Air pollutants and acid rain
Key idea: Incomplete combustion produces toxic carbon monoxide. Sulfur dioxide and nitrogen oxides form acids in rain, damaging buildings and ecosystems; these pollutants also harm breathing.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Name the common pollutants and their formulae.
- Compare how carbon monoxide, nitrogen oxides and sulfur dioxide form.
- Balance carbon forming carbon monoxide and choose the oxygen supply.
- Write the equation and check its atoms.
- Explain how a blocked air inlet affects combustion and breathing.
- Follow sulfur from coal to damage to limestone.
- Choose the nitrogen source, temperature condition and effect.
- Explain nitrogen oxide formation in engines and lightning.
- Match each exhaust gas to its effect.
- Correct the acid-rain explanation.
- Explain how a volcano could damage a limestone church.
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ContinueBurning fuels releases more than carbon dioxide and water. This lesson follows three air pollutants, carbon monoxide, nitrogen oxides and sulfur dioxide, from where they come from to the harm they do: poisoning, acid rain and damage to breathing and buildings.
It builds on two earlier ideas. When a fuel burns completely in plenty of oxygen, its carbon becomes carbon dioxide and its hydrogen becomes water. Acids react with carbonates to form a salt, water and carbon dioxide.
The common air pollutants
An air pollutant is a substance in the air at a level that harms living things or damages materials. These are the ones to know:
| Pollutant | Formula |
|---|---|
| carbon monoxide | CO |
| methane | CH₄ |
| nitrogen oxides: nitrogen monoxide and nitrogen dioxide | NO and NO₂ |
| ozone (at ground level) | O₃ |
| sulfur dioxide | SO₂ |
| unburned hydrocarbons (fuel that escapes without burning) | for example C₈H₁₈ |
Methane is a greenhouse gas. The rest of this lesson follows three pollutants from their sources to their effects: carbon monoxide, nitrogen oxides and sulfur dioxide.
Where three pollutants come from
Carbon monoxide: incomplete combustion
A fuel that contains carbon burns completely when there is plenty of oxygen. All its carbon becomes carbon dioxide:
With too little oxygen, burning is incomplete. Some of the carbon becomes carbon monoxide, and some may be left as soot (carbon):
Compare the equations: incomplete combustion uses less oxygen for the same fuel. Any carbon-containing substance can burn incompletely, including petrol in a car engine, gas in a cooker or heater, charcoal and wood. It is most likely where the air supply is poor, such as a blocked burner or a closed room.
Nitrogen oxides: lightning and engines
Nitrogen and oxygen in air do not normally react. At very high temperatures they do, forming nitrogen monoxide, which then reacts with more oxygen to form nitrogen dioxide:
Two places are hot enough: a lightning flash (natural), and the inside of an internal combustion engine, such as a car engine (human). The nitrogen comes from the air drawn into the engine, not from the fuel.
Sulfur dioxide: volcanoes and fossil fuels
Volcanoes release sulfur dioxide naturally. Fossil fuels such as coal and oil contain small amounts of sulfur, so burning fossil fuels, for example in power stations, also releases it:
Check your understanding 1 Balance carbon monoxide before explaining
For carbon forming carbon monoxide, find the smallest whole-number coefficients in C + O₂ → CO, and identify the combustion condition.
Show answer
The coefficients are 2, 1 and 2. Carbon monoxide can form with limited oxygen; complete combustion produces carbon dioxide. Now construct the complete equation and check its atoms.
Check your understanding 2 Carbon burning in too little oxygen
Write a balanced equation for carbon burning incompletely to form carbon monoxide. Check the atoms and state the combustion condition.
Work on paper or in the optional notes. Compare coefficients, atoms and the stated combustion condition.
Show answer
2C(s) + O₂(g) → 2CO(g): each side has two carbon and two oxygen atoms. A limited oxygen supply permits incomplete combustion; complete combustion forms carbon dioxide instead.
Carbon monoxide is toxic
Carbon monoxide is colourless and has no smell, so you cannot tell that it is there. When it is breathed in, it combines with haemoglobin in red blood cells more strongly than oxygen does. Less oxygen is then carried around the body. Exposure can cause headaches and dizziness; severe exposure can cause unconsciousness and death. The effect depends on concentration and exposure time. A correctly maintained burner needs an adequate air supply for complete combustion, and its combustion gases must be removed safely. Ventilation alone does not make a faulty appliance safe.
Worked example 1
Trace carbon monoxide from a faulty heater
Problem
Show solution step by step
Identify the source condition
Method
Decide whether the burning is complete or incomplete.Reason
The blocked air inlet limits the oxygen supply.Working
Answer
too little oxygen → incomplete combustionName the pollutant
Method
Write the equation for incomplete combustion of methane.Reason
With less oxygen, the carbon forms carbon monoxide instead of carbon dioxide.Working
Answer
2CH₄ + 3O₂ → 2CO + 4H₂OState the effect
Method
Explain the harm to people in the room.Reason
“It causes pollution” does not say what the harm is.Working
Answer
Carbon monoxide combines with haemoglobin, so the blood carries less oxygen.Explain the chemical condition
Working
Answer
With an adequate oxygen supply in a correctly operating burner, methane can burn completely to carbon dioxide and water instead of forming carbon monoxide.
Quick check
Acid rain
Rain is naturally slightly acidic, because it dissolves a little carbon dioxide from the air. Acid rain is rain that is more acidic than this. It forms from sulfur dioxide and nitrogen dioxide:
- Sulfur dioxide and nitrogen dioxide are released into the air.
- They dissolve in water droplets in clouds and react with the water and oxygen to form acids: sulfuric acid and nitric acid.
