Item RG009003: As the water at the bottom of a cup is heated, it becomes less dense than the cooler water above it. The difference in densities causes the warmer water to rise, carrying its thermal energy with it.

A student fills a cup with room temperature water. Then she places the cup over a flame to heat the water.

Which of the following statements correctly describes what happens as the water is being heated?

  1. As the water at the bottom of the cup is heated, its thermal energy increases. Thermal energy then rises to the top of the cup separate from the water molecules. This heats the water at the top of the cup.
  2. As the water at the bottom of the cup is heated, heat molecules are produced. Heat molecules then travel from the bottom of the cup to the top of the cup and carry thermal energy with them as they rise. This heats the water at the top of the cup.
  3. As the water at the bottom of the cup is heated, it becomes less dense than the cooler water above it. The difference in densities causes the warmer water to rise. As the warmer water rises, it carries with it its thermal energy. This heats the water at the top of the cup.
  4. While the water is over the flame, the temperature of all of the water increases at a constant rate, which increases the thermal energy of all of the water at the same time.