The Hard Sought, "Grape in a Microwave" Explanation
In reference to: Create Super Heated Plasma with a Grape
Far be it from us to destroy anyone’s nerdy fascination with lighting stuff on fire with a microwave—one of the ultimate proofs of commitment to your geekdom—and what better object to show off this dangerous, potentially poisonous exhibit than with a grape?
The experiment is simple. Take a seedless grape and slice it down its length, making sure not to cut all the way through (this part is important!), so you leave just a small amount of skin connecting the two halves. Put it face-up in a microwave, turn it on for 30 seconds, and presto! A ball of flame.
So what the heck is going on in that thing? Grapes are packed full of electrolytes, an ion-rich liquid (also known as "grape juice") that can conduct electricity. Each grape-part serves as a reservoir of electrolytes, connected together by a thin, weakly conducting path formed by the skin. Microwaves produce the energy that shove the stray ions in the grape back and forth very quickly between the two halves.
As a consequence, the current that's produced pumps excess energy into the skin bridging the grapes, heating it up to 3000 degrees and eventually bursting into flame. Meanwhile, the traveling electrons arc through the flame and across the gap, which ionizes the air around the grape creating a bright blue burning plasma (phew!).
And what about the poisonous gas tainting your roommate’s dinner? Well, he's talking about the ozone (O3) generated when the air inside the glass is ionized (think lightning storm). While not directly poisonous, ozone in high doses can cause issues with your lungs and just isn't the best thing you could breath. And the smell isn't all that appealing either.
There's also the possibility of damaging your microwave with all this grape microwaving. But keep it just under "explosive ball of flame" and you should be ok. Good luck!