Pollutants from Burning Fuels
Burning fuels is the major source of atmospheric pollutants. Most fuels contain carbon and/or hydrogen, and many — coal especially — also contain some sulfur. What comes out of the chimney or exhaust depends on the fuel and on how much oxygen is available. You first met complete and incomplete combustion of hydrocarbons in C7: with plenty of oxygen a hydrocarbon burns to carbon dioxide and water; starved of oxygen it gives carbon monoxide and soot instead.
With plenty of oxygen, a hydrocarbon burns cleanly to CO2 and water. Starve it of oxygen and you get a sooty yellow flame producing carbon monoxide and carbon particles instead.
- Carbon dioxide (CO2) — from complete combustion of the carbon in the fuel. (A greenhouse gas, not a “classic” pollutant, but a product of burning.)
- Carbon monoxide (CO) and soot (carbon particles) — from incomplete combustion, when there is not enough oxygen. Unburned hydrocarbons are released too; soot and unburned hydrocarbons form particulates.
- Sulfur dioxide (SO2) — from the sulfur impurities in the fuel burning.
- Oxides of nitrogen (NOx) — nitrogen and oxygen from the air react together at the high temperatures inside an engine.
You don’t need to recall combustion equations, but you should be able to predict the products from the fuel and the conditions: a hydrocarbon with plenty of oxygen → CO2 + water; with limited oxygen → CO and/or carbon (soot) + water; if the fuel contains sulfur → also SO2.
- Thinking NOx comes from the fuel. It comes from nitrogen and oxygen in the air reacting at the high temperature of the engine.
- Getting incomplete combustion wrong. With too little oxygen a fuel gives carbon monoxide and/or carbon (soot) as well as water — and often some CO2 too. The exam point is that CO and soot only appear when oxygen is limited.
- Forgetting the sulfur. SO2 only forms if the fuel contains sulfur — it isn’t made from the air.
🧪 Exam-style questions
Explain how carbon monoxide is produced when petrol is burned in a car engine.
Show answer
- There is not enough oxygen for complete combustion (incomplete combustion). 1 mark
- So the carbon is only partly oxidised, forming carbon monoxide (CO) instead of carbon dioxide. 1 mark
Describe how oxides of nitrogen are produced when fuel is burned in an engine.
Show answer
- The high temperature inside the engine 1 mark
- makes nitrogen and oxygen from the air react together to form oxides of nitrogen (NOx). 1 mark
Which pollutant is produced from sulfur impurities in a fuel? Tick (✓) one box.
A hydrocarbon burns in plenty of oxygen. What are the products? Tick (✓) one box.
Properties & Effects of Pollutants
Each pollutant causes a specific problem, and the exam wants the property linked to the effect. Learn them as a table — and keep them separate from greenhouse gases.
| Pollutant | Property | Problems it causes |
|---|---|---|
| Carbon monoxide (CO) | toxic, colourless and odourless | binds to haemoglobin so blood carries less oxygen; not easily detected; fainting, coma, death |
| Sulfur dioxide (SO2) | acidic gas | respiratory problems; acid rain |
| Oxides of nitrogen (NOx) | acidic gases | respiratory problems; acid rain |
| Particulates (soot & unburned hydrocarbons) | tiny solid particles | global dimming (reflect sunlight); respiratory/health problems |
Carbon monoxide binds to the haemoglobin in red blood cells in place of oxygen (and more strongly), so the blood carries less oxygen — which is why this colourless, odourless gas is so dangerous.
Sulfur dioxide and oxides of nitrogen dissolve in rainwater to make it acidic. Acid rain damages buildings and statues (especially limestone and metals), harms trees and aquatic life in lakes and rivers, and damages crops. Limestone is a carbonate, so acid rain reacts with it.
Sulfur dioxide comes from sulfur in fuels (especially coal); oxides of nitrogen form from nitrogen and oxygen in the air at the high temperature of car engines. Both dissolve in rain to make it acidic — eroding limestone buildings, harming trees and crops, and making lakes too acidic for fish.
- Greenhouse gas vs pollutant. A greenhouse gas (CO2, methane, water vapour) causes global warming. A pollutant (CO, SO2, NOx, particulates) is not part of clean air and causes problems near the ground — toxicity, smog, breathing problems, acid rain.
- Global dimming, not warming. Particulates cause global dimming by reflecting sunlight back into space — a different effect from the greenhouse warming caused by gases.
- CO is dangerous because it’s undetectable. Colourless and odourless means people don’t notice it — that’s the exam point, alongside it being toxic.
🧪 Exam-style questions
Give two reasons why people may be unaware that carbon monoxide is present. Tick (✓) one box.
Which two pollutants cause acid rain? Tick (✓) two boxes, then press Check.
What environmental problem is caused by particulates (soot) in the atmosphere? Tick (✓) one box.
Four coal samples contain different amounts of sulfur: A 0.4%, B 1.8%, C 0.9%, D 2.5%. Which sample produces the most acid rain per kilogram, and why? Tick (✓) one box.