Atmospheric pollutants from fuels: The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles and unburned hydrocarbons may also be released that form particulates in the atmosphere.

The idea

Burning a fuel releases a mixture of substances into the atmosphere. Complete combustion of a hydrocarbon in plenty of oxygen produces carbon dioxide and water vapour. If oxygen is limited, carbon monoxide and solid particles such as soot can form instead. Sulfur impurities in fuels produce sulfur dioxide, and the high temperature of combustion makes nitrogen and oxygen from the air react to form oxides of nitrogen, for example nitrogen monoxide, NO, and nitrogen dioxide, NO₂. Unburned hydrocarbons may also escape. Solid particles and unburned hydrocarbons together form particulates, which are tiny solid or liquid particles suspended in the air.

For example: Burning diesel in a vehicle engine can release CO₂, H₂O, CO, NOₓ, SO₂ and particulates, depending on engine conditions and fuel composition.

What earns the marks

• Carbon dioxide and water vapour are released when carbon- and hydrogen-containing fuels burn completely.
• Carbon monoxide can be released when combustion is incomplete, meaning oxygen supply is limited.
• Sulfur dioxide is released when a fuel containing sulfur impurities is burned.
• Oxides of nitrogen form because nitrogen and oxygen in the air react at the high temperatures of combustion.
• Solid particles and unburned hydrocarbons may be released and form particulates in the atmosphere.
• The mixture of gases and particles released depends on the fuel composition and the conditions of burning.

Common mistakes and how to put them right

• Stating that carbon monoxide forms during complete combustion.

  Put it right: Carbon monoxide forms when combustion is incomplete, often because oxygen supply is limited.

• Saying oxides of nitrogen come from nitrogen in the fuel.

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

• Describing particulates as gases.

  Put it right: Particulates are tiny solid particles and unburned hydrocarbons suspended in the atmosphere.

Exam tips

• Name the five gases listed in the statement and give the condition or source for each, such as limited oxygen for carbon monoxide.
• Distinguish clearly between gases and particulates when describing atmospheric pollutants from fuels.
• Use balanced chemical equations or word equations to show how a named product forms from the fuel or from air.