In complete combustion, which two substances are primarily formed?

• A. Carbon dioxide and Water
• B. Carbon monoxide and Water
• C. Oxygen and Nitrogen
• D. Hydrogen and Oxygen

Answer: (A)

Complete combustion in the presence of plenty of oxygen oxidizes all the carbon in the fuel to carbon dioxide and all the hydrogen to water. This happens because oxygen from the air combines with the fuel’s carbon to form CO2 and with the hydrogen to form H2O, leaving those two substances as the main products. For example, burning methane: CH4 + 2 O2 → CO2 + 2 H2O shows CO2 and H2O as the products.

If there isn’t enough oxygen, incomplete combustion can occur, producing carbon monoxide or even carbon, which is why carbon monoxide would appear in other contexts. Oxygen and nitrogen are components of air rather than products of combustion, and hydrogen and oxygen would be reactants or separated gases rather than the typical products of complete burning.