Which or the following products could be formed during incomplete combustion of a hydrocarbon. i.Carbon ii. Hydrogen III. Carbon (Il) oxide

Analyze the question: We need to identify which products can form when a hydrocarbon burns in a limited supply of oxygen (incomplete combustion).

Concept:

• Complete combustion (plenty of oxygen): hydrocarbon + O₂ → CO₂ + H₂O
• Incomplete combustion (limited oxygen): the carbon is not fully oxidised, so you get carbon monoxide (CO), and/or soot (carbon, C), along with water.

Equations:

• CH₄ + 3/2 O₂ → CO + 2H₂O (carbon(II) oxide formed)
• CH₄ + O₂ → C + 2H₂O (carbon/soot formed)

Check each statement:

• i. Carbon: Yes. It appears as soot (a black deposit), as seen on a smoky kerosene lamp or a candle flame with limited air.
• ii. Hydrogen: No. Hydrogen in the hydrocarbon is oxidised easily and ends up as water (H₂O), not as hydrogen gas.
• iii. Carbon(II) oxide: Yes. It is the poisonous gas CO, produced in incomplete combustion (for example, from generator exhaust or charcoal burning in a closed room).

So the correct statements are i and iii only.

**Answer: C. I and III only.

Exam tip: Remember that incomplete combustion gives C, CO, and H₂O, while complete combustion gives CO₂ and H₂O. Hydrogen gas is never a product of burning a hydrocarbon in oxygen. Also remember that CO is dangerous because it binds to haemoglobin, a point that often appears in follow-up questions.