Science activities for families to share
You might have noticed the insect in this drawing skittering across the surface of a creek or a pond.
It’s called a water strider because of its ability to move across the surface of water. A property of
water called surface tension makes this possible. Read on to learn more about surface tension and
then experiment with this phenomenon yourself.
Sticking Together
Teachers: Cut out this page and photocopy it for your students.
Everything, including water, is made of tiny particles
called molecules. The molecules in liquids and
solids are attracted to each other. In water, the
molecules are strongly attracted and pull towards each other. This attraction between
molecules is called cohesion.
You can’t see water molecules because
they are too small, but you can see the effect
of them being attracted toward each other. You
can see this effect when rain falls on an object,
like a car or a leaf, and the water pulls together
into beads or droplets. The molecules pulling
together form water drops.
Surface Tension
Water molecules that are surrounded by other
water molecules are attracted to the molecules
in every direction. Water molecules that are found at the top surface of some water, such as the top of
a cup or the surface of a pond, have only a small number of water molecules in the air above them.
These surface water molecules are attracted downward by the water molecules below them. The
strong attraction below causes the water to have something like a strong invisible “skin” at the top
surface. The scientific name given to this “skin” is surface tension.
Surface tension is enough to support some objects. It is what supports the water strider as it moves
across the surface of the water. Surface tension can be helpful to people as well. It helps keep water
from a water fountain in a neat stream rather than spraying all over the place.
Surface Tension Can Be Broken
The attraction of water molecules to each other can be disturbed by adding soap. Soap interferes with
the attraction between water molecules and breaks the surface tension. Soap has a lower surface
tension than water.
Now try some activities that demonstrate
surface tension.
(see reverse)
Kathleen Damonte teaches seventh-grade science at
Julius West Middle School in Rockville, Maryland.
November/December 2003 47
Bulging Water
Materials:
• Plastic or glass cup
• Water
–50 pennies
• 25–
Time needed:
10 minutes
Directions:
1. Fill the cup of water as close to the top as you can.
2. Make a prediction about how many pennies you think you will be
able to add to the cup before water spills over the edge.
3. Add pennies, one at a time, into the cup. Slip them in carefully
without splashing. Count the pennies as you add them. Continue
adding pennies as the water bulges over the top of the cup.
4. Stop adding pennies when the water spills over the edge. The water spills over when the
surface tension breaks.
Questions:
• How many pennies could you add to the cup?
• How close was your prediction to your actual results?
• What did the water look like just before it spilled over the cup? Describe what you saw.
Soap and Surface Tension
Materials:
• 3 toothpicks
• Liquid dishwashing
soap
• Water
• Shallow bowl
Time needed:
Directions:
1. Fill the shallow bowl with water and let it sit until the water is
completely still.
2. Carefully place two toothpicks parallel to each other about 1 cm
apart on the surface of the water (see picture below).
3. Dip the end of the third toothpick into the soap.
4. Touch the end of the toothpick with the soap into the space between
the two toothpicks and observe what happens.
toothpicks
5. If you want to try this activity again, wash out the bowl and start with new water.
Once surface tension has been broken by the soap, it won’t come back.
10 minutes
Questions:
• How did you know the surface tension of the water was broken?
• Try this activity again using a sprinkle of pepper on top of the water
instead of the toothpicks. What do you think will happen to the pepper?
Try it and find out.
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Science and Children