Microwave ovens are tuned away from water resonances to allow deep penetration and gradual energy deposition with depth.
Melting occurs when the various binding energies of the solid's constitutent units are exceeded by enough of the tail of thermal energy. Discrete pure structures usually have narrow melting ranges unless they decompose first. (Consider incommesurrate melting for mullite.) Mixtures can have broad melting ranges for the variety of interactions possible. Polymers may have problems disentangling into a melt or fitting into a crystal lattice. Glycerin is a solid at ambient temp but it viciously undercools. Cast TNT must be thermally annealed or it changes shape and density over time as it settles into a more stable structure.
Choline chloride has mp=302 C, urea has mp=133 C. A 1:2 mixture freezes at 12 C. Transition metal oxides (e.g., rust) do not dissolve in ordinary solvents without reaction. They all dissolve in this stuff. Go figure.
