Doing an experiment that demonstrates molecule space may make the concept of water density more clear. Fill a jar as full as possible with marbles. Then, fill another jar about halfway with marbles. Invite students to shake the jars and ask them in which jar the marbles move more freely. The packed jar is like cold water, in which the molecules are closer together and cannot move as easily; the half-full jar is like hot water, in which the molecules are spread out and move quicker because of the open space.
This experiment will show how less-dense hot water is able to permeate and mix easily with cold water. Fill two bottles with water, one hot and one cold. Place red food coloring into the hot water and blue food coloring into the cold water. Then, take a notecard and lay it on top of the opening of the cold bottle. Press it firmly and quickly tip the bottle upside down on top of the hot bottle. Both openings should be on top of each other with the note card in the middle. Slowly slide the notecard out from the middle. It is OK if some water spills during the process. Watch as the less dense, faster-moving hot water begins to move into the more dense, slower moving cold water. Hot water also naturally rises, and the molecules will quickly begin to fill in the empty spaces of the cold water.
Fill two cups or beakers with water, one really hot and the other really cold. Place some red food coloring into the hot water and some blue food coloring into the cold. Then, transfer the cold water into a graduated cylinder or tube, followed slowly by the hot water after that. This may take time to get just right, but the hot water should stay layered on top of the cold water. Because hot water is less dense than cold water, it sits on top of the cold water. Hot water also naturally rises; since it is the top layer, there are no other layers for it to rise through.
Our oceans are made up of salt water. Understanding the densities of both saltwater and freshwater will help understand how they react with one another. Repeat the previous experiment, but add salt to the cold water. Continue experimenting by mixing different amounts of salt and varying the temperatures of the water. Notice how the results are similar, but not exactly the same. Salt and temperature affect the density of water. Even though they might produce similar results in each experiment, the density of water always changes depending upon the amount of salt and water temperature.