Expanding Ivory Soap

Did you ever make a bar of Ivory soap foam up in a microwave?

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It’s easy.


  • 2 bars of soap, Ivory and something else. I used Dial.
  • A microwave-safe plate. I used a pie plate because it has high edges.
  • A microwave


  • Set the other bar of soap in pie plate and put in the microwave.
  • Set microwave on high for 2 minutes.
  • After two more minutes to let it cool down, remove from the microwave.
  • Repeat with the Ivory soap.

What Should Happen?

The Ivory soap bar will foam up to about six times its normal size.

Not the kind of light, watery foam that comes from bubble bath.

But, a brittle, foamed version of the soap itself.

The other soap will barely foam up at all.

The fresher, or newer, the Ivory soap bar, the more it will foam.

I had a bar of soap that was about ten years old.

It still foamed up to about three times its size.

Why Is This Happening?

Air is whipped into Ivory soap during the manufacturing process.

The heat softens the soap.

It also heats the air inside the soap, which expands and turns the softened soap into a brittle foam that looks like marshmallow crème.

The reason old soap doesn’t expand as much is because it is drier.

There is less water in the soap to evaporate, and less air to expand.

Similarly, other soaps are not whipped during manufacture so there is less air to expand.

You are demonstrating Charles’s Law, which states that gas expands when it is heated.

Though you can see this physical change, there is no chemical change.

The soap still washes your hands.

What Good Is It?

The popping of popcorn also demonstrates Charles’s Law, as moisture inside the kernel expands and pops through the outer shell.

Foam is a material that traps gas inside, like the foam at the top of heated milk.

Other foams include whipped cream, Styrofoam and shaving cream.

Styrofoam is a polystyrene foam invented at Dow Chemical that resists moisture, is light and buoyant.

It has been used to make life rafts for the Coast Guard, home insulation, cups to hold hot liquids, and to anchor flowers in florists’ arrangements.

You can click on Expanding Ivory soap to see a Youtube video of this experiment, thanks to spanglerscience.com

Thanks to happyhooligans.ca for the expanding Ivory soap activity.

Thanks to chemistry.about.com for an explanation of the scientific principles demonstrated.


Carol Covin, Granny-Guru, Grandma to two awesome grandchildren

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers



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