Atmospheric pollutants from fuels — AQA GCSE Combined Science
Test yourself on Atmospheric pollutants from fuels with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Atmospheric pollutants from fuels explained
When fuels burn, the elements inside them react with oxygen and release gases into the atmosphere.
Read the full explanation
Carbon in the fuel forms carbon dioxide, which is a greenhouse gas. Hydrogen forms water vapour. If the fuel contains sulfur impurities, sulfur dioxide is produced, and this can dissolve in rain to form acid rain. At high temperatures in engines, nitrogen and oxygen from the air react to form nitrogen oxides, which also contribute to acid rain and to photochemical smog. Incomplete combustion, caused by too little oxygen, produces carbon monoxide, a toxic gas, and solid particles called particulates, which can cause breathing problems and reduce visibility. A balanced equation such as CH₄ + 2O₂ → CO₂ + 2H₂O shows complete combustion, while limited oxygen gives CO instead of CO₂.
Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur.
Fuels are substances that release energy when burned, and their composition explains which products form. Most fuels are based on carbon and/or hydrogen: coal is mainly carbon with variable impurities, while natural gas is largely methane, CH₄, so it is rich in both elements. Many fuels also contain sulfur compounds as impurities, especially coal and crude-oil fractions. When a fuel burns, its carbon atoms can form carbon dioxide, CO₂, or carbon monoxide, CO, its hydrogen atoms form water vapour, H₂O, and any sulfur forms sulfur dioxide, SO₂. Knowing the elements present therefore lets you predict the combustion products and the pollutants released. For example, burning sulfur-containing coal produces SO₂, which can contribute to acid rain, whereas burning pure methane cannot produce SO₂ because no sulfur is present.
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.
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.
describe how carbon monoxide, soot (carbon particles), sulfur dioxide and oxides of nitrogen are produced by burning fuels
Burning fuels can release four named atmospheric pollutants. Carbon monoxide and soot form when a fuel burns in a limited supply of oxygen, a process called incomplete combustion. For example, methane burning in too little air can produce carbon (soot) and carbon monoxide instead of just carbon dioxide and water. Sulfur dioxide forms when sulfur impurities in fuels such as coal or petrol oxidise during burning. Oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperatures found in car engines and furnaces. You should be able to describe each production route clearly, linking the pollutant to its source and the conditions needed, and use formulae such as CO, SO₂ and NOₓ where helpful.
predict the products of combustion of a fuel given appropriate information about the composition of the fuel and the conditions in which it is used.
Combustion products depend on what the fuel contains and on how much oxygen is available. Fuels made mainly of carbon and hydrogen burn in plentiful air to give carbon dioxide and water: methane + oxygen → carbon dioxide + water, CH₄ + 2O₂ → CO₂ + 2H₂O. If oxygen is limited, carbon monoxide and/or carbon particles (soot) also form, because carbon is only partly oxidised. A fuel containing sulfur produces sulfur dioxide, which contributes to acid rain. Nitrogen in the air can react with oxygen at the high temperatures of a flame to give nitrogen oxides. To predict products, identify the elements in the fuel, decide whether oxygen supply is plentiful or limited, then combine each element with oxygen: carbon → CO₂ or CO, hydrogen → H₂O, sulfur → SO₂.
Your focus
- Name the main products of complete and incomplete combustion of fuels.
- Describe the environmental or health effect of carbon dioxide, sulfur dioxide, nitrogen oxides, carbon monoxide and particulates.
- Write a balanced equation for the complete combustion of a simple fuel such as methane.
Show all 15 objectives
- State that most fuels contain carbon and/or hydrogen.
- Identify that some fuels, including coal, may also contain sulfur.
- Use the composition of a fuel to predict possible combustion products.
- List the gases that may be released into the atmosphere when a fuel is burned.
- Explain how carbon monoxide, sulfur dioxide and oxides of nitrogen are formed during combustion.
- Describe how solid particles and unburned hydrocarbons form particulates in the atmosphere.
- Describe how carbon monoxide and soot are produced by the incomplete combustion of fuels in a limited supply of oxygen.
- Describe how sulfur dioxide is produced from sulfur impurities in fuels during burning.
- Describe how oxides of nitrogen are produced when nitrogen and oxygen from the air react at high temperatures in engines or furnaces.
- Predict the products formed when a named fuel burns in a plentiful or a limited supply of oxygen.
- Deduce the products of combustion from information about a fuel's composition, including carbon, hydrogen and sulfur content.
- Write word and balanced symbol equations for the complete and incomplete combustion of simple fuels.
Atmospheric pollutants from fuels exam tips
Marking Points
- States that burning fuels releases gases into the atmosphere, making combustion a major pollution source.
- Names carbon dioxide as a product of complete combustion and links it to the greenhouse effect.
- Names sulfur dioxide from sulfur impurities and links it to acid rain.
- Names nitrogen oxides formed at high temperature and links them to acid rain or smog.
- Names carbon monoxide and particulates from incomplete combustion and gives a harmful effect of each.
- Uses a word or balanced symbol equation to show complete or incomplete combustion.
- Most fuels contain carbon and/or hydrogen as their main combustible elements.
- Coal is mainly carbon but also contains variable impurities, including sulfur compounds.
- Hydrocarbon fuels such as methane contain hydrogen as well as carbon.
