Water and microwaves

[Santas little helper]

From what I understand water absorbs microwave frequencies better than any
other.But why is that ? Are there some theoretical considerations which will
give you this result or has it been obtained through experimentation ? And what
about other materials ? Can we determine which range of electromagnetic frequencies
they absorb best ? What would happen if I put olive oil in a microwave oven ?

[Schneibster]

The frequency emitted by microwave ovens is indeed tuned to one that water molecules will react strongly to. And yes, this is a theoretical result of the precise size and shape of the water molecule. And the precise phase is selected so that maximum heating will occur in the center of the turntable.

The original discovery was by a researcher who was working with waveguide and RADAR, and happened to have a candy bar in his pocket; when he discovered his candy bar had melted, he traced his movements back, and discovered this use for microwaves. However, those were a different frequency than is used in the familiar kitchen appliance.

If you put olive oil in the microwave, be incredibly careful- it can become hot enough to cause third degree burns (charred flesh- literally; this is not a joke or exaggeration, please be extremely careful) with no sign at all that it is in any way hot. Water can be heated beyond the boiling point without any sign- and explode into steam in your face when you touch the container. I suggest reviewing several sites on microwave safety before trying any experiments whatsoever.

[I. C. Unicorns]

I think you need to clarify. Do you mean water absorbs microwave frequencies better than other frequencies or water absorbs microwave frequencies better than other substances absorb microwave frequencies?

The absorption vs. transmission of the EM energy by water should increase as the frequency increases.

[Schneibster]

Huh! I was wrong. Actually, it works on any polar molecule, by vibrating it; and the frequency they use isn't even the best one for that. It's one that's convenient to produce, and easy to shield against; and it works, though not as well as it would if it were higher frequency.

this site (http://www.amasci.com/weird/microexp.html#myth) has the straight scoop on it. Warning: you do these experiments, you're ON YOUR OWN. Don't do them in MY kitchen, please!

[Corona688]

Huh! I was wrong. Actually, it works on any polar molecule, by vibrating it; and the frequency they use isn't even the best one for that. It's one that's convenient to produce, and easy to shield against; and it works, though not as well as it would if it were higher frequency. They picked a frequency that's a bit off to provide more even heating. If they picked a perfect frequency, it would get absorbed by the outside and not heat the inside; you'd char and vaporize the outer layers of water-containing things yet leave the inside frozen.

[Loren Pechtel]

I think you need to clarify. Do you mean water absorbs microwave frequencies better than other frequencies or water absorbs microwave frequencies better than other substances absorb microwave frequencies?

The absorption vs. transmission of the EM energy by water should increase as the frequency increases.

The frequency used in microwave ovens is resonant with the water molecule, it will be absorbed much better than frequencies on either side of it. This resonance is not chance, they picked the best frequency for the purpose.

[Santas little helper]

Do you mean water absorbs microwave frequencies better than other frequencies
That's what I meant.

The frequency used in microwave ovens is resonant with the water molecule, it will be absorbed much better than frequencies on either side of it. This resonance is not chance, they picked the best frequency for the purpose.
Not according to others in the thread.

Any molecule which is "polar" and has positive and negative ends will be rotated back
and forth to align with the changing e-field of the radio waves in the oven.
I take it then that the physical principle in effect in a microwave
oven is different than the physical principle in spectroscopy (absorption
lines).Could someone explain to me the difference between the two ?

If you put olive oil in the microwave, be incredibly careful- it can become hot enough to cause third degree burns
Can one use a microwave oven for frying then ?

[I. C. Unicorns]

I take it then that the physical principle in effect in a microwave
oven is different than the physical principle in spectroscopy (absorption
lines).

Oh absolutely. The microwaves make the molecules of the food vibrate, microscopic vibration is macroscopic temperature. Molecules (and everything else) can move in three manners: translation, rotation and vibration. Some molecules, like fats, are long compared to water, so they can store more rotational and vibrational energy than water. That's why butter gets hot faster than water in a microwave.

absorption lines occur when looking at white light that has passed through a gas. Certain wavelengths of light are absorbed by the gas which cause electrons to jump to higher energy states in the atoms.

I'm not an expert, so don't quote me.

[Shadowy Man]

absorption lines occur when looking at white light that has passed through a gas. Certain wavelengths of light are absorbed by the gas which cause electrons to jump to higher energy states in the atoms.

