I've seen many explanations regarding this topic where they explain it in the sense of the anomalous expansion of water, but I actually want an answer where I can understand the different forces at play in the molecular level.

I've seen a paper where it is said that when the pressure is applied the h-bonds contract thus making them stronger but this also makes the covalent bond weaker which results in higher probability of the hydrogen to get separated from one of the two oxygen atoms which in turn decreases the energy required to break the bonds therefore lowering the melting point of ice. Is this explanation correct?


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The potential energy between the O atoms in the H bond has the form of an anharmonic potential an example of the shape is the Morse potential or a Lennard-Jones 6-12. The energy is zero at large O-O separation and moves to a minimum as this distance is reduced (the point at which a the H bond exists with the H atom between the two O atoms, typically 0.15-0.25 nm). As the O-O distance is further reduced the potential rises again to zero then positive potential energy. The H bond thus forms in a lop-sided potential well, with the H atoms between the two O atoms. (I imagine the effect on the covalent bonds would be less as they are stronger and shorter than the H bond.)

Suppose that the distance between the O-O atoms is reduced then the potential rises and becomes closer to zero or repulsive (positive) potential. Suppose that the pressure causes this to happen just a little and is such that the H bond energy is now only slightly below zero. At this point the thermal energy (as molecular vibrations) could in places be enough to increase the energy above zero and so break the H bond, at which point the solid would start to melt. The structure beginning to break down could further collapse rather like a building collapses by bending and twisting if it structural girders are removed.

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    $\begingroup$ But if I apply this to CO2 the consequence should much more drastic as the molecules are attracted by London forces(which are weaker than h-bonds). But the phase diagram of CO2 contradicts with this fact $\endgroup$
    – Romcade
    Commented Jun 13, 2020 at 14:56
  • $\begingroup$ Yes, this is a good point, but the vdw are (probably) at shorter range so the effect if any would be limited to far higher pressures. Hopefully someone will know of some experimental data to sort out the actual mechanism. $\endgroup$
    – porphyrin
    Commented Jun 13, 2020 at 18:05
  • $\begingroup$ The key is that when water freezes increase its volume. Only substances that increase their volume when solid should melt if pressure is applied. $\endgroup$
    – C.X.F.
    Commented Jun 14, 2020 at 22:32
  • $\begingroup$ @blu potatos can you list some experimental data to support your suggestion ? $\endgroup$
    – porphyrin
    Commented Jun 15, 2020 at 7:17
  • $\begingroup$ If a substance increase its volume when it freezes that means that within the structure there are gaps between molecules (see Cristal structure of ice) and when pressure is applied you are forcing molecules to be close to each other something that is detrimental for keeping their solid structure that involves more spacing between the molecules. $\endgroup$
    – C.X.F.
    Commented Jun 15, 2020 at 10:41

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