Incomplete combustion of a hydrocarbon often produces which pollutant?

• A. Oxygen
• B. Carbon dioxide
• C. Water
• D. Carbon monoxide

Answer: (D)

The key idea is that how completely a hydrocarbon burns depends on how much oxygen is available. With plenty of oxygen, the fuel fully oxidizes to carbon dioxide and water. When oxygen is limited, oxidation stops partway, so carbon monoxide forms (and sometimes soot). Carbon monoxide is a pollutant because it is toxic and binds strongly to hemoglobin, blocking oxygen delivery in the body. Oxygen is a reactant, not a product, and while carbon dioxide and water can be produced, they come from complete combustion, not the incomplete case. So carbon monoxide is the pollutant produced in incomplete combustion.