The temperature of the water remains constant. The pressure acting on the water remains constant. The number of atoms in the water remains constant. The distance between water molecules remains constant.
I gave my students this multiple choice question: When 1 gram of water is evaporated, the volume of the water vapor increases but the mass remains constant. Why does the mass of the water remain constant?

Only one answer is correct among these 4 choices:
- The temperature of the water remains constant.
- The pressure acting on the water remains constant.
- The number of atoms in the water remains constant.
- The distance between water molecules remains constant.

Could you tell me which one is correct?

The number of atoms in the water remains constant.