Have you ever heard that hot air rises? That's true! As air heats up, its molecules expand and spread out, making the air less dense than it was before. It floats up through the denser cooler air. As the warm air rises it starts to cool off and its molecules move closer together, causing it to sink again. This circulation is called convection, and the rising and falling of the air are called currents. Convection currents are part of what causes different kinds of weather. (You'll find out how in the next experiment.)
We can't see convection in the air; do you think water might act the same way? Do this experiment to find out!
You should have an adult help you with the hot water and the knife.
Materials
Large glass jar or beaker
Small cup or beaker (it needs to fit inside the jar)
Food coloring
Knife
Plastic wrap
Rubber band
Water
Procedure:
Fill the small cup or beaker with very hot (almost boiling) water and add several drops of food coloring. Stretch the plastic wrap smoothly over the cup and seal it with the rubber band. (The plastic wrap will puff up--this is because the hot air above the water is expanding!)
Fill the jar almost full with cold water from the tap.
Use a pair of tongs to set the cup of hot water in the bottom of the jar.
Slice open the plastic wrap with the knife and watch what happens! (One long gash should do it.)
What happened? The hot water was less dense than the cold water surrounding it, so it rose to the top in a convection current. What happens as the colored water gets to the top? Does it stay there? Why or why not?
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