Advanced Placement (AP) Chemistry Practice Exam

Substances that exhibit only London dispersion forces typically exist in which physical state at room temperature?

1. Solid

2. Liquid

3. Gas

4. Plasma

The correct answer is: Gas

Substances that exhibit only London dispersion forces typically exist in the gaseous state at room temperature due to the weak nature of these forces. London dispersion forces, also known as van der Waals forces, arise from temporary fluctuations in electron distribution within atoms or molecules, resulting in transient dipoles that induce attractions between neighboring particles. In substances that rely solely on London dispersion forces, these forces are not strong enough to hold the particles together in a solid or liquid state at room temperature. As a result, they tend to have low boiling and melting points, which allows them to exist as gases under normal conditions. For example, noble gases like helium, neon, and argon are held together only by London dispersion forces and are found in the gaseous state at room temperature. In contrast, substances held together by stronger intermolecular forces, such as hydrogen bonds or dipole-dipole interactions, are more likely to be liquids or solids at room temperature. This fundamental understanding of intermolecular forces helps explain the physical states of different substances under varying conditions.