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Molecule Madness: A Water Experiment

Fifth Grade Physical Science Activities: Molecule Madness: A Water Experiment

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Water is a great molecule that is truly under-appreciated by teens (as well as the rest of us!). Just about every chemistry student knows the chemical formula H2O and can draw a rough diagram of a water molecule, but does your teen really understand how unique water is? Put your budding chemist’s water knowledge to the test with this fun activity!

What You Need:

  • Water
  • Dish soap
  • Small dish
  • Paper clips

What You Do:

  1. Fill a clean, soap-free bowl with water.
  2. See how many paper clips you can float on top of the water. Floating paper clips takes a little practice, but once you get it, start counting!
  3. Once several clips are floating, add a couple drops of dish soap to the water and watch what happens.
  4. Try floating a paper clip now that the water has soap in it. What happens?

What's Going On?

Water is a polar molecule. When the two positively-charged hydrogen atoms attach to the negatively-charged oxygen atom, a stable water molecule is created. However, the atoms attach in the “bent” shape most chemistry students can draw. This does not allow for an equal “pulling” or sharing of the electrons between the three atoms, allowing the hydrogen atoms to develop a slight positive change and the oxygen atom to develop a slight negative charge.

The difference in charge across the molecule is called a “dipole”. Think of it like a magnet, which has a north and south pole. Most of us understand the concept that “opposites attract,” allowing the north pole of a magnet to attract the south pole of another magnet. The same thing occurs between polar water molecules. The slightly positive hydrogen side of one water molecule attracts the slightly negative oxygen side of another water molecule, creating a natural “attraction” between the molecules.

So what does this have to do with floating paper clips on water? Water molecules on the surface of a bowl of water are attracted to each other, which creates surface tension and allows the paper clips to float. When you add soap to water, however, the surface tension breaks. Soap molecules consist of nonpolar, dipole-free bodies and a polar head. When soap mixes with water, the polar heads attach to the surface of the water and the nonpolar bodies tend to point upwards, away from the water, reducing the water’s surface tension. Thus, the paper clips sink and it becomes difficult to float new clips!

Lori Stewart is a freelancer specializing in the development of science education materials. As a high school science teacher, Lori had several students place first and second in NASA's Student Involvement Program national competition.

Updated on May 23, 2013
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