Key concepts
Microwave Radiation
Chemical Polarity
Dipole Rotation

Ever wonder how a microwave oven cooks your food? A microwave uses—you guessed it!—microwaves, a form of high-frequency electromagnetic radiation. Think radio waves, just with a much shorter wavelength. Here's the cool thing about microwaves: They're absorbed by certain kinds of matter, whereas other kinds are left alone. In this activity, we'll try cooking up some foamy soap in order to illustrate how microwave radiation influences different types of matter.

Soap is a handy combination of chemicals that do some very cool things when you use it. Soap molecules have both a polar and a nonpolar part. What' is polarity? A molecule is described as polar when it has two or more areas of different electrical charges. A water molecule is a perfect example: it has a negative pole created by the negative charge its oxygen atom and a positive pole created by the positive charges of its two hydrogen atoms.* By having a polar and a nonpolar part, soap can bind to both water and dirt. This is what makes it such an effective cleanser!

Microwave ovens work by causing molecules that have two opposing poles to spin rapidly. Because of their polarity, molecules like water will constantly align themselves with a magnetic field they're subjected to. Microwave radiation creates a magnetic field that oscillates—which means that the field is constantly changing its orientation (direction the positive and negative charges face). Those shifts make polar molecules like water start spinning as they try to keep up with the changing charges. As the molecules spin, they generate heat. This process is known as dipole rotation. ("Dipole" simply means "having two poles.")

Have you ever used a microwave to boil water and found yourself wondering why the air in the microwave doesn't get hot like the air in a conventional oven? Microwave radiation causes water molecules go nuts, but the air itself isn't directly heated because its molecules aren't as polar as the molecules that make up water. This help explains why you can't make a pie's crust crispy in the microwave,* but it's easy to burn one to oblivion in a conventional oven! Polarity also explains why many plastics, glasses and ceramics are considered "microwave safe"—their molecules in these substances aren't very polar, so they aren't as disturbed by the magnetic fields shifting.

• One bar Ivory brand soap
• One bar Dial soap
• Two paper plates
• Microwave

• Note the weight of each bar of soap, which should be written on the packages.
• Unwrap each bar.
• Place each on a paper plate, noting which one is which.

• Microwave the Ivory soap for two minutes, carefully observing it during the whole time. What happens to the soap? Why do you think this might occur? 
• Remove the plate with the soap from the microwave. Use caution, as it may be hot.
• Microwave the Dial soap for two minutes, carefully observing. Does this soap react differently than the first one? Why do you think this might be?
• Again, be careful when removing the heated plate.
Extra: Before microwaving, weigh each bar of soap. Which is heavier? If one is heavier than the other, what do you think accounts for this difference, besides size? Do you think this might help to explain what we saw when we "cooked" the soap bars in the microwave?

Observations and results
The Ivory soap should have produced an impressive amount of foam compared with the Dial. Ivory has thousands of tiny air pockets in it—that's why it should have weighed less! These pockets of air expanded when they got hot, creating the soap bubbles that made up our foam.

But what caused the air to get so hot in the first place? As we discussed, microwave ovens are pretty bad at heating air. But part of our soap molecules are polar, so the microwave radiation caused these molecules to spin and build up kinetic energy, which created heat. This resulting heat was conducted from the melting soap to the air pockets, causing the pockets to expand and create some impressive foam.

More to explore:
Foaming Soap , from
Microwave Madness , from
Exploding Marshmallows , from
Chemical Polarity , from Wikipedia

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*Editor's note (May 30, 2013): These sentences were edited after posting either to correct
or clarify the original content.