# Fuels

Fuels are one of the most important substances on Earth. They cook your food, heat your home and keep the cars and trains running. Without fuels the world would be a very different place.

A fuel is a substance that reacts exothermically with oxygen. The energy contained in the fuel is released when it burns.

### Fossil Fuels

Many of the fuels we use in everyday life are obtained from fuels called fossil fuels. These fuels are mostly hydrocarbons – compounds that contain the elements carbon and hydrogen only but contain some impurities which can lead to pollution when we burn them.

Fossil fuels like coal, crude oil and natural gas have been formed over millions of years from dead plant and animal remains which have been buried.

### Crude oil formation

Millions of years ago small animals and plants died and fell to the bottom of the sea. Their remains were covered by mud.

### Step three

The mud eventually turned to rock. This rock put a lot of pressure on the dead animals and plants. Rocks around them also heated them up. Together the heat and the pressure turned the remains into crude oil. It was important that no air or oxygen was present.

Fossil fuels are finite (non-renewable) energy resources. Their supply is limited and they will eventually run out. Fossil fuels do not renew themselves, while fuels such as wood can be renewed as trees capture energy from the sun in chemical reactions. This is the energy that is released when they burn.

Fossil fuels release carbon dioxide gas when they burn which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.

Coal and oil contain sulfur impurities. When these fuels burn, the sulfur burns too which releases sulfur dioxide (SO2) gas. Sulfur dioxide causes breathing problems for living creatures and contributes to acid rain.