Give an example of a saturated hydrocarbon and an unsaturated hydrocarbon.

• A. Saturated: methane; Unsaturated: ethane.
• B. Saturated: ethane; Unsaturated: ethene.
• C. Saturated: ethene; Unsaturated: ethane.
• D. Saturated: ethane; Unsaturated: ethyne.

Answer: (B)

Saturated hydrocarbons have only single bonds between carbon atoms, so they hold as many hydrogen atoms as possible. Unsaturated hydrocarbons contain one or more carbon–carbon multiple bonds (double or triple), which lowers the number of hydrogens the molecule can have.

Ethane is a simple saturated hydrocarbon with only single bonds, C–C and C–H. Ethene has a carbon–carbon double bond, C=C, which makes it unsaturated and gives it fewer hydrogens, C2H4. This pair clearly contrasts a fully saturated structure with a molecule containing a double bond, so it’s a clear, correct example.

Other pairings either use two saturated hydrocarbons (no unsaturation shown), label an unsaturated molecule as saturated, or mix in a triple bond (also unsaturated) without illustrating the contrast as plainly.