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Microwave Soap—It's Not Magic, It's Science!

[[{"type":"media","view_mode":"media_large","fid":"39140","attributes":{"class":"media-image","typeof":"foaf:Image","style":"","alt":""}}]]By Becky Wolfe, Science Programs Manager

NOTE: Before starting this activity, please remember to be careful when pulling your soap out of the microwave.  Any time you microwave something, it gets hot! The soap will too!

As one of the museum’s science educators, I spend a lot of time researching and trying interesting science experiences for our visitors.  Microwaving a bar of Ivory soap is one of the demonstrations that had me yelling at my family across the house, “Hey! You have to come see this! It’s so cool!” I even threw a second bar in the microwave, just so our family could watch the reaction a second time.  

So is it magic? Nope—just a bit of physics. SPOILER ALERT: I’m going to start talking about reaction, so if you haven’t tried your soap in the microwave and want to be surprised, stop here and come back when you are done. (Follow the directions in the blog post Saturday Science: Microwave Soap!) 
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Back to the reaction. So what causes the soap to expand into a big fluffy mass? First, it’s important to know how Ivory is made.  All soap is a mixture of the soap ingredients such as glycerin  (this varies by type and is often proprietary), water and air. In the case of Ivory, it has a bit of extra air whipped into the soap mixture. This is why it floats in the bath tub. If your soap has cooled down, touch the microwaved soap and notice the texture. Is it the same as the bar of soap? You might notice that it’s crumbly and a bit drier, in addition to having expanded greatly in size. This is a big clue has to what is happening with the soap. 

SoapThe soap expands due to Charles’s Law, named for the scientist who is credited with the calculations to explain what happens to gases when heated. This law explains that when a gas is heated, its volume expands. When soap is microwaved, the air trapped inside of the soap heats up and starts to expand. The water inside the soap also heats up and will turn to steam. As the gases grow in volume, they push on the remaining soap ingredients, expanding the bar from something small and compact to a fluffy blob.  This is also why the soap feels a bit drier. The water in the soap has turned to steam, and has been released from the soap. 
The same reaction you see with the soap happens when you make popcorn. Water inside of the kernel expands as the kernels are heated. Eventually, the water becomes hot enough that it turns to water vapor or steam.  As the steam expands, it pushes on outer walls of the kernel until they burst exposing the yummy insides for the popcorn kernel. I actually think the microwaved soap looks like a large piece of popcorn.  But don’t eat the soap!

Some of you may be asking what to do with your soap bar, now that it’s a big fluffy bar. There are a number of sensory activities that use crumbled ivory such as ghost mud. You can also use the soap like, soap. 

P.S. A number of you may be wondering why I didn’t mention the “accident” that caused the invention of Ivory.  A few years ago, the makers of Ivory, Proctor and Gamble, shared documents from their archives that showed the process their chemists used to develop floating soap. While a great story of a mistake leading a great invention, no worker accidently whipped more air into the soap.