Phase transition expands molecular distances

Steps:

• At boiling point, temperature remains constant, so average kinetic energy stays the same.
• Added heat overcomes intermolecular forces, allowing molecules to move farther apart.
• This separation increases spacing between molecules without altering their size or count.
• Result: liquid converts to gas with greater molecular volume.

Why C is correct:

• In gases, molecules are much farther apart than in liquids, as defined by the ideal gas law (PV = nRT), where volume expands at constant T and P.

Why the others are wrong:

• A: Average kinetic energy depends on temperature, which is constant at boiling point.
• B: Molecular size remains unchanged during phase change; it's an intrinsic property.
• D: Total number of molecules is conserved in a closed system during evaporation.

Final answer: C