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Polluted Runoff: Nonpoint Source Pollution

Darby Duck, the Aquatic Crusader - What is Surface Tension?

A drop of water is small, but it is made of even smaller parts called molecules. Water molecules have bonds that hold them together. At the surface of the water, the molecules hold on to each other even more tightly because there are no molecules pulling on them from the air above. As the molecules on the surface stick together, they form an invisible "skin" called surface tension.

Water striders and other insects can walk on water without sinking. The surface tension is strong enough to hold them. The insects' feet make dents in the surface tension, but it doesn't break. Here are some experiments to show that surface tension is there, even if you can't see it.


  • cooking oil
  • water


  1. Rub a few drops of cooking oil on your hand.
  2. Let water from a faucet (tap) run over your hand then turn off the faucet. What happens?
  3. Wash your hands with soap. Does this make the oil go away?

How it Works

The water molecules stick together tightly and will not mix with the oil on your hand. Since water molecules are attracted to each other so strongly, they formed small balls or drops which rolled over your oil coated hand. There is an invisible "skin" of surface tension around each drop.

Soap molecules are attracted to both water and oil. One end of the soap molecule sticks to oil, the other end sticks to water. The soap breaks up the surface tension and keeps the oil drops mixed in with the water so that the oil can wash off your hand.