How to Make a Bar of Soap ERUPT in the Microwave! Kids Activities Blog

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Erupting Soap is a super fun and easy science experiment for your kids! If you think baking soda and vinegar eruptions are cool, then you’re going to love what happens to a bar of Ivory soap when you put it in the microwave.

Erupting Soap

How to Make Erupting Soap In the Microwave

What you need:


Bar of Ivory Soap

  • A bar of Ivory soap (no substitutions allowed)
  • A microwave safe plate
  • A microwave

Yep, that’s it!

I first asked my son what he thought might happen if we put a bar of soap in the microwave. He naturally said that it would melt. Most soaps will melt, but Ivory soap is different because of the way it is formed. More on that later…

What to Do:

Put your bar of soap on the plate and microwave it for 2 minutes.

Ivory soap in microwave

The action starts right away as the soap quickly starts to grow.

When it stops growing you can stop the microwave, although it won’t harm anything if it runs for the full 2 minutes.   The soap just won’t grow any bigger at that point.

My son was absolutely giddy watching this for the first time…and every time after that. I must admit I haven’t gotten tired of watching erupting soap either!

happy boy

When the soap had finished erupting, this is what we got.

Ivory Soap out of microwave

What’s going on?

There is a scientific principle called Charles’ Law which states that the volume of a gas directly increases with an increase in temperature. So the hotter air gets, the more space it wants to take up, and the more pressure it will produce in order to take up that space.

Finger painting with Ivory Soap

Ivory soap is an unusual kind of soap, in that it has a lot of air pockets in it.

There is also a lot of moisture in Ivory soap. When it is heated, the soap softens but before it gets close to melting, the moisture in the bar gets hot and turns to gas (steam). Add that to the already present air particles throughout the bar and you’ve got a lot of steam trying to get out. As the steam pushes its way out, it expands the soap.

Related: Here’s a simple animation of Charles’ Law to help explain how volume and temperature are directly related.

Other soaps are not as porous as Ivory soap because they do not have air pockets throughout. Therefore, the steam isn’t able to build up inside it and instead the soap just melts.

Except for the loss of water, this is still Ivory soap. No real chemical change took place. The soap is puffed full of air so we had fun crumbling it up, and then we whisked in a bit of water and made “soap paint”.

crumbling ivory soap

We painted on styrofoam trays both with paintbrushes, and with our hands.

painting with Ivory Soap

Once the “Wow Factor” died down just a little, we decided to get a little more scientific so we pulled out a scale.

We weighed a whole bar of ivory soap: 78g., and an erupted bar of Ivory soap: 69g.

The erupted bar weighed less due to moisture evaporation.

weighing Ivory Soap

Other Observations:

1. The soap has expanded six or more times its original size, but actually weighs less now because of water that has evaporated. Amazing!

2. If you microwave half a bar of Ivory soap, the cut side of the bar will expand significantly more quickly and with more force than the uncut side. In this experiment above, the force of the expansion out of the cut side was so strong that it flipped the bar from its side to an upright position so that the eruption from the cut side was then facing upward.

3. The plate was hot all over after a minute and half.  However, the plate was significantly hotter directly under the expanded soap. Microwaves focus on heating water molecules, so the water in the soap heated quickly and made that part of the plate hotter.

Did you know? We wrote a science book!

This post contains affiliate links.

Our book, The 101 Coolest Simple Science Experiments, features tons of awesome activities just like this one that will keep your kids engaged while they learn. How awesome is that?!

The 101 Coolest Simple Science Experiments

Hope you enjoyed making a bar of soap erupt in the microwave! Tell us your kid’s reaction in the comments!

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