Candle wax is a mixture of hydrocarbons. When a candle burns completely in a good supply of air, complete combustion occurs. Explain what is meant by the term hydrocarbon, state the products formed during complete combustion of a hydrocarbon, and explain why incomplete combustion is considered more dangerous than complete combustion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Candle wax is a solid mixture of hydrocarbons derived from crude oil. When a candle is lit in a well-ventilated room, the wax undergoes complete combustion. However, in a poorly ventilated room, incomplete combustion can occur.
Model answer (5 marks)
A hydrocarbon is a compound made only of carbon and hydrogen atoms.
Complete combustion of a hydrocarbon gives the products CO₂ and H₂O.
In incomplete combustion, CO is produced. CO is dangerous because it binds to haemoglobin, reducing the blood’s ability to carry oxygen and can cause death by poisoning.
Complete combustion of a hydrocarbon gives the products CO₂ and H₂O.
In incomplete combustion, CO is produced. CO is dangerous because it binds to haemoglobin, reducing the blood’s ability to carry oxygen and can cause death by poisoning.
Examiner tips
- Use the definition of hydrocarbon exactly; mention only C and H. State both CO₂ and H₂O as products of complete combustion. Explain that CO is toxic and why it is hazardous – mention haemoglobin binding or oxygen transport.
- common_mistakes
- :
- Writing ‘oxygen’ instead of ‘CO₂’ as a product of complete combustion. Forgetting to mention that CO is produced in incomplete combustion. Using vague terms like ‘dangerous’ without explaining the haemoglobin effect.
Mark scheme (5 marks)
- A hydrocarbon contains hydrogen and carbon only
- Complete combustion produces carbon dioxide
- Complete combustion produces water
- Incomplete combustion produces carbon monoxide, which is toxic/poisonous
- Carbon monoxide is dangerous because it reduces the blood's ability to carry oxygen (or: it binds to haemoglobin / it can cause death by poisoning)
Key terms in this question
hydrocarbon · complete combustion · incomplete combustion
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →