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My understanding is, if you put ice into a >0 degrees environment, it will be 0 degrees as it melts and if you put water into a <0 degree environment, it will 0 degrees as it freezes. but what happens if you put water and ice in to an environment maintained exactly at 0 degrees. I'm inclined to think it should be said water freezes below 0 and ice melts above 0, I'd go so far, pending a satisfactory answer of course, is to suggest, what happens to water and ice at 0 degrees is nothing at all. Please put me out of my misery on this :-D

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My understanding is, if you put ice into a >0 degree environment it will be 0 degrees as it melts and if you put water into a <0 degree environment it will 0 degrees as it freezes.

Correct. If you place ice in a container where the temperature is above $0\ \mathrm{^\circ C}$, the ice will first heat up to $0\ \mathrm{^\circ C}$, and then undergo phase transition to a liquid. The same works the other way around, as you stated. Placing water in a sub-zero temperature will cause the water to cool down to $0\ \mathrm{^\circ C}$ and then it will freeze (form a solid). See picture below:

image from http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/home_energy/heating_housesrev3.shtml

But want happens if you put water and ice in an environment maintained exactly at 0 degrees.

This really depends on the other qualities of water. If you add salt in to the water, it will remain as a liquid at $0\ \mathrm{^\circ C}$. This is due to freezing-point depression, which is described reasonably well in Wikipedia.

If you are considering pure water (no salt or other solvents mixed), then you will get an equilibrium of water and ice. It might be easier to understand, if you keep in mind that although the overall temperature is $0\ \mathrm{^\circ C}$, there will be some variation between single molecules, some having a lower amount of thermal energy and some having a higher amount.

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  • $\begingroup$ If you add salt to an ice/water mixture the temp of the water will drop below 0 C. The ice too must be lowered in temperature. This melts some of the ice. Then mixture now extracts heat from the environment to warm up to 0 C thus melting all the ice. $\endgroup$ – MaxW Feb 22 '17 at 23:05
  • $\begingroup$ Thanks but wouldn't the equilibrium of water and ice be, the ice parts being below zero and the water parts above zero? or would this equilibrium have a different state? $\endgroup$ – Jim G Feb 26 '17 at 6:31

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