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Hah! Finally all these chemistry classes I'm taking can go to good use!
So here's how microwaves work: The frequency they operate at, ~2.4ghz, is the frequency at which water molecules start to vibrate. When you increase the average kinetic energy of a system, say the molecules in a cup of water, you get temperature increase -- that is basically what temperature is. So in your microwave you have essentially superheated water so that it goes above its boiling point at 1 atmosphere pressure. Now if you've ever been in chemistry lab you might notice that in boiling things you often place boiling chips into the flask or whatever you're apparatus to distill/heat liquids is in. This is because the boiling chips are rocks with many jagged locations on them that are called nucleation sites; these allow for the phase change from liquid to gas in the high temperature liquid when it gets to the boiling temperature. Like what everyone else said, in smooth containers where you have few imperfections, you can see this superheating in effect.
What's cool is that the opposite is also true. You can cool liquids below their freezing temperature if the liquid is very pure, if the container has no nucleation sites so that crystals cannot form. Then when you disturb it the whole thing freezes up. Same reasoning there too.
Superheated and supercooled, are keywords if you want to find out more.
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