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Today I learnt that microwaves heat food by blasting electromagnetic waves through the water molecules found in the food.

Does that mean food with 0% moisture (if such a thing exists - dried spices?) will never receive heat from a microwave oven? And how in that case is a microwave able to melt plastics etc., which contain no obvious water?

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    $\begingroup$ I won't leave an answer, as I'm only guessing, but one might ask the following questions: is it really possible for a food item to be moisture free? If a plastic is porous, might it absorb some water to its surface while in the air? Most importantly: what is special about water that makes it heat up under microwave irradiation? I think the answer is that $H_2O$ has some vibrational modes that correspond to the frequency of the microwaves. If this is the case: could the polymers in a plastic also have modes which correspond to microwave frequencies? $\endgroup$
    – Lachy
    Dec 3, 2014 at 1:46
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    $\begingroup$ This is why if you sprinkle a little water on top of a cookie and put it in the microwave, it will taste amazing. $\endgroup$
    – Señor O
    Dec 3, 2014 at 15:42
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    $\begingroup$ @SeñorO I'm not following your line of reasoning... $\endgroup$
    – Michael
    Dec 3, 2014 at 20:07
  • $\begingroup$ @Michael Normally if you put a cookie in the microwave, the only part that really heats up are the chocolate chips. If you sprinkle some water on the whole cookie, it heats up the doughy parts and makes it warm and gooey like it just came out of the oven. Any excess water on the cookie will vaporize in the process. $\endgroup$
    – Señor O
    Dec 3, 2014 at 21:01
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    $\begingroup$ @SeñorO I see. So the taste of the cookie is affected by its temperature and viscosity. $\endgroup$
    – Michael
    Dec 3, 2014 at 22:14

3 Answers 3

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Microwave heating is largely caused by the changing electric and magnetic fields (i.e. the "microwaves") which are emitted by your microwave oven affecting polar molecules. As the direction of the electric field changes over time, the polar molecules (often, of water) attempt to follow the field by changing their orientation inside the material to line up along the field lines in an energetically favorable configuration (namely, with the positive side pointing in the same direction as the field lines). As these molecules change direction rapidly (millions of times per second at least), they gain energy - which increases the temperature of the material. This process is called dielectric heating.

However, water is not the only polar molecule in the world. You can test for yourself that most plastics don't heat in a microwave while most glass and ceramic objects do. So, a microwave oven melting your plastic bowl has more to do with it over-heating your food than over-heating that food's container.

EDIT: After doing some research to address some questions brought up in the comments to this post, I've found some very interesting information about why glass and ceramics heat up in the microwave which I will share here.

First of all, according to this article from the Royal Society of Chemistry so-called "earthenware" ceramics are fired at categorically lower temperatures than "stoneware." As a result, a non-negligible quantity of water molecules remain inside the now-seemingly-dry "earthenware," while the vast preponderance of water molecules in "stoneware" have been removed as a result of the higher firing temperature. The conclusion is that earthenware ceramics heat up in the microwave because they have the polar water molecules in them which undergo dielectric heating. On the other hand, stoneware (and apparently porcelain) will not heat in the microwave due to their respective lack of water molecules. Either way, I still wouldn't recommend microwaving your grandmother's porcelain china to find out.

Second, glass' molecular structure is apparently locally tetrahedral but without long-range order (i.e. it is an amorphous solid) which means that there tend to be spaces in the molecular structure of the glass to accommodate ionic impurities (mostly sodium, see this explanation of how glass is made to get an idea of the chemicals that go into the final product). These impurities are only loosely bound and are able to move around within the amorphous structure of the glass. These ions of sodium or other elements have a net charge (they are ions after all) which means that the oscillating electric field produced by the microwave oven causes the ions to jostle back and forth, gaining energy. The idea is very similar to the rotations of polar molecules (which have an electric dipole but no net charge), but the mechanism is different (namely, translational energy rather than rotational energy).

So in summary, ceramics apparently heat up because they still contain some water, while glass heats up mostly because of the presence of semi-free, charged ions.

