Sink or Swim

Here's an experiment, borrowed from the United States Environmental Protection Agency's website, that helps demonstrate how detergents and other soaps affect our streams.

Sometimes detergents and other soaps get into our streams and lakes. This happens when people wash cars in their driveways and allow the dirty wash water to go down a storm drain. The dirty water travels through the storm pipes, untreated, and ends up in the nearest stream or lake. Once there, the detergents and soaps can do some real damage to the water's surface habitat.

Car wash bucket soap hose BS

What you'll need

  • A clean bowl (free of detergent)
  • Paper clip or sewing needle
  • Fork or tweezers
  • Liquid dish detergent

What to do

  1. Fill the bowl with water.
  2. Put a paper clip or needle on the tines of the fork, or hold it with tweezers. Gently place the paperclip or needle on the surface of the water. Be patient and careful. You will be able to get the clip or needle to sit on top of the water! Can you see the surface tension bend under the paper clip or needle?
  3. Add one or two drops of liquid dish detergent to the water near (not on top of) the paperclip or needle. What happens?

What just happened?

The paperclip or needle was resting on top of the surface tension. This "skin" supported the clip or needle and kept it from sinking. When you added the detergent, the soap weakened the attraction the water molecules had for each other. This caused the surface film to disappear. Then the paperclip or needle sank.