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?
Best Answer
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.