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Surface Tension: Why Some Insects Can Walk on Water

By — John Wiley & Sons, Inc.
Updated on Dec 10, 2010

Water tends to form beads or drops. This ability of water molecules to stick together, a property known as surface tension, is due to the mutual attraction of water molecules. One side of each water molecule has a slight positive charge; the other side has a slight negative charge. The attraction of two molecules is maintained by a hydrogen bond. The high surface tension of water forms a kind of ''skin'' on the top of water. Lightweight insects such as water striders can scoot across the water's surface without sinking. In the following activity, you will examine the property of surface tension.

Materials

Penny; Medicine dropper; Cup of water; Paper towel

Activity

  1. Place the penny on a paper towel so the head side of the penny faces up.
  2. Using the medicine dropper, slowly add small drops of water to the penny. Count the number of drops as you add them.
  3. Notice what happens to the water as more and more drops pile up on top of the penny.
  4. Continue this process until water finally spills over the side of the penny.

Follow-Up Questions

  1. How many drops of water were you able to place on the penny before it ran over the side?
  2. Describe the appearance of the water on top of the penny just before it spilled over the side.
  3. What finally caused the water to break through the ''skin''?

Answers

  1. Answers will vary, but the number of drops will most likely be greater than students expect.
  2. Water forms a dome on top of the penny.
  3. The weight of the water overcame the pull of attraction holding the water molecules together.

Extension

Stir a small amount of hand soap into the cup of water. Dry the penny and repeat this activity using the soapy water. Count the number of drops the penny can hold. Write a statement about how soap affects the surface tension of water. How would it affect the water strider's ability to walk on water?

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