A sample of water is boiling as heat is added at a constant rate. Which statement describes the potential energy and the average kinetic energy of the water molecules in this sample?
(1) The potential energy decreases and the average kinetic energy remains the same.
(2) The potential energy decreases and the average kinetic energy increases.
(3) The potential energy increases and the average kinetic energy remains the same.
(4) The potential energy increases and the average kinetic energy increases.
Answer and Explanation:
During boiling, molecules must gain enough energy to separate from one another (overcome intermolecular forces holding them together) and enter the gas phase.
During phase changes, the temperature of the substance does not change.
Since temperature is the measure of the average kinetic energy, kinetic energy of boiling water remains the same, leaving us with choices 1, and 3. Since the molecules gain enough energy to separate from one another, their potential energy (energy due to position) increases. Therefore, choice (3)The potential energy increases and the average kinetic energy remains the same is the correct answer choice.