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

Which of the following best describes an unsaturated molecule?

1. A molecule containing only single bonds

2. A molecule containing carbon-carbon double bonds

3. A molecule that is fully bonded

4. A molecule with no functional groups

The correct answer is: A molecule containing carbon-carbon double bonds

An unsaturated molecule is characterized by the presence of one or more carbon-carbon double or triple bonds. These double or triple bonds create points of unsaturation in the molecule, allowing it to potentially react with additional atoms or molecules. This contrast against saturated molecules, which contain only single bonds and are fully "saturated" with hydrogen atoms, highlights the key feature of unsaturated compounds: their ability to undergo reactions that can add more atoms across the double or triple bonds. Choosing B acknowledges the presence of a carbon-carbon double bond, which clearly defines the unsaturated nature of the molecule. In contrast, options that describe a molecule with only single bonds, being fully bonded, or lacking functional groups do not capture the essence of unsaturation. Each of these options relates to saturated molecules or those lacking reactive sites, which are not representative of unsaturation in organic chemistry.