Do microwaves break hydrogen bonds? We're told that the photon energy of microwave frequency radiation ($\sim 10^{-5}~\text{eV}$) is not high enough to break hydrogen bonds.  But if that's true, how does dielectric heating of water work? Liquid water is a network of polar molecules held together by H bonds so that they CAN'T rotate, in concert with the microwave beam or anything else ....
Seems like this is a problem of the two ways of looking at radiation -- classical wave vs photon stream -- being incompatible.
 A: In a solid or liquid we have collective vibrations of the whole system. It can be useful to think of these as quasiparticles called phonons, that is when we add vibrational energy to the system as a whole it generates a phonon, or conversely a phonon can decay and emit energy.
Black body radiation is (mostly) the emission of photons from the collective vibrations i.e. the decay of phonons to emit photons, and dielectric heating is the reverse process i.e. the absorption of photons to create phonons. This is what happens in your microwave oven. The heating is due to the excitation of the collective vibrations, not to the interaction of photons with hydrogen bonds. The quanta of these collective excitations (i.e. their phonons) are generally very small so they can absorb photons of even very low energies.
In real materials the collective vibrational modes are anharmonic oscillators so they all interact with each other and the vibrational energy is distributed between them in accordance with the Boltzmann distribution. That means the vibration energy from the absorbed microwave photons is quickly equilibrated with higher energy modes such as the vibrational excitations of hydrogen bonds, and it can break those bonds. This means energy can be absorbed in small units from microwave photons and still build up sufficiently to break the much higher energy hydrogen bonds.
A: I would think that microwaves are only sufficient to factor into the rotational modes of the water molecule and that they couldn't "break" per se, the hydrogen bond. Even if they did, I think the Van Der Walls forces would make a quick repair!
A: You don’t need an individual quantum of energy to break any waves; you just need the energy to be absorbed.
It’s like pushing someone on a swing (with rigid arms on the swing). You don’t need to be strong enough to push them so they make a complete loop after a single push: it would be enough to keep pushing at the right time, and if there is sufficiently low friction in the system you will eventually push them “over the top”.
The vibration / rotation modes of water molecules are coupled - excite one, and over time some of the energy transfers to other modes. So the entire system absorbs energy, until bonds start to break. In fact, even without adding energy bonds are breaking (and forming) all the time - again, because the energy of an individual water molecule will change as they “jiggle”.
We can say something about the average energy of a molecule - but usually not about the energy of the individual molecule.
Does that make sense?
