Fossil fuels

1. The three fossil fuels are coal, natural gas and petroleum.
2. They formed over millions of years from the remains of plants and animals that were buried and then squeezed and heated deep underground.
3. Each fossil fuel exists in a different state at room temperature.
   1. Coal is a black solid that formed mainly from the remains of ancient land plants.
   2. Petroleum, also called crude oil, is a thick, dark liquid that formed mainly from the remains of tiny sea organisms.
   3. Natural gas is a colourless gas that is often found trapped in porous rock above petroleum deposits.
4. Fossil fuels are burned to release thermal energy, which is used for heating, cooking, transport and generating electricity.
5. Fossil fuels are non-renewable, because they take millions of years to form and are being used up far faster than they are replaced.
6. Petroleum is also the main source of the raw materials used to make plastics, solvents and many other chemicals.

Hydrocarbons

1. A hydrocarbon is a compound that contains hydrogen and carbon only.
2. The atoms in a hydrocarbon molecule are held together by covalent bonds.
3. The word only is the key part of the definition, so a compound that contains any other element is not a hydrocarbon.
   1. Methane, ${CH}_4$, ethane, $C_2{H}_6$, and ethene, $C_2{H}_4$, are all hydrocarbons.
   2. Ethanol, $C_2{H}_{5}OH$, is not a hydrocarbon, because it also contains oxygen.
   3. Chloromethane, ${CH}_{3}Cl$, is not a hydrocarbon, because it also contains chlorine.
4. A hydrocarbon is a compound, so do not call it a mixture or an element.
5. Most of the substances in petroleum and natural gas are hydrocarbons, which is why fossil fuels are the main source of hydrocarbons.

Natural gas and methane

1. Methane is the main constituent of natural gas, which means it is the substance present in the greatest amount.

2. Natural gas is mostly methane, with smaller amounts of other hydrocarbons such as ethane and propane.

3. Methane has the formula ${CH}_4$, with one carbon atom joined to four hydrogen atoms by single covalent bonds.

   $$\begin{array}{ccccc}& & H& & \\ & & |& & \\ H& -& C& -& H\\ & & |& & \\ & & H& & \end{array}$$

4. Methane is the simplest hydrocarbon, because it has only one carbon atom.

5. Natural gas is piped to homes and power stations, where the methane is burned for heating, cooking and generating electricity.

Petroleum

1. Petroleum is a mixture of hydrocarbons.
2. In a mixture the substances are not chemically joined, so each hydrocarbon in petroleum keeps its own properties.
3. The hydrocarbon molecules in petroleum range from very small molecules with one carbon atom to very large molecules with more than 50 carbon atoms.
4. Most of these hydrocarbons are alkanes, which are carbon chains of different lengths.
5. Because the hydrocarbons have different chain lengths, they have different boiling points.
6. Petroleum is not useful straight from the ground, because each use needs hydrocarbons with particular properties, such as a runny liquid that ignites easily for car engines.
   1. The different boiling points allow petroleum to be separated into useful parts, as you will see with fractional distillation.