Say I have a refrigerator running at -30C and inside the refrigerator is a container containing 100mL of pure ethanol with a 100mL block of pure H2O floating in it.
Pure ethanol has a freezing point of -114C, A 50/50 ethanol water mix has a freezing point of -40C and water has a freezing point of 0C.
So my question is, as time goes towards infinity, what will happen to the system. On the one hand, the ice shouldn’t melt as it’s below the melting point of ice. On the other hand, it’s above the melting point of the ethanol-water mixture.
The ice will remain frozen and the ethanol will remain a pure liquid. The freezing point of an alcohol/water mix has no connection to the freezing point of the water in this case.
I’m not sure that’s true in this particular case. I believe pure ethanol very strongly attracts and holds water.
So I think here, at least some of the ice will gradually melt/sublimate/dissolve into the alcohol, at least to some percentage. I suppose there might be a more-or-less stable equilibrium at some point, though I don’t know how to calculate it.
This is all assuming that the container is airtight. Otherwise, the ice will just (gradually) sublimate into the very dry freezer air, and eventually you’ll have ninety-something percent alcohol (with some water), and no ice at all (except on the freezer walls).
I disagree. The ice will not melt as much as dissolve into the ethanol. This will be an endothermic process and the temperature of the mixture will drop below -30 C. To what degree will depend on the heat transfer into the vessel, surface area of the ice block, etc. Once all the ice has dissolved, the temperature will rise to -30 C and you will have a liquid 50/50 mixture of water and ethanol.
You can see this effect to some degree with some water, ice, and salt. Make up an ice bath, so you have a nice mixture of water and ice all at 0 C. Dump in some salt and stir it up and you should now have a sub-zero mixture of ice and water.