Sticking Together

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.

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.

Bulging Water

Materials:

• Plastic or glass cup
• Water
• 25–50 pennies

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:
10 minutes

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).

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.

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.

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.

Kathleen Damonte teaches seventh-grade science at Julius West Middle School in Rockville, Maryland.

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