Which of the following is released from car exhaust due to incomplete combustion of fuel?

1. Analyze the question The question asks which gas comes from car exhaust specifically because the fuel burns incompletely. The key phrase is "incomplete combustion."

2. Explain the concept Petrol and diesel are hydrocarbons (made of carbon and hydrogen).

• Complete combustion (plenty of oxygen): Hydrocarbon + O₂ → CO₂ + H₂O
• Incomplete combustion (limited oxygen): Hydrocarbon + limited O₂ → CO (carbon monoxide) + H₂O (and sometimes soot, which is carbon)

Inside a car engine, oxygen is often not enough to burn all the fuel fully, so carbon monoxide is formed.

3. Go through the options

• A. Carbon dioxide: Product of complete combustion, so it is not the answer.
• B. Carbon monoxide: Product of incomplete combustion. ✔
• C. Nitrogen dioxide: Forms when nitrogen and oxygen in the air react at the very high temperatures in the engine. It is not caused by incomplete combustion of the fuel.
• D. Oxygen: It is a reactant, not a pollutant produced by burning.
• E. Sulphur dioxide: Comes from sulphur impurities in the fuel, not from incomplete combustion.

4. Answer The correct option is B. carbon monoxide.

Why it matters: Carbon monoxide is poisonous. It binds to haemoglobin in the blood much more strongly than oxygen does, forming carboxyhaemoglobin, which reduces the oxygen carried around the body. This is why you should never run a car or generator in a closed space.

5. Exam tip Remember: "Incomplete = Carbon monoxide; Complete = Carbon dioxide." The word "mono" means one, so CO has just one oxygen, which shows less oxygen was available.