But if you did microwave spectroscopy through water vapor you should be able to see the absorption lines. I think it is the rotational modes that are excited by the microwaves. You can also see absorption by water in the infrared, but I think those are the vibrational modes. I'm a UV spectroscopist, so I'm not as sure about the water lines.

[hagiograph]

Not an expert either, but...

IN INFRARED SPECTROSCOPY: If I recall corrctly, as the O-H bond stretches and relaxes it occurs at such a frequency as to allow for the Plank criteria to be met (delta E = hf) where absorption of a photon of the right wavelength (around 3 or so microns) is allowed due to the frequency of the stretching.

In Microwaves, it sounds as if an earlier post is correct, that it abosrbs because of rotational movement of the water molecule. Earlier it was mentioned the outgrowth of microwaves from radar research in the 1940's, apparently early radar was problematic precisely because they were using microwave-wavelength radiation which was absorbed by atmospheric water and could not be used on cloudy or rainy days. AGain, IIRC.

-h.

[gnojek]

Take some carbon powder and stick it in a commercial microwave oven and watch it reach 1000C in 10-20 seconds. If it's in a glass container it will melt it.

[Santas little helper]

Molecules (and everything else) can move in three manners: translation, rotation and vibration.
Isn't vibration translation back and forth ? Or do you mean something different with the term ?

In Microwaves, it sounds as if an earlier post is correct, that it abosrbs because of rotational movement of the water molecule.
That was my understanding also.But this leads to the question: doesn't the "other" kind
of absorption , the one responsible for absorption lines also affect temperature ?

[Corona688]

doesn't the "other" kind of absorption , the one responsible for absorption lines also affect temperature ? Sure. It's just not dominant in microwave ovens.

[Santas little helper]

So I guess that even if we put some material made out of non polar molecules in
a microwave oven its temperature would increase.Would it be noticeable ?

[Corona688]

Depends on what it is. I accidentally microwaved an empty plate once. It got kinda warm, but not very. A pie plate would arc all over the place and melt and char.

[Shadowy Man]

Isn't vibration translation back and forth ? Or do you mean something different with the term ?

Not in the context of that post. Translation would be the bulk movement of the molecule. Vibration would be the oscillation of the atoms relative to each other - i.e. in the reference frame of the translating molecule.

Think this:

O---H---O
...O-H-O
O---H---O

or

O-H----O
O----H-O

there's a third dimension I'm not including here, so there are bending modes of the H2O molecule as well.

[hagiograph]

doesn't the "other" kind
of absorption , the one responsible for absorption lines also affect temperature ?

Again, don't take my word for gospel, but I think the key is that the vibration that accounts for absorption from bonds vibrating is a different amount of energy change than what you see in translation of entire molecules. If I'm thinking correctly, then it would require different wavelengths to absorb and hence increase the temperature.

In other words, the wavelength that can be absorbed by ONE type of motion (bond vibration) is different from the wavelength that can be absorbed by the other. The H-O bond vibrates at a freqency that can absorb IR radiation of 3um, while microwaves are longer (1mm to 30cm, if I found the right reference). So they don't fulfill the same parts of the Planck criteria.

IF there is a bond in there that does absorb in that 1mm to 30cm range then it would cause heating based on a molecular bond, vs translation of the entire molecule.

And "absorption lines" can mean different things. If you pass radiation through a mass of material you can absorb different wavelengths by different mechanisms. If you pass a microwave through a cloud of various chemicals including water, you should get an absorption band due to the absorption by water molecules. If you pass an IR beam through a water cloud then you'll get an absorption at 3400cm-1 due to the O-H bond in the water molecules.


-h.

[Schneibster]

The molecule, if I understand it, is more like

H H
\ /
O

and can also "flop" back and forth. Let's see if that comes out when I post it.

[Schneibster]

Yeah, it came out kinda OK, if you move the oxygen atom to the right a bit; IIRC, the angle is much less acute than that, but there aren't any symbols that I can use that have that low an angle. :P

[Shadowy Man]

The molecule, if I understand it, is more like

H H
\ /
O

and can also "flop" back and forth. Let's see if that comes out when I post it.

Yeah, that's why I mentioned the third dimension and the bending modes.

The microwaves excite the rotational modes though, and most probably these can excite translational motions (i.e. increase temperature) through collisions with other molecules.

[Berthold]

In a very diluted gas (nearly hard vacuum) microwave spectra are indeed line spectra. Herzberg got the Nobel Prize in chemistry for using them to establish very exact molecular geometries. In condensed (=liquid or solid) condition material absorbs these wavelengths as "bands", because the molecules interact with each other.