Molecules in a vapor state move faster than molecules in a liquid state.

• A. Sometimes
• B. Not enough information
• C. True
• D. False

Answer: (C)

When a substance is in the vapor (gas) state, its molecules are far apart and experience only weak intermolecular forces, so they can move freely and reach higher speeds at a given temperature. In a liquid, molecules are closely packed and continually tugged on by neighboring molecules, which slows their motion through frequent collisions and attractions. Because of this contrast, for the same temperature the vapor-phase molecules tend to move faster than liquid-phase molecules. (Also, lighter molecules move faster than heavier ones at the same temperature, which reinforces this general trend for a given substance.)