Teaching Strategies

"Sinkholes"
Students observe what happens when ice-cold water mingles with warm water

by Janice VanCleave

Just as the air in the atmosphere shifts and changes, warms and cools, so does the water that fills oceans, lakes, rivers, and streams. In spring, frozen lakes begin to thaw. Do students think that water moves differently depending on its temperature? Tell them that water molecules in water contract, or bunch up, when it turns cold. When these molecules become cold enough, they are packed so closely together that the water turns into ice. When the ice warms and melts, the molecules expand, or move farther apart. Water's behavior also alters with smaller shifts in temperature. The movement of water due to temperature differences is called convection currents. These currents occur in oceans and lakes but can also be observed in small amounts of liquid. Have your intermediate students try the following experiment to determine how temperature affects the movement of water. Here's how they can do it.

Materials:

Pencil
5-ounce (150 ml) paper cup
1 quart (liter) small-mouthed jar
Masking tape
Warm tap water
2 ice cubes
Eyedropper
Food coloring

Procedure:

1. Direct youngsters to use a pencil point to punch four small and evenly-spaced holes around the bottom of the paper cup.
2. Set the paper cup right side up in the jar so that the top rim rests on the mouth of the jar.
3. Place a small piece of tape on the outside of the jar to mark a spot about 1/2 inch (1.25 cm) above the holes in the cup.
4. Lift the cup out of the jar and fill the jar to the top of the tape with warm tap water.
5. Set the paper cup back in the jar and place the two ice cubes in the cup. Wait one minute.
6. Add three drops of food coloring to the water in the paper cup. Avoid the ice cubes.

Results:

Colored water leaves the cup, curling as it flows downward.

Why?

When the warm water enters the cup, it is cooled by the ice cubes. The cold water in the cup contracts, and the warm water in the jar expands. The contraction of the cold water makes it denser than the warm water. For this reason, the cold water escaping through the holes sinks through the warm water in the rest of the jar.


Excerpted with permission of the publisher John Wiley & Sons, Inc., from Janice VanCleave's 201 Awesome, Magical, Bizarre, & Incredible Experiments. Copyright 1999 by Janice VanCleave.

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