In Lower School you may have seen pictures like this...

... which suggest that oil formed from the remains of large animals like fish and even crocodiles.

It is true that oil comes from once living material, but almost all of that was plankton - microscopic plants and animals.

When the material died it sank to the sea-floor and was unable to rot due to lack of Oxygen.

Eventually it became buried by metres of sand.

As the sand above built up into sandstone, the heat and pressure increases.

And the plankton slowly became Hydrocarbons - molecules containing only Carbon and Hydrogen

These float up in the space between sand grains until they get trapped by a later of impermeable rock - usually shale (mudstone).

The diagram below looks like the oil is in some sort of underground cave, but actually it is between the sand grains inside the sandstone

Since the oil came from plankton which lived millions of years ago, we can call oil the remains of an ancient biomass.

We said Hydrocarbons only contain Hydrogen and Carbon.

The simplest Hydrocarbons are called Alkanes - these contain only single bonds.

You need to be able to name and draw the first four Alkanes - Methane to Butane.

The easiest way to know how many Carbon and Hydrogen atoms to draw is to learn the general formula C_nH_2n+2

This means that the number of Hydrogen atoms in an Alkane is always twice the number of the Carbon atoms plus two.

Displayed formulae of the first 4 Alkanes - each is -CH₂-longer than its predecessor.

We only need to be able to name the first 4 Alkanes.

They will all end with -ane to signify that they are Alkanes.

They begin with a prefix that tells you how many Carbons are in the chain.

C1 = Meth-, C2 = Eth-, C3 = Prop-, C4 = But-, C5 = Pent-

After 4 the prefixes are the same as for Polygons in Maths (Pent -, Hex-, Hept-, Oct-, Non-, Dec- etc)

We can remember the first 5 as: My Evil Plane Bombed Paris

The examiner may ask you for the molecular formula of an Alkane with, for example, three Carbon atoms.

In which case, you apply the general formula (C_nH_2n+2 ) and get

But you may also be asked to draw a displayed formula.

In which case you link the three Carbon atoms with single bonds, then add Hydrogen atoms until every Carbon atom is surrounded by four bonds.

There's no reason why you couldn't be asked to do the same for an Alkane with 6 or 8 Carbon atoms.

But you can't be asked to name these.