- The acids fall to the ground in rain.
Carbon monoxide and carbon dioxide are not the causes of acid rain.
Effects on breathing
The breathing harm comes from inhaling polluted air, rather than from rain falling on the skin. Sulfur dioxide and nitrogen dioxide irritate the airways. They also form fine sulfate and nitrate particles that can be inhaled. These pollutants can cause breathing difficulties and make asthma worse. The same emissions can therefore cause two problems: acid rain damages materials and ecosystems, while the gases and fine particles in the air harm breathing.
Effects on buildings
Limestone and marble are mainly calcium carbonate. The acid in acid rain reacts with it, so stone buildings and statues slowly wear away and lose their detail:
Acid rain also speeds up the corrosion of metal structures, such as iron bridges and railings.
The way to reduce acid rain is to release less sulfur dioxide and nitrogen dioxide in the first place: burn less fossil fuel, and remove sulfur from fuels before they are burned.
| Source | Pollutant | Effect |
|---|---|---|
| incomplete combustion of a carbon-containing fuel | carbon monoxide | toxic: less oxygen is carried by the blood |
| lightning; internal combustion engines | nitrogen oxides | acid rain; irritates the airways |
| volcanoes; burning fossil fuels | sulfur dioxide | acid rain; irritates the airways |
Worked example 2
Trace acid rain from a coal-fired power station
Problem
Show solution step by step
From the fuel to the gas
Method
Follow the sulfur in the coal.Reason
Burning oxidises the sulfur.Working
Answer
S + O₂ → SO₂: sulfur dioxide leaves the chimney.From the gas to the acid
Method
Describe what happens in the clouds.Reason
Sulfur dioxide dissolves in cloud droplets and reacts with water and oxygen.Working
Answer
sulfur dioxide → sulfuric acid → acid rainFrom the acid to the damage
Working
Answer
The statues are limestone or marble (calcium carbonate). Sulfuric acid in the rain reacts with the calcium carbonate, so the surface slowly wears away.
Quick check
Guided practice 3
Nitrogen oxides from a car and from a storm
Problem
Choose each part of the chain
Hints
Hint 1
reactants
Look at the equation N₂ + O₂ → 2NO. Where do both reactants come from?Hint 2
condition
This reaction does not happen in air at ordinary temperatures.Show solution step by step
The source
Method
Identify the reactants and the condition.Reason
Both nitrogen and oxygen are in air; they react only when very hot.Working
Answer
Inside a car engine and in a lightning flash, nitrogen and oxygen from air form NO, which reacts with more oxygen to form NO₂.The effect
Working
Answer
Nitrogen dioxide dissolves in cloud droplets and forms nitric acid, so it causes acid rain. It also irritates the airways.
Quick check
Check your understanding 3 Explain both sources and their effect
Nitrogen dioxide is measured beside a busy road and after a thunderstorm far from any road. Explain both sources, including the reactants, temperature condition and formation of nitrogen dioxide. Give one effect. Work on paper or in the optional notes.
Show answer
Nitrogen and oxygen in air react at the very high temperatures inside engines and lightning flashes: N₂ + O₂ → 2NO. Nitrogen monoxide reacts with more oxygen: 2NO + O₂ → 2NO₂. Nitrogen dioxide dissolves in cloud droplets and reacts with water and oxygen to form nitric acid, contributing to acid rain. It also irritates the airways. Nitrogen comes from air, not petrol; high temperature differs from the limited oxygen that produces carbon monoxide.
Check your understanding 4 Match each gas to its own effect
Car exhaust contains nitrogen dioxide and may contain carbon monoxide. Choose which contributes to acid rain and the characteristic harm from carbon monoxide.
Show answer
Nitrogen dioxide contributes to acid rain; carbon monoxide reduces oxygen transport by blood. Correct the complete exhaust explanation in the next activity.
Spot the mistake 4
“Carbon monoxide causes acid rain”
Learner claim
A learner writes: “Car exhaust causes acid rain because it contains carbon monoxide.” Find the error and write a correct explanation on paper or in the optional notes. Compare the pollutant, its source and its effect.
Correct the explanation
Show solution step by step
Find the error
Method
Check which gases form the acids in acid rain.Reason
Acid rain contains sulfuric acid and nitric acid. Neither contains carbon.
Working
Answer
The acids come from sulfur dioxide and nitrogen dioxide.
Give the correct link
Working
Answer
Car exhaust contributes to acid rain because the hot engine makes nitrogen oxides, and nitrogen dioxide forms nitric acid in clouds. Carbon monoxide in the exhaust is toxic, but it does not form acid rain.
Common mistake
Car exhaust contains several pollutants. Name the one that causes the effect in the question.
Check your understanding 5 A volcano and a limestone church
A town with no factories and very little traffic lies near an active volcano. Its limestone church is wearing away faster than a similar church in another country. Suggest an explanation.
Work on paper or in the optional notes. Compare the natural source, gas, acid and reaction with limestone.
Show answer
The volcano releases sulfur dioxide. The sulfur dioxide reacts with water and oxygen in clouds to form sulfuric acid, which falls as acid rain. The acid reacts with the calcium carbonate in the limestone, so the church wears away. Sulfur dioxide has a natural source as well as human ones.
Common mistakes
- Saying carbon dioxide or carbon monoxide causes acid rain. Sulfur dioxide and nitrogen dioxide do.
- Saying the nitrogen in nitrogen oxides comes from the fuel. It comes from the air drawn into the engine.
- Saying carbon dioxide is toxic in the same way as carbon monoxide.
- Writing “causes pollution” instead of naming the effect, such as acid rain or breathing difficulties.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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