Aug. 27, 2003

Clingers

Why can water flow at an angle?

Materials:

• scissors
• ruler
• cotton kite string
• 1 cup (250 milliliters) measuring cup with handle
• water
• drinking glass
• cookie sheet

Procedure:

1. Cut an 18-inch (45 centimeter) length of string.
2. Tie one end of the string around the top of the measuring cup's handle.
3. Fill the cup with water.
4. Wet the entire length of the string with water.
5. Set the glass in the center of the cookie sheet.
6. Lay the string over the spout of the measuring cup.
7. Hold the free end of the string against the inside of the glass.
8. Separate the cup and glass so the string is tight.
9. Raise the bottom of the cup about 12 inches (30 cm) from the cookie sheet.
10. Slowly pour the water out of the cup.

Results:

The water flows down the string into the glass.

Why?

Water molecules attract each other. This force of attraction between like molecules is called cohesion. Under the surface of the liquid, each molecule is attracted by other molecules that pull on each other in all directions. At the surface of the liquid, the molecules are only pulled to the sides and downward. This unbalanced force tends to draw the water molecules at the surface closer together, forming a tough, elastic, skinlike film across the water. This tendency of water molecules to cling together at the surface is called surface tension. The water in the wet string attracts the molecules in the falling water. The surface tension on the outside of the flowing water holds the water close to the string as it flows down the slanted string.

Let's Explore:

1. Would the height of the measuring cup affect the results? Repeat the experiment changing the height of the cup. Keep a record of the various heights and their results by standing a yardstick (meterstick) behind the cup. Science Fair Hint: Have a helper take photographs as you test the results at different heights. Use these photographs with a written report of the results as part of the project display.
2. Would the length of the string affect the results? Repeat the experiment, changing the string's length. Test with shorter and longer lengths of string. Observe and record any difference in the flow of water.
3. Would the results be the same if a different type of string was used? Repeat the experiment using strings made from different materials.
4. How important is it to wet the string? Repeat the original experiment with dry string.

From Janice VanCleave's Molecules: Spectacular Science Projects (ISBN 0471-55054-X) by Janice VanCleave. Published by John Wiley & Sons. Copyright © 1993 by John Wiley & Sons.

This material is used by permission of John Wiley & Sons, Inc.

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