Advanced Placement (AP) Biology Practice Exam

What makes hydrocarbons stable?

1. Strong ionic bonds between carbon atoms

2. Weak intermolecular forces between molecules

3. High melting points

4. Highly soluble in water

The correct answer is: Weak intermolecular forces between molecules

Hydrocarbons are considered stable primarily due to the weak intermolecular forces present between their molecules. These forces include London dispersion forces, which arise from temporary dipoles occurring due to the movement of electrons around the nucleus. While these interactions are relatively weak, they allow hydrocarbons to remain in a stable form without easily breaking apart under normal conditions. This stability is particularly significant when considering that hydrocarbons are predominantly nonpolar molecules, which do not interact strongly with polar solvents like water. This lack of strong attractions contributes to their stability. Additionally, the covalent bonding within the hydrocarbon molecules themselves, particularly the carbon-carbon and carbon-hydrogen bonds, is strong, contributing to the overall stability of the molecule. However, the focus here is on the interactions between independent molecules. The other choices address characteristics that do not fundamentally explain the stability of hydrocarbons. Strong ionic bonds are not present in hydrocarbons, as they primarily contain covalent bonds. High melting points can be a characteristic of some hydrocarbons but are not a defining factor of stability in the context of molecular interactions. Finally, high solubility in water is not relevant for hydrocarbons, as they are typically not soluble in polar solvents.