- Sulfur is present in some fuels as an impurity rather than as the main fuel component.
- The elements present in a fuel determine which combustion products can form, for example sulfur in a fuel can lead to sulfur dioxide.
- Combustion products include carbon dioxide and/or carbon monoxide from carbon, and water vapour from hydrogen.
- 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.
- Carbon monoxide is produced by incomplete combustion of a carbon-containing fuel when oxygen supply is limited.
- Soot consists of carbon particles and is also produced by incomplete combustion of fuels.
- Sulfur dioxide is produced when sulfur impurities in fuels burn and oxidise, forming SO₂.
- Oxides of nitrogen are produced when nitrogen and oxygen from the air react at high temperatures, for example in vehicle engines.
- Complete combustion in plenty of air produces carbon dioxide and water, so the four pollutants require either limited oxygen, fuel impurities or high temperature.
- Each pollutant should be linked to a named fuel or context, such as coal, petrol, diesel or natural gas, to show understanding of production.
- Identify the elements present in the fuel from the information given, for example carbon, hydrogen and possibly sulfur.
- State that complete combustion in a plentiful supply of oxygen produces carbon dioxide and water.
- State that incomplete combustion in a limited supply of oxygen produces carbon monoxide and/or carbon (soot) as well as carbon dioxide and water.
- Deduce that a sulfur-containing fuel produces sulfur dioxide, which can cause acid rain.
- Recognise that nitrogen oxides can form when nitrogen and oxygen from the air react at the high temperature of a flame.
- Write word equations, and balanced symbol equations where required, for the combustion reactions predicted.
Examiner Tips
- 💡Link each pollutant to its source and its effect in a three-part chain: fuel, gas, consequence.
- 💡Use correct names and formulae, for example CO₂, SO₂, NOₓ and CO, and balance any equation you write.
- 💡If asked to evaluate, compare pollutants by harm, for example carbon monoxide is toxic while carbon dioxide contributes to climate change.
- 💡Read the question carefully: 'atmospheric pollutants' means gases and particles released into the air, not waste left in the engine.
- 💡Link each element in a fuel to the product it can form: carbon to CO₂ or CO, hydrogen to H₂O, sulfur to SO₂.
- 💡Use the phrase 'may also contain some sulfur' when describing fuels, because sulfur content varies between fuels.
- 💡When asked to predict pollutants, first identify which elements the fuel contains, then name the corresponding oxides.
- 💡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.
- 💡Structure each description as fuel plus condition gives pollutant, for example petrol burning in limited air produces carbon monoxide and soot.
- 💡Use the correct formula for each pollutant where you can: CO, SO₂ and NOₓ, and write carbon for soot.
- 💡Check that you have covered all four pollutants, because exam questions often award credit for each separate production route.
- 💡Underline the composition of the fuel and the words describing the oxygen supply before you start predicting products.
- 💡Use the phrase 'plentiful supply of oxygen' for complete combustion and 'limited supply of oxygen' for incomplete combustion to make your reasoning clear.
- 💡Check that every element in the fuel appears in a product, and balance any symbol equation by counting atoms on each side.
Common Mistakes
- Saying carbon dioxide causes acid rain. Correction: sulfur dioxide and nitrogen oxides are the acidic gases; carbon dioxide is a greenhouse gas.
- Writing that carbon monoxide is produced when there is plenty of oxygen. Correction: carbon monoxide forms during incomplete combustion when oxygen is limited.
- Confusing particulates with gases. Correction: particulates are tiny solid particles, often carbon, released in smoke.
- Stating that all fuels contain sulfur; correction: sulfur is only present in some fuels, such as coal and heavier oil fractions, as an impurity.
- Claiming that coal is a pure element; correction: coal is mainly carbon but contains variable impurities, including sulfur compounds.
- Assuming water vapour forms only if hydrogen gas is burned; correction: any fuel containing hydrogen, such as methane, produces water vapour when burned.
- Stating that carbon monoxide forms during complete combustion; correction: carbon monoxide forms when combustion is incomplete, often because oxygen supply is limited.
- Saying oxides of nitrogen come from nitrogen in the fuel; correction: they form when nitrogen and oxygen in the air react at high combustion temperatures.
- Describing particulates as gases; correction: particulates are tiny solid particles and unburned hydrocarbons suspended in the atmosphere.
- Saying carbon monoxide is produced by complete combustion; correction: carbon monoxide is a product of incomplete combustion when oxygen is limited.
- Confusing soot with carbon dioxide; correction: soot is solid carbon particles, while carbon dioxide is a colourless gas.
- Stating that sulfur dioxide comes from nitrogen in the air; correction: sulfur dioxide comes from sulfur impurities in the fuel, whereas oxides of nitrogen come from nitrogen and oxygen in the air at high temperature.
- Assuming every fuel gives only carbon dioxide and water: correct this by checking the fuel's composition and the oxygen supply, since sulfur gives sulfur dioxide and limited oxygen gives carbon monoxide or soot.
- Writing carbon monoxide as the product of complete combustion: correct this by linking carbon monoxide to incomplete combustion with insufficient oxygen.
- Forgetting that nitrogen oxides come from the air rather than the fuel: correct this by noting that nitrogen and oxygen react together at flame temperatures.