International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following is a product of both complete and incomplete combustion of alkanes?

1. Water

2. Carbon Monoxide

3. Hydrogen

4. Carbon Dioxide

The correct answer is: Water

In the combustion of alkanes, both complete and incomplete combustion occurs, and one of the consistent products in both processes is water. During complete combustion, alkanes react fully with oxygen to produce carbon dioxide and water. However, in incomplete combustion, where there isn't enough oxygen, alkanes yield carbon monoxide, carbon, and water. The formation of water is common in both scenarios because it is a byproduct of the hydrogen present in the alkane reacting with oxygen; this reaction doesn't rely on the completeness of the combustion process. While carbon dioxide is produced during complete combustion, it does not appear in significant quantities during incomplete combustion. Similarly, carbon monoxide is a byproduct of incomplete combustion but is not formed during complete combustion. Hydrogen is not a combustion product but rather a reactant since alkanes are hydrocarbons composed of hydrogen and carbon. Thus, water serves as a product consistently arising from both types of combustion.