil formed from ancient sea life buried under rock for millions of years
- Fractional distillation separates crude oil into useful fractions using different boiling points
- Smaller molecules rise higher in the fractionating column because they have lower boiling points
- Crude oil products are used as fuels (petrol, diesel) and feedstock (to make plastics)
- Crude oil is finite - once it's used up, we can't replace it