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I'm researching about the Mpemba effect and I came across an explanation saying that the effect occurs because at higher temperatures, the hydrogen bonds between the water molecules stretch and store energy and when the water starts to cool, the hydrogen bonds go back to normal and release the stored energy on top of the normal cooling process, that's why hot water freezes faster than cold water.

I'm quite new to hydrogen bonds, so I don't really understand how hydrogen bonds can store energy. I thought bonding was something like a magnet attracting something, the further you get, the weaker the attractive force becomes. Or am I understanding it wrong?

The article that I saw this in is Hot water freezes faster than cold water; physicists solve the Mpemba effect

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  • $\begingroup$ Hydrogen bonds are like other chemical bonds. They can stretch and then get contracted. It is like a teeter-totter, or like a spiral spring, or a leaf spring. If you give them some energy they will stretch far away from the equilibrium position, and oscillate later on. . $\endgroup$
    – Maurice
    Mar 20 '21 at 15:44
  • $\begingroup$ @Maurice What do you mean by oscillate? $\endgroup$
    – prata
    Mar 20 '21 at 16:20
  • $\begingroup$ Even if you assume that, while returning to their low energy state they will have to release energy working against freezing. The only way could be something relegated to kinetics.... $\endgroup$
    – Alchimista
    Mar 20 '21 at 16:29
  • $\begingroup$ @Brien Lim. Oscillation is like a vibration. A bond AB between two atoms A and B is continuously vibrating between a short and a long separation distance AB. At low temperature the amplitude of the vibration (= difference between longest and shortest AB distance) is small, maybe a couple of percent of the average AB bond. If energy is transmitted to this bond, it starts vibrating or oscillating more. The amplitude of the oscillation is larger. $\endgroup$
    – Maurice
    Mar 20 '21 at 16:43
  • $\begingroup$ We should read the original paper, at least about the freezing effect. H-bond vibrate as all the other bonds, with the difference that they have lower strength and are less defined (in the sense that a sample will have a multitude of slightly different ones). $\endgroup$
    – Alchimista
    Mar 21 '21 at 9:54
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Yeah this question is actually surprisingly difficult to to answer. But let me explain it this way. In thermodynamics you can't have negative temperature. Thus anytime you are getting heat, you are actually gaining electrons. When you gain electrons these chemical bonds change and rearrange themselves, which is how they can store energy.

We do not think about this normally because we look at hydrogen and we just see a proton and electron. But in reality, unless hydrogen is in an extreme environment, it likely has absorbed energy in some manner.

For instance, if you put a glass of water under concentrated sunlight it will absorb photons, these photons will be absorbed by the moving electrons inside the atom. Which we understand as "heat", moreover If there are more electrons than water molecules than it will start to evaporate because they run out of room.

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