Is the Glass of Water Full?

This experiment illustrates the large surface tension of water. Water molecules tend to attract each other. In the bulk of the solution, water molecules pull on each other equally in all directions. But at the surface the water molecules are pulled into the water because there are no water molecules above the surface to pull in the opposite direction.

 

Materials

Plastic cup

80-100 paper clips

Water

Liquid detergent

 

What To Do

Fill the plastic cup of water "completely" full of water. Have students predict what will happen if a paper clip is carefully added to the cup. Try it. Now have students predict how many paper clips may be added before the cup overflows. Ask the students to keep track and add as many paper clips as possible. Add soap to the cup of water and try the experiment again. 

 

Questions

1. How many paperclips can be added before the water spills? What is keeping the water from spilling?

2. Look at the interface of a solution of water and air. Put a piece of black paper behind the container you are studying to see the interface more clearly. Describe what you see at the top layer of the water solution. This indentation of the liquid is called the meniscus of the solution.

 

Summary

The surface tension causes the surface of the liquid to act like a thin film on top of the liquid. This thin film keeps the water from flowing over the edge of the cup. Liquid detergent lowers the attraction between the water molecules and therefore, the water overflows quickly with the addition of paper clips. EXTENSION Water striders use the surface tension of water to "walk" across the liquid surface of ponds and rivers. Check out how these insects live. What would happen to them if the water in the stream became polluted with detergents?

 

Sources

"Science Is . . . A Source Book of Fascinating Facts, Projects and Activities." Susan V. Bosak, Scholastic, 1991, ISBN 0-590-74070-9.

"Teaching Chemistry with Toys: Activities for Grades K-9." Jerry L. Sarquis, Mickey Sarquis, John P. Williams, McGraw-Hill, ISBN 0-07-064722-4.

"How Science Works." J. Hahn, Dorling Kindersley, London, 1991, ISBN 0-7621-0249-7.

"The Science Explorer: Exploratorium." Pat Murphy, Ellen Lages, Linda Shore, and The Exploratorium, Henry Holt and Company, 1996, ISBN 0-8050-4536-8.

© S. Olesik, WOW Project, Ohio State University, 2000.