In the lab, Ann cools a closed container containing 50 grams of steam until all of the steam becomes a liquid. She then cools the container until the liquid water freezes. Which remains constant during all of these changes?

The mass

Explanation:

The only thing that will remain constant during the change of state is the mass of the water molecules.

Steam consists of water molecules in a gaseous state. The mass remains constant as it is cooled from steam to liquid water in a closed container and then frozen to become ice, still within a closed system.

The cooling system is just exchanging energy with the surrounding without exchanging any matter. Hence, the mass of the water molecules remains the same throughout the change of state.