Microwaving liquid N2

Thank you, both, but I have to say that John’s explanation was more fun, all those dancing molecules. :smiley:

Solid water is denser than liquid nitrogen?

I thought the absorption of microwave frequencies was due to rotation of the molecule.

If the molecule is polar, like H2O, then the molecule will tumble as the EM wave ‘passes by’. The dipole interacts with the oscillating electric field.

If the molecule is non-polar, like N2, then the electric field component of the EM radiation does not affect the molecule.

Vibrational frequencies are usually in the infra-red region.

Yep. Ice is about .92 g/cm[sup]3[/sup], liquid nitrogen is about .8 g/cm[sup]3[/sup]

Hey, explanations I’ve seen have stressed the fact that liquid water DOESN’T have resonances. The weak bonds between the molecules spoil any resonant effects.

Yes, water VAPOR (especially in vacuum) is going to have sharp resonances and absorbtion lines. But don’t assume that the same applies to liquids. Analogy: a bronze bell has a narrow resonance line, but if you bury it in dirt the resonance is spoiled.

Now water molecules are certainly polar, and if exposed to strong e-fields will tend to rotate (which disrupts the bonds to neighboring molecules, so strong rotation will heat up the liquid.) But this effect isn’t frequency dependent. We could just as well place our mug of coffee right near the antenna of a 1000 watt UHF transmitter and the radio waves would heat it just as hot. The rule is that “polar molecules are heated by high frequency e-fields”, as opposed to “resonant molecules absorb EM energy of a particular frequency.”

I think the stuff about water resonance is a kind of “urban legend” among technical people. It probably started out because of those water absorbtion lines found in interstellar space by radio astronomers. But that only applies to single molecules, those not in contact with neighbors.

Very true, bbeaty. I knew some people who worked in a facility that designed and tested high powered radars. Sometimes, just for kicks, they’d stick their lunches in the anechoic chamber and fire up whatever radar they were testing. Not the most efficient use of a multi-million dollar piece of equipment, perhaps, but an entertaining one.

The term ‘resonance’ usually applies to coupling of oscillators. It is better to use ‘absorption’ in the descriptions here.

Liquid water still does absorb in the infrared region, however liquid water has a broadened and shifted absorption band due to intermolecular forces.
-O-H…O-H…O- (close enough for just one line)
So I dont think the bell analogy isn’t very good. I think more in terms of balls on springs. Continuing the analogy with liquid water, maybe the balls/springs have crazy-string sprayed over them.

Pure rotational transitions are in the microwave region. Water has a permanent dipole and thus shows a pure rotational spectrum. ie it absorbs microwave. Nitrogen does not.

Also, nitrogen has no vibrational mode * that changes the dipole moment* and thus cannot absorb in the IR region either.

Well, sort of right, sort of wrong. Liquid water has an absorption spectrum with definite peaks and troughs in it. It is frequency dependent. And some of the peaks are sharp enough to be called “resonant frequencies”. Cite. Look especially at the absorption peak at around 3 um.

The closest peak to the 2.45 GHz operating frequency is at about 10 GHz for liquid water at 0 [sup]o[/sup]C, rising to 100 GHz for liquid water at 100 [sup]o[/sup]C. Cite

OT: I love this place. I rarely have anything to contribute, but so much to absorb (or is it resonate? ;))

About ten years ago, I took a 1 pint plastic bottle with a flip-top and filled it about a 1/4 full. Then, I took some liquid Nv2 and poured enough to fill it up half way. The Nv2 was floating above the water (as I expected) and boiling away furiously and spraying a large vapor trail out of the container. I reasoned that if the water was between the plastic and Nv2, it keep the container becoming brittle.

I closed the flip-top and threw it into a large trash container figuring it would give a loud pop. Well, as I found out the hard way that flash boiling in a closed container was a very bad idea. I didn’t get hurt, fortunately, but I won’t do it again either. :eek:


“I reasoned that if the water was between the plastic and Nv2, it keep the container becoming brittle.”

“I reasoned that if the liquid Hv2Owater was between the plastic wall and the liquid Nv2, it would keep the container from becoming brittle.”


I’m talking about the frequency where most microwave ovens operate. Wasn’t that clear?

That’s long IR!!! What does that have to do with microwave ovens purportedly being set to a resonance peak? I bet water has UV and gamma absorbtion lines, but that has no bearing on my previous message either.

In other words, “I’m right.” :slight_smile:

Read my original message. I’m was complaining about people who claim that microwave ovens heat water because of resonance. In reality your kitchen oven’s frequency is NOT set to a water resonance peak. If it was, then the radiation wouldn’t penetrate bulk water such as meat, coffee, etc. And besides, interactions between water molecules spoil the peaks in the many-cm band so they aren’t even absorbtion lines, they’re huge broad humps.

Microwave ovens work by having the polar molecules trying to follow an e-field, they aren’t pumping a molecular resonance. Your cites say as much.

I was pointing out that the same does apply to liquids.

As I pointed out, the effect is frequency dependent.

Again, I pointed out that it does apply to liquids.

No, it wasn’t, especially when you were talking a complete lack of frequency dependence. I guessed that that might be what you were talking about, which was why I mentioned the operating frequency.

The two specific points I addressed were:

  1. Water vapour might resonate, but liquid water doesn’t.
  2. RF absorption by liquid water is independent of frequency.

On both points, you were simply wrong.

If you wanted to be right, and you wanted to say something like that, then you should have just said something like that.

In other words, I understood the point you were trying to make, but disagreed completely with the generalisations and mistakes you made along the way.