This is a fun activity that only needs a bar of Ivory brand soap and a few minutes in the microwave. My kids first thought it was really silly that we were putting soap in the microwave and then were excited and surprised when they saw what happened.
All you really do is microwave the soap. I cut our bar of soap into 4 pieces and then microwaved one piece at a time. I set the soap chunk in the middle of a microwave safe plate and set the microwave for about 2 minutes. You will want to huddle around to watch because this is where the surprising part happens. The exact timing will depend on your microwave, but the soap will start to expand and “explode” as it heats up. Once it stops expanding, you can stop the microwave and bring the soap out to play with. It will be hot when you first remove it from the microwave, so you will need to give it a minute or two to cool. After that, though, it’s fun to play with and crumble.
Note: It really does have to be Ivory brand. There is something unique about their production method that mixes a lot of air pockets into the soap. These air pockets are what makes this work (and are also why Ivory soap floats). Not all soap is produced this way.
Here is what’s going on. When you put the soap in the microwave the soap itself and all of the little air pockets that were mixed in start to heat up. When gasses heat up, they expand and take up more space. When gasses cool down, they take up less volume. This relationship between temperature and volume is described by Charles’ Law, which says that V1/T1= V2/T2. This basically says that temperature and volume will increase together or decrease together. As those little pockets of air in our soap heat up their volume should increase too. For the volume of the trapped air pockets to increase with the temperature, the soap has to expand out to make room. Pretty cool!
The soap is still fine to use, or there is another fun way to continue the play. If you add 1 cup of warm water to the crumbled soap (from the entire bar) and then pour that mixture over a roll of unrolled toilet paper, it will turn into a play-doh type consistency. My kids had fun mashing it all together and playing with it. They also seemed to enjoy unrolling an entire roll of toilet paper! ha!