Young Naturalist Lesson:

 Students explore the behavior of molecules in warm and cool water by watching how colored water at one temperature disperses through water of another temperature.

Curriculum Focus:

• Science

Materials:

• paper towels
• tall, clear container (large beaker, glass pitcher or 2-liter bottle with top cut off)
• food coloring
• hot and cold water
• snack-size recloseable bags
• scissors
• small, heavy object such as a golf ball, magnet or stone
   

Activity:

   Review the Young Naturalist story. Check for student understanding about molecules and how they behave differently as water freezes into ice. If you performed the experiments included in the story, discuss results. Explain to students that liquid water molecules also behave differently when they are heated or cooled. When water is heated, its molecules speed up. They bounce off each other and spread out, or expand. As a result, hot water is less dense than cold water. Ask students to predict what would happen if hot water were added to cold water-which water would form the top layer (hot water, which is less dense) and which would form the bottom layer (cold water, which is more dense)? Make sure students understand you are discussing water in its liquid, not solid, state to avoid confusion. Tell students they can see this result for themselves by doing a simple experiment with colored water. You may wish to divide students into groups. The directions are as follows: Cover the work area with paper towels.

Fill a clear container halfway with very cold water. Mix a few drops of food coloring with very warm water and pour it into a recloseable snack-size bag. Add a small, heavy object to the bag to give it weight. (If you need to make the bag smaller to fit in the container, fold it in half and wrap a rubberband around it.) Drop the bag into the container of cold water. It should sink to the bottom. While you are at eye level with the bag, carefully snip it open with scissors. What happens to the colored water? Where does it go? Press on the bag to release colored water if needed. Count to 20. What is happening to the colored water? Repeat this experiment, but fill the container with hot water (not too hot--you will be sticking your hand in it!) and the snack bag with cold water. Where does the colored water go when you cut open the bag?

  Discuss results of the experiment as a class. Students should observe that the warm colored water rises rapidly to the surface of the container of cold water. This is due to the fact that the warm water is less dense and therefore floats. When the bag is filled with cold colored water, it barely rises out of the bag before sinking through the warm water. This is because the cold water is more dense than the warm water. Ask students what the container would look like after five minutes in either scenario (the colored water and clear water approach equal temperature, and as a result the color should spread evenly throughout the container.)
 

Extension idea:
Review the concept that hot water molecules have more space between them than cold water molecules. Explain that materials dissolve in water as their own molecules fill the spaces between water molecules (which makes it seem as though they are disappearing, but they are held in solution). Observe how warm water is "better" at dissolving a material such as hot chocolate mix than cold water is.