Melting ice: reversible or irreversible? 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?
 A: 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.
A: 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. 
