Earth science

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Video

Materials

• 4 identical glass jars with openings that are at least 4 cm (1 1/2 in) in diameter
• Food coloring in two different colors
• 2 playing cards (or other pieces of stiff and glossy paper, for example from a catalog cover)
• 1 teaspoon
• Water

Short explanation

Cold water is heavier than warm water and therefore sinks to the bottom. At the same time, hot water rises to the surface for the same reason. If the cold water is already at the bottom, no mixing of water will take place.

Long explanation

When a chemical substance (for example water) is cold, it means that the particles the chemical substance consists of (for example water molecules) move very little. On the other hand, when a chemical substance is hot, the particles move a lot and collide with each other all the time, which means that they push each other away and are on average further apart. In cold water, the water molecules are closer together than in hot water, and therefore cold water has a higher density than hot water.

The same thing happens in a gas or a gas mixture, such as air. In hot air, the oxygen and nitrogen molecules, and the other particles air consists of, are further apart than in cold air. Therefore, hot air is lighter than cold air. Cold air sinks while hot air rises.

This demonstration thus shows both what is happening in the oceans and in the atmosphere. In the oceans water sinks when it's carried by currents to colder latitudes and is cooled down. In the atmosphere, air rises when it's heated by Earth's surface (which is heated by the sun).

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if the water in both bottles has the same temperature?
• What happens if the water in the two bottles only differs slightly in temperature?
• What happens if you use ice water and boiling hot water in the bottles?
• What happens if you hold the bottles so that they lie on their sides instead?

Variation

You can also experiment with a small drinking glass and a large bowl. Fill the glass to the brim with warm water, color the water in any color, and cover the glass with plastic wrap. Secure the plastic wrap with a rubber band. Then fill the bowl with cold water, color it with some other color, and place the small glass on the bottom of the bowl. Then make holes in the plastic foil with a pencil and see how the water seeps out. Then do the same, but now with cold water in the glass and hot in the bowl, and see how the water stays in the glass.

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