What pollutants come from burning fuels, and what harm do they cause?
Atmospheric pollutants from burning fuels; carbon monoxide, soot, sulfur dioxide and oxides of nitrogen; their sources; and their effects on health and the environment.
A focused answer to AQA GCSE Chemistry 4.9.3, covering the pollutants released when fuels burn, including carbon monoxide, soot, sulfur dioxide and oxides of nitrogen, where they come from, and the harm they cause to health and the environment.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to identify the pollutants released when fuels burn, explain how they form (complete and incomplete combustion, and impurities), and describe their effects on human health and the environment. The exam reward comes from matching each pollutant to its precise origin and its precise harmful effect.
Complete and incomplete combustion
When a fuel burns with plenty of oxygen (complete combustion), it gives carbon dioxide and water and releases the most energy. With too little oxygen (incomplete combustion), the fuel is not fully oxidised, so it also produces carbon monoxide and soot (carbon particulates), and releases less energy. The amount of oxygen available is therefore what decides whether these incomplete-combustion pollutants form.
Carbon monoxide
Soot (particulates)
Soot is solid carbon particles from incomplete combustion. It causes respiratory (breathing) problems when inhaled, can cause global dimming by reflecting sunlight back into space, and makes buildings dirty.
Sulfur dioxide
Oxides of nitrogen
The contrast between these two is a common exam point: sulfur dioxide comes from an impurity in the fuel itself, while oxides of nitrogen come from the air being heated, not from the fuel.
Effects of acid rain
Acid rain (from sulfur dioxide and oxides of nitrogen) damages buildings and statues (especially limestone, which reacts with acid), harms trees and plants, and acidifies lakes, harming aquatic life such as fish.
Reducing the pollutants
Knowing the origin of each pollutant points to how it can be reduced. Sulfur dioxide can be cut by removing sulfur from fuels before they are burned, or by using flue-gas desulfurisation to remove it from power-station emissions. Carbon monoxide and soot can be reduced by ensuring complete combustion, with plenty of oxygen, and by fitting catalytic converters that oxidise carbon monoxide to carbon dioxide. Oxides of nitrogen, which form from the air at high engine temperatures, are also reduced by catalytic converters that convert them back to nitrogen and oxygen. These measures explain why modern vehicles and power stations release far less pollution than older ones, even though they burn similar fuels, and they are a common application-style exam point.
Try this
Q1. Explain why carbon monoxide is dangerous. [2 marks]
- Cue. It is toxic and reduces the blood's ability to carry oxygen, and it is colourless and odourless.
Q2. State two pollutants that cause acid rain. [2 marks]
- Cue. Sulfur dioxide and oxides of nitrogen.
Q3. State the source of the sulfur that forms sulfur dioxide when a fuel burns. [1 mark]
- Cue. Sulfur impurities in the fuel.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA car engine burns petrol, a hydrocarbon fuel. Explain how incomplete combustion produces carbon monoxide and soot, and describe one harmful effect of each on health or the environment.Show worked answer →
A 4-mark Paper 2 question on incomplete combustion pollutants.
Incomplete combustion (1 mark): when there is not enough oxygen, the fuel is not fully oxidised, so instead of all carbon dioxide it produces carbon monoxide and soot (carbon particulates). Carbon monoxide effect (1 mark + 1 mark): it is a toxic gas that binds to red blood cells and reduces the amount of oxygen the blood can carry, and it is colourless and odourless so it is hard to detect. Soot effect (1 mark): solid particulates cause respiratory (breathing) problems, and can cause global dimming and make buildings dirty.
Markers reward the not-enough-oxygen cause and a correct harmful effect for each pollutant.
AQA 20214 marksSulfur dioxide and oxides of nitrogen are both pollutants from burning fuels. Explain how each is formed, and describe how they lead to acid rain and its effects on the environment.Show worked answer →
A 4-mark question on sulfur dioxide, oxides of nitrogen and acid rain.
Sulfur dioxide (1 mark): formed when sulfur impurities in the fuel burn. Oxides of nitrogen (1 mark): formed when nitrogen and oxygen from the air react together at the high temperatures inside an engine. Acid rain (1 mark): both gases dissolve in rainwater to form acidic solutions, producing acid rain. Effects (1 mark): acid rain damages buildings and statues, harms trees and plants, and acidifies lakes, harming aquatic life.
Markers reward the different origins (impurity for sulfur dioxide, air at high temperature for oxides of nitrogen) and at least one acid-rain effect.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)