A Staff Report from the Straight Dope Science Advisory Board

What would happen if you put liquid nitrogen in a microwave?

January 14, 2003

Dear Straight Dope:

I have a question that neither of my high school science teachers could answer for sure, including my physics teacher, who scored 1590 on his SATs (he also found an error on the test) and is a member of Mensa. If I were to put liquid nitrogen in my microwave, what would happen? Would it superheat and instantly explode or would it just boil off like if it was in regular air? By the way, my teacher suggested for me to try this experiment.

SDSTAFF Alphaene replies:

Here at work I am within three feet of both a microwave and 240 liters of liquid nitrogen. However, even if I leave my job I'd like my boss to be a reference. And if he said "everything was fine until he superheated liquid nitrogen and destroyed the lab microwave," it may not reflect positively on me.

SDSTAFF Chronos replies:

When I saw this question, I was going to answer based on the resonance properties of water and nitrogen molecules. Microwaves work by exciting resonances in water molecules, and the water, once excited, heats up everything else in the frozen dinner you're cooking. Since water is an asymmetric triatomic molecule, it has three distinct rotational modes and three more vibrational modes, making it easy to find resonances. Nitrogen, on the other hand, is a symmetric diatomic molecule, so it only has one distinct rotational mode and one vibrational mode. That makes it much harder to match a resonance for nitrogen, so the microwave probably wouldn't do anything to it.

That's how I was going to answer. But that's boring. Here at the Straight Dope, we do experiments.

I went to the instructional lab supervisor here and asked if I could have some liquid nitrogen for, uh, "instructional purposes." Having obtained the substance, I then proceeded to pour equal quantities into two identical paper cups. One was left outside as a control, and the other was placed in the microwave, which was set for one minute.

It turns out that my prediction was half-correct. Liquid nitrogen is, as I suspected, transparent to microwave radiation. The effect of this transparency, however, is not that "nothing happens." The effect is the same as when a microwave oven is run containing only air: a spectacular light show of sparks and flashes. Although my view of this phenomenon was somewhat hindered by the skull-and-crossbones sign on the door of the microwave, I decided that it would be nonetheless prudent to cut short the experiment after approximately one second. Afterward the liquid level in both cups remained the same, to within the limits of measurement. When an empty cup of the same sort was placed in the oven, similar pyrotechnics were produced. We may therefore conclude that, so far as the microwave oven is concerned, liquid nitrogen behaves in the same way as the atmospheric nitrogen normally present in the cooking chamber. 

Later it occurred to me that there was no reason to halt the experiment prematurely.  A cup of liquid nitrogen would hardly be expected to catch fire, so the only concern would be damage to the magnetron tube. Since the next experiment proposed for this microwave was, "What happens to a microwave oven dropped from the ninth story?" the fate of the tube was likewise not an issue. Unfortunately, this thought did not occur to me until after my office-mates had already scattered the liquid nitrogen on the floor to see the Leidenfrost effect, so further tests were impossible.

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