The combustion of fuels is a major source of atmospheric pollutants.

The idea

Combustion means burning a fuel in oxygen, releasing energy. When the fuel is a hydrocarbon, complete combustion gives carbon dioxide and water vapour. In a limited supply of oxygen, incomplete combustion also produces carbon monoxide and solid carbon particles. Fuels may contain sulfur, so burning them releases sulfur dioxide, and the high temperature inside an engine or furnace lets nitrogen and oxygen from the air combine to form oxides of nitrogen. These products enter the atmosphere and cause problems such as acid rain, respiratory harm and global climate change. For example, burning petrol in a car engine releases carbon dioxide, water vapour, carbon monoxide, nitrogen oxides and particulates. This makes fuel combustion a major source of atmospheric pollutants.

What earns the marks

• Combustion is the reaction of a fuel with oxygen, releasing energy as heat and light.
• Complete combustion of a hydrocarbon produces carbon dioxide and water vapour only.
• Incomplete combustion, caused by insufficient oxygen, produces carbon monoxide and solid carbon particles as well as carbon dioxide and water.
• Sulfur impurities in fuels burn to form sulfur dioxide.
• High temperatures in engines and furnaces allow nitrogen and oxygen from the air to react, forming oxides of nitrogen.

Common mistakes and how to put them right

• Thinking that all combustion is complete and produces only carbon dioxide and water.

  Put it right: State that a limited oxygen supply causes incomplete combustion, which also forms carbon monoxide and carbon particles.

• Believing that oxides of nitrogen come from impurities in the fuel.

  Put it right: They form when nitrogen and oxygen in the air react at high temperatures.

• Writing that combustion destroys matter or makes pollutants disappear.

  Put it right: Atoms are conserved, so fuel and oxygen atoms rearrange into the product gases and particles.

Exam tips

• Link each pollutant to its origin: carbon dioxide and water from complete combustion, carbon monoxide and carbon from incomplete combustion, sulfur dioxide from sulfur impurities, and nitrogen oxides from hot air.
• Use the phrase limited supply of oxygen when explaining incomplete combustion, because it shows the cause rather than just naming the products.
• When asked why combustion is a major source of pollutants, refer to the huge quantities of fuel burned in vehicles, power stations and homes, not just to one example.