Crude Oil, Hydrocarbons and Fuels

Crude oil comes from dead things…..yes you heard me right. Over millions of years the remains of plants and animals have turned into oil under extreme pressure. We’ve then extracted it by drilling and pumping.

So What Exactly Is Crude Oil

Well, it’s hard to say exactly – it’s a mixture of so many different compounds! Most of these compounds are hydrocarbon molecules (i.e. made up of hydrogen and carbon atoms only) – which are basically just fuels like petrol, diesel, kerosene etc.

These different compounds are not chemically bonded together, and each bit of the mixture will have kept its original properties, which means they can be separated by physical methods like distillation. You then get each bit of the mixture to come out unchanged!

Splitting Out Crude Oil Into Groups of Hydrocarbons Using Fractional Distillation

A fractioning column works continuously (the examiners expect you to know this is a continuous process for some reason!). Basically crude oil is pumped into it and the smaller molecules in the crude oil rise to the top. They crude oil vaporises and the different compounds condense at different levels and are constantly tapped off:



The fractions that come out of each bit have different properties but they have a few things in common:

  • The fractions of crude oil are hydrocarbons called alkanes.
  • Alkanes are just chains of carbon atoms surrounded by hydrogen atoms.
  • The different alkanes have chains of different lengths
  • In an exam you should be able to recognise alkanes but you only need to know the names of the first four:

first four alkanes

  • Carbon atoms form four bonds, hydrogen form one. As you can see in the diagrams above, all the atoms have formed as many bonds as they can – this means they are saturated. The bond shown with a straight line are covalent.
  • There is a general formula for alkanes


So if a alkane molecule has 10 carbon atoms, then it has 22 hydrogen atoms.

Alkane Trends

  • The shorter the molecule the more viscous (i.e. runny) it is
  • The shorter the molecule the more flammable it is
  • The shorter the molecule the lower its boiling point is

You can see the fractional distillation graph above for some uses. As you can see the different qualities of the alkanes make them useful for different things.


The fractions from crude oil burn g cleanly which makes them good as a fuel – just think about everywhere you use petrol! Parts of crude oil are also used in central heating systems and in power stations to generate electricity. Crude oil can also be turned into plastics and the raw materials for other chemicals – so there’s loads of scientists trying to get it out the ground!

There are alternatives to crude oil fractions – e.g. nuclear or wind power, ethanol powered cars and solar energy to heat water – but we’re so set up for crude oils with petrol cars, petrol stations and large industries devoted to crude that often it is the cheapest and easiest thing to use.

Crude oil fractions tend to be more reliable that wind and solar power too and it doesn’t have the nasty radioactive waste that nuclear does.

However, it will run out one day as it is a non-renewable fuel. Some say this will happen in 25 years – others thing longer. New reserves are found constantly and as tech improves we can get to more oil. Some say we should stop using crude fractions for things like transport and only use it for essential things like medicines.

It will take time to develop alternative fuels that can deliver enough energy to us and for us to adapt things to use these sources.

Renewable energy, like wind, solar and tidal power are being developed by some as an alternative.

Additionally, oil spills can kill off local environments and poison local wildlife.

Lastly, you have to burn oil to release its energy…and this is a major cause of global warming, acid rain and global dimming.