Sink or Swim (Surface Tension)

What happens when you weaken the hydrogen bonds in water?

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There are times in science where changing one variable can be the difference between fire and smoke, conducting and insulating, and in this case, sinking and swimming. In the Sink or Swim experiment, you’ll learn how adding one variable to water will change the amount of surface tension the water has. You’ll see that anything you add to water may quickly affect whether something sits on water or not.


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Here's What You'll Need

  • Two glasses
  • Spoon
  • Water
  • Dish soap
  • Paper

Let's Try It

  1. Fill two glasses equally full with water.

  2. In one of the cups, add about 1 oz (30 mL) of dish soap and gently stir the solution.

  3. Create two identical balls of paper that can fit into your glasses of water.

  4. Gently drop one paper ball into the plain water and drop the other paper ball into the glass with the soapy water solution.

  5. You will quickly observe that the paper balls react differently to the two fluids. In fact, one paper ball begins to sink while the other sits atop the water!

How Does It Work

It may appear that one paper ball is floating on the water while the other ball sinks, but it isn’t entirely about density. Instead, what you are observing is a difference in the surface tension of the water. Soap is a surfactant, or a compound that lowers the surface tension of a liquid. Soap, in particular, decreases the surface tension of water by weakening the hydrogen bonds that make water such a special substance.

This lower surface tension has two direct effects when it comes to the paper ball. First, the lower surface tension means that the paper can’t sit atop the water’s surface, allowing more of the water to come in contact with more of the paper. Second, the weakened hydrogen bonds mean that the water is more likely to soak into the porous paper, making the paper much more dense and causing it to sink.

Science Fair Connection

Weakening hydrogen bonds is fun, but it isn’t a science fair project yet.  You can create a a science fair project by identifying a variable, or something that changes, in this experiment.  Let’s take a look at some of the variable options that might work:

  • Try testing different amounts of soap. You can time the amount of time it takes for the paper ball to be completely submerged.
  • Try to find the best surfactant possible. Try different brands of soap, or even other solids and liquids to see which acts as the best surfactant.
  • Try changing the shape or size of the paper.  Time how long it takes different sizes or shapes to be submerged.

Those are just a couple of ideas, but you aren’t limited to those! Try coming up with different ideas of variables and give them a try.  Remember, you can only change one thing at a time.  If you are testing different surfactants, make sure that the other factors are remaining the same!

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