The Amazing expanding bar of soap

Who would have thought that soap could be so much fun? Not only does Ivory soap float in water, but if you heat it in a microwave it will expand into a mass of soap about three times larger than the bar. This experiment also generates some strong soapy smells – so be prepared!

What you need:

  • Bar of Ivory™ Soap and another brand of soap
  • Paper towel
  • Microwave oven
  • Bowl of water


What to do:

  1. Drop the bars of soap into the bowl of water. Notice how one floats and one doesn’t.
  2. Remove the Ivory soap from the water. Break it in half and notice if there are any air pockets.
  3. Place the Ivory soap in the middle of a paper towel and place in a microwave on HIGH for 2 minutes.
  4. Observe what happens to the soap! Don’t over cook your soap soufflé! Allow your soap to cool for a minute before touching it.

What’s the science?
Ivory soap floats because it has air pumped into it while it is being made. It also contains water, both in the form of water vapor trapped in air bubbles and water in the soap matrix itself. The heating of the water that is inside the soap causes the expanding effect. As the water vaporizes, air bubbles are formed. The heat causes trapped air to expand and the soap to soften and become pliable.

This is an example of Charles’ Law which states that as the temperature of a gas increases so does its volume. When the soap is heated, the molecules of air move faster and move farther apart from each other. This causes the soap to puff up and expand.

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