I am looking into whether the melting of ice (or any substance for that matter) at constant pressure and temperature is reversible or irreversible. Different sources say different things, and it may well depend on specific conditions. But is it generally said that melting is reversible or irreversible?

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    $\begingroup$ Reversible process refers to those process in which we can return to the initial step by changing the external condition infinitesimally; you can reverse the melting by infinitesimally changing the external temperature ie. by freezing. So, indeed, IMO, I'd take melting of as reversible. $\endgroup$
    – user36790
    Mar 27, 2016 at 12:26
  • $\begingroup$ I'd add to the above comment by saying that physical changes, i.e., ones where there is no change in the structure of the component atoms, is always a reversible change. The reversal can always be brought about by addition or removal of energy! $\endgroup$ Mar 27, 2016 at 12:58
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    $\begingroup$ @user36790 It is probably worth pointing out (correct me if I am wrong) that for your argument to hold, the temperature of the surroundings must be at $0^\circ~\textrm{C}$ (assuming atmospheric pressure). I think following your argument that if the temperature of the surroundings is above this then it won't be reversible. $\endgroup$ Mar 27, 2016 at 13:18
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    $\begingroup$ @Quantumspaghettification: Use ^\circ~\textrm{C} in place of \degrees C. $\endgroup$
    – user36790
    Mar 27, 2016 at 13:19
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    $\begingroup$ At the melting point it is reversible - you can add or subtract enthalpy to shift the amount of liquid vs solid, but the free energy of each phase remains the same. $\endgroup$
    – Jon Custer
    Mar 27, 2016 at 15:11

2 Answers 2


To cause a phase change from a solid to a liquid (melting of ice) heat is required. That requires exposure of the solid to an environment whose temperature is greater than the solid. Heat transfer over a finite temperature difference is irreversible.

However, the process can theoretically be made reversible if the temperature difference is infinitesimal. Problem is the rate of heat transfer is proportional to the temperature difference (for conduction and convection) or proportional to the difference in the temperatures to the fourth power (for radiation). That would make the time to melt infinite for a reversible process.

All reversible processes are idealizations. All real processes are irreversible. Perhaps this is one of the reasons you find "different sources say different things".

Hope this helps.

  • $\begingroup$ This is the correct explanation. $\endgroup$ Mar 5 at 15:33

It's irreversible. The reason can be easily understood when you look the molecular properties of water

The presence of a charge on each of these atoms gives each water molecule a net dipole moment. Electrical attraction between water molecules due to this dipole pulls individual molecules closer together, making it more difficult to separate the molecules and therefore raising the boiling point. This attraction is known as hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at timescales faster than 200 femtoseconds. However, this bond is sufficiently strong to create many of the peculiar properties of water, such as those that make it integral to life.

This hydrogen bonds are mixing the water, and are the reason for the lower volume of fluid compared to ice.

If you have absolute pure water, it will melt in 0 degrees in normal pressure, but you might need to cool it down to -48 degrees celcius to make it solid again. The reason is the ice lattice structure, which needs more volume than the water.

Ofcourse everything is sort of Reversible; it depend's only from definitions. But here I mean with irreversibilty that the we expect the same atom's to take the exact same positions they had before; -this is impossible. As they are not even on same molecules, they were before.

  • $\begingroup$ "But here I mean with irreversibilty that the we expect the same atom's to take the exact same positions they had before" - that's not what reversibility of a thermodynamic process means. $\endgroup$
    – ACuriousMind
    Apr 17, 2016 at 19:47
  • $\begingroup$ It seems clear to me that the definition of reversibility adopted is the one pointed out by @MAFIA36790. In this case his answer is correct. $\endgroup$
    – Diracology
    Apr 17, 2016 at 19:49
  • $\begingroup$ @ACuriousMind Ok, so the pure water/ice melting at zero degrees but freezing at -48.3 is reversible with your thermodynamical means. :-) Pls. draw us the carnot-diagramm.... $\endgroup$
    – Jokela
    Apr 17, 2016 at 19:54
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    $\begingroup$ @Jokela The answer is wrong. It is not a matter of "how we define reversibility". At the melting point (constant $T$ and $P$, as the OP specifies) the chemical potential of ice is the same as that of liquid water. This makes the ice/water system an equilibrium state and the interconversion between phases a reversible process. $\endgroup$
    – Themis
    Aug 24, 2019 at 22:29
  • $\begingroup$ @Themis It's theoretically possible, that you have frozen distilled water which has needed to be struck to to get the lattice structure creation started because though supercooled down to -20 didn't get it frozen. When this ice is then heated up to 0 deg and then melted to 0 deg water and then cooled down to - 1 water again, it very well might remain liquid, if it's just the reversibility of temperature which you are looking. $\endgroup$
    – Jokela
    Aug 25, 2019 at 5:45

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