Erupting Soap is a super fun and easy science experiment for your kids! Using just a bar of Ivory soap and your microwave, you and your kids can have a quick and easy science experiment that will delight everyone. Use this simple science activity for kids at home or in the classroom.

Erupting Soap (text) - yellow plate in the microwave with a large mound of expanding Ivory soap (white) on top - Kids Activities Blog

How to make Erupting Soap in Microwave

Kids of all ages will think this science experiment is cool! You are going to love what happens to a bar of Ivory soap when you put it in the microwave.

Related: Baking soda and vinegar eruptions

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…

*Adult supervision is needed for this science experiment.*

Ivory Soap Experiment – Ingredients Needed

Bar of Ivory Soap shown intact and on an orange background
No substitutions on the soap! It has to be Ivory…
  • A bar of Ivory soap (no substitutions allowed)
  • A microwave safe plate
  • A microwave

Yep, that’s it!

Video: microwaving ivory soap

Instructions on Ivory Soap Science Experiment

Step 1

Step 1 - Ivory soap growing in microwave
Look at what is happening to the Ivory soap!

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

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

Step 2

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.

child watching the Ivory soap science experiment in the microwave smiling - Kids Activities Blog
Mom, this is so cool!

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!

Finished Ivory Soap Eruption

Ivory Soap out of microwave
This is what our Ivory soap eruption looked like!

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

Why does this Microwave Soap Erupt?

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.

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

Child with moist Ivory soap all over fingers - Kids Activities Blog
Ivory soap has more moisture than other soaps.

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.

Charles & Gay-Lussac’s Law

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

crumbling ivory soap - two images of making ivory soap flakes after experiment
Playing with Ivory soap after it has cooled.

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

painting with Ivory Soap after the science experiment is over and the soap has cooled
We added more moisture and did some “painting” with the leftover soap.

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

Related: Things to make with soap

Does Ivory Soap Get Lighter after Being Heated?

We weighed a whole bar of ivory soap before and after the eruption experiment to see if it got heavier or lighter after heating it up.

weighing Ivory Soap on a scale before and after the science experiment - Kids Activities Blog
Look at what the bar of Ivory soap weighs!

The weight of a bar of Ivory soap:

  • Ivory soap bar weight before experiment: 78 grams
  • Ivory soap bar weight after the experiment: 69 grams

The erupted bar weighed less due to moisture evaporation.

Other Observations from Ivory Soap in Microwave

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.

Ivory Soap Microwave FAQs

Is it safe to microwave Ivory soap?

“Ivory Gentle Bar Soap gives you a safe, pure clean trusted for generations. Our simple soap is free of dyes and heavy perfumes, is dermatologist tested, and continues to be so pure, it floats! …Dermatologist Tested, Free of dyes & heavy perfumes…99.44% Pure.”
-Ivory Soap website(Gentle Bar Soap, Original Scent)

When you ask if it is safe to microwave Ivory soap, the answer will be no because of the dangerous chemicals. We can not find any dangerous chemicals. So, please understand that some think this is dangerous, but have not given us the reason why.

How long do you put a bar of Ivory soap in the microwave?

2 minutes is the recommendation for the amount of time to let Ivory soap stay in the microwave.

What to do with Ivory soap after microwave?

Once your Ivory soap cools, you can play with it.

Yield: 1

How to Make Soap in the Microwave ERUPT

Ivory soap in microwave experiment for kids - erupting soap on plate

This simple science experiment for kids needs just three simple things: Ivory soap bar, microwave-safe plate & microwave. With adult supervision and just 2 minutes you can see how Ivory soap erupts in the microwave and expands into a white fluffy eruption! Let's chat about the science behind all the fun.

Active Time 2 minutes
Total Time 2 minutes
Difficulty Medium
Estimated Cost $1

Materials

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

Tools

  • Microwave

Instructions

  1. Remove the wrapping from your bar of Ivory soap.
  2. Set your Ivory soap bar on a microwave safe plate in the microwave.
  3. Set on high for 2 minutes in the microwave.
  4. Watch what happens.
  5. Let cool before touching the Ivory soap.

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Did you know? We wrote a science book!

The 101 Coolest Simple Science Experiments book by the authors of Kids Activities Blog
This experiment is in our science book!

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?!

More Science Fun from Kids Activities Blog

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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15 Comments

  1. I am NOT going out at eight o’clock at night to buy Ivory soap so we can try this….but that’s only because the roads are icy! Hopefully they thaw in the morning because this is just too cool not to try — thanks for sharing it!

  2. This also works with marshmallows- but be very careful since unlike soap, marshmallows _will_ burn. Also marshmallows are elastic, so they will shrink as they cool, and the sugar will crystallize and become stiff.
    We had a lot of fun using toothpicks to join the marshmallows into snowmen and faces and igloos, and then watching them morph into nightmarish forms.
    Again, use caution, and supervise closely.

  3. We’ve done this and it’s loads of fun…. my kids were totally amazed! Loved your explanation of what’s happening!

  4. Wow! I’d forgotten about Charles’ law. What an excellent explanation of the science. My kids (ages 4, 6, and 8) are going to love this!

  5. It is the first time I hear about this law. 🙂 I know Murphy’s law since it’s happens to many times to me lol I will absolutely do it with my kids! This is so much fun ! Thank you!

  6. I have only one word to describe it: AMAZING! You explained it in a simple way and so interesting. It’s definitely on my to do list for this weekend. My son loves everything scientific 🙂