The average size of the molecules decreased. The average size of the molecules increased. The average kinetic energy of the molecules decreased. The average kinetic energy of the molecules increased.
I gave my students this multiple choice question: Which statement best describes the molecules in a sample of water that was heated from a liquid to a gas?

Only one answer is correct among these 4 choices:
- The average size of the molecules decreased.
- The average size of the molecules increased.
- The average kinetic energy of the molecules decreased.
- The average kinetic energy of the molecules increased.

Could you tell me which one is correct?

The average kinetic energy of the molecules increased.