Carbon is deposited in the exhaust pipes of cars because of

Analyze the question: Why does soot (carbon) collect in a car's exhaust pipe?

Concept: Petrol is a mixture of hydrocarbons (mainly octane, C₈H₁₈). When hydrocarbons burn, the products depend on how much oxygen is available:

• Complete combustion (plenty of oxygen): 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O Only carbon dioxide and water form, with no soot.

• Incomplete combustion (limited oxygen): C₈H₁₈ + O₂ → CO + H₂O (carbon monoxide) or → C (soot) + H₂O Carbon monoxide and free carbon (soot) form.

Solution:

• Inside the engine cylinder, the air supply is often not enough to burn all the fuel completely. Some carbon is therefore released as fine black soot and deposited in the exhaust pipe.
• A. Additives: these are meant to improve burning (e.g., anti-knock agents). They are not the main cause of carbon deposits.
• B. Incomplete combustion of petrol: this is correct, because insufficient oxygen leaves unburnt carbon.
• C. Presence of carbon in petrol: petrol contains hydrocarbons (compounds of carbon and hydrogen), not free carbon. This option is misleading.
• D. Contamination with diesel: this is not the general cause.

Answer: B. Incomplete combustion of petrol.

Exam tip: Whenever you see "soot," "black smoke," or "carbon monoxide," think incomplete combustion = limited oxygen. Remember: Lots of O₂ gives CO₂ + H₂O; little O₂ gives CO + C (soot).