Kinetic Energy Equivalence at Boiling Point

Steps:

• Recall that boiling point is 100°C where liquid water turns to vapor at equilibrium.
• Understand kinetic energy relates to molecular motion and temperature.
• At the same temperature, all molecules in a sample have the same average kinetic energy regardless of phase.
• Compare statements to this principle to identify the correct one.

Why A is correct:

• At boiling point, both liquid water and vapor are at 100°C, so their molecules have the same average kinetic energy per the kinetic theory of gases.

Why the others are wrong:

• B: Evaporation happens at any temperature below boiling point via surface molecules gaining enough energy.
• C: Freezing doesn't change molecular mass; water molecules remain H₂O with the same weight.
• D: Molecules retain vibrational kinetic energy in ice; they don't lose all motion.

Final answer: A