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  • $\begingroup$ I'm not sure how well the molecular model applies to glass. The core mechanism is that the matter to be heated has internal electric fields due to the non-uniform charge density. If the external electric field alternates, the internal field will never have the ability to align. $\endgroup$
    – MSalters
    Dec 3, 2014 at 14:57
  • $\begingroup$ I don't really like this kind of answer : I find it misleading because I don't think that this explains the dielectric losses. The proof is that the magnetic losses are maximum when the angle between the field and the polarization is the highest. $\endgroup$
    – TZDZ
    Dec 3, 2014 at 15:55
  • $\begingroup$ I guess ceramics can be heated in the oven because they contain water, not because they are polar. Also, I believe glass is not heated in the oven (didn't do a controlled experiment, but I remember heating a glass of tea, and the upper part of the glass was colder). $\endgroup$
    – user27542
    Dec 3, 2014 at 18:35
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    $\begingroup$ A good piece of ceramics that has been fired correctly ought to be entirely free of moisture. Now maybe the material being heated is the glaze and not the clay, but that's a different issue. Glass is another matter. There are an enormous number of different glass formulae, some of which will heat in a microwave and some of which won't. I'm not an expert on the chemistry of all glass and all ceramics, but I'll try to do a little research once I can get on a computer (not just my phone). In the meantime, feel free to put an empty glass or coffee mug into your microwave and see for yourself. $\endgroup$
    – Geoffrey
    Dec 3, 2014 at 20:44
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    $\begingroup$ If you are going to do this experiment, be careful as to the plastic you use; some (e.g. the melamine used in "unbreakable" plates and cookware) will not only heat up considerably but can also crack explosively and/or release formaldehyde. Stinks up your kitchen for ages. $\endgroup$
    – Jon Hanna
    Dec 3, 2014 at 23:44
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This Isn't About Water

Microwave heating actually has nothing to do with the moisture content of items. It has everything to do with the amount of electric dipoles (polar molecules) in the item of concern. Water molecules (with many other organic molecules) happen to be electric dipoles. (That is, one side of the molecule has a positive charge and the other side has a negative charge.)

When the oven uses a microwave to make an electric field, all electric dipoles move to align with that field. If the direction of that field quickly flips, you give these dipoles kinetic energy. As you increase a group of molecules' kinetic energy, you increase the temperature of that group.

Any material containing significant amounts of electric dipoles will heat in a microwave. Also, resonance of water molecules have nothing to do with heating food in microwave ovens. The oscillations of the waves in microwave ovens is too slow for resonance to play a role.

For further reading, check out wikipedia's articles on Microwave Ovens and Dielectric Heating, which should answer your questions more throughly.

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    $\begingroup$ The question that the otherwise fine answers of both PipperChip and of @Geoffrey miss is that why the motion of polar molecules in a microwave field is absorptive? After all, they move in sync along the electric field lines that can be quite well modeled as harmonic motion with friction. What causes the friction? $\endgroup$
    – hyportnex
    Dec 8, 2014 at 0:58
  • $\begingroup$ @user31748 Actually, the temperature of a system is just a statistical measure of the overall internal energy. The molecules become excited rotationaly which increases the internal energy thus increasing the temperature. You should probably take that friction analogy with a grain of salt. $\endgroup$
    – Geoffrey
    Dec 12, 2014 at 1:07
  • $\begingroup$ I was told to cut down on salt... Ok the internal energy increases, but the question remains: granted the increase of internal energy what is the cause dissipation. Somehow the normally and coherently oscillating dipoles become disordered. How? In the case of ferromagnetism the "friction" is between neighboring domains, but what about dielectrics? I think, and this is just an uninformed guess, the neighboring dipoles interact electrically and that interaction propagates. What is the real dissipation mechanism? $\endgroup$
    – hyportnex
    Dec 12, 2014 at 1:17
  • $\begingroup$ There is not just perfectly coherent oscillatory motion of the molecules going on - there is always some superpower random motion. So there is a chance of two molecules hitting each other - the faster they rotate the larger the transfer of energy. This in turn increases the incoherence which sustains the dissipation mechanism. $\endgroup$
    – Floris
    Dec 12, 2014 at 3:57
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    $\begingroup$ As I said, I do not understand, but I do know that the microwave oven's operating frequency has absolutely nothing to do with water or any other material resonant frequency whatsoever. Any coincidence is coincidental. The ISM (industrial-scientific-medical) band frequencies given out by the FCC were determined by regulatory/bureaucratic/interference considerations not by physics. $\endgroup$
    – hyportnex
    Jan 1, 2015 at 17:27
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I don't think it's dielectric heating .. it's more akin to inductive heating .. the establishment of eddy currents (largely surficial) which heat through resistance heating. Best material would be moderately electrically conductive substance .. hence ceramics containing metal atoms heat - where ceramics containing exclusively metal-oxide atoms - don't!

It's an AC environment so it would be more accurate to refer to the object's reactive impedance (and not it's ohmic resistance).

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