During which of the following phase changes does the average distance between molecules always increase?

• A. Gas to liquid (condensation)
• B. Liquid to gas (vaporization)
• C. Solid to liquid (melting)
• D. Liquid to solid (freezing)

Answer: (B)

Verified step by step guidance

1. Understand that the average distance between molecules depends on the phase of the substance: solids have molecules closely packed, liquids have molecules less tightly packed, and gases have molecules far apart.
2. Analyze each phase change to determine if the molecules move closer together or farther apart:
3. For gas to liquid (condensation), molecules move from a dispersed gas phase to a more compact liquid phase, so the average distance decreases.
4. For liquid to gas (vaporization), molecules move from a compact liquid phase to a dispersed gas phase, so the average distance increases.
5. For solid to liquid (melting), molecules move from a rigid, closely packed solid to a less ordered liquid, so the average distance increases but not as much as vaporization; for liquid to solid (freezing), the distance decreases as molecules become more tightly packed.