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[noodle0117]

Let's say I have a thermally insulated container (heat cannot get in or out) filled with water that is exactly at 0^oC.
I have also placed a lump of ice at exactly 0^oC in the water where it is completely submerged.



Now if we were to ignore the effects of any shrinking/expanding of ice/water:

How much ice will melt? (All, some, or none)
How will the temperature of the water change? (increase, decrease, or no change)

[kaijyuu]

I believe nothing would change. The ice would stay ice and the water would stay water.


Water changing from a high energy state to a low one is exothermic. So were some of the water to freeze, it'd heat up some of the other water, resulting in a balance. Since it's already uniform temperature, it's already balanced.

[palsch]

It would be dependent on pressure from what I can see.

You have a fixed volume at a fixed temperature (net energy). The pressure the system is under will determine exactly how much water freezes or ice melts.

In a fairly complicated manner of course. Water is weird stuff.

[lordcooper]

Wouldn't it all be ice at 0 degrees?

[Urist Imiknorris]

I agree with kaijyuu.

[Loud Whispers]

'Tis a dynamic equilebrum. The water should freeze. The ice stays frozen. The temperature stays the same.

Water changing from a high energy state to a low one is exothermic. So were some of the water to freeze, it'd heat up some of the other water, resulting in a balance. Since it's already uniform temperature, it's already balanced.

Exothermic is only applicable to a reaction that releases heat, this is just a physical change occurring from the lack of energy. It's basically a tub full of ice and more ice with the same amount of heat-energy.

[kaijyuu]

Hrm, well I'm remembering stuff from way back in high school chemistry. I do specifically recall that state changes can be exothermic or endothermic, though (the chemistry teacher specifically mentioned it being one of the reasons steam is dangerous). Maybe I'm remembering wrong.

[MonkeyHead]

Dont forget "latent heat". Water at zero degrees still needs energy removed from it to change state (exothermic), and the ice will need energy added to it to melt it before the temperatre rises above zero (endothermic)... IIRC these two quantites will be equal, and seeing as the system is hermetically sealed and hence 100% isolated from the rest of the universe, there should be no overall noticable change due to the whole system being in a dynamic equilibrium.

[The Fool]

Assuming it was pure water with no salt, the water should all be either frozen or liquid, not a bit of both. If every molecule is uniformly 0 degrees Celsius and the same liquid it could only be one state. As written I'd say that nothing would change since it's all the same temperature, therefore no energy to transfer.

[Miggy]

When things freeze/thaw, there are two processes going on:

*The things are heated or cooled, emitting or attaining heat energy. This energy is described by its heat capacity, its mass and the temperature it changes.

*The things release/obtain energy in order to build or break the solid state that is the frozen water. When water is frozen, it is more stable than when it is a liquid, so therefore it will release energy freezing (an exothermic reaction), or require energy applied to it when thawing (an endothermic reaction).

In this case, both ice and water are at 0C, and we'll assume 1 atm because otherwise this question wouldn't be "interesting". This is the exact freezing point of water, the only energy involved is the energy required to arrange the water molecules into the ice crystal, and the subsequent "heating" of the surrounding water or ice when this takes place.

Seeing as no energy can be wasted, you will always have the same mass of ice in the box, and always have the same mass of water in the box. But the entire thing will not freeze, since doing that will release a lot of energy, enough to thaw all of the ice that has just formed. And the ice will not melt, since melting the ice will make the water reduce its temperature, making it sporadically form new ice crystals.

EDIT: If you instead of 0C and 1 atm had 0,01C and 0,006 atm you would be at water's triple point, where not only would water and ice be capable of spontaneously interconverting, but so would water and steam, and steam and ice. You would be able to pose the same question with not only a box with water and an ice cube, but a box with water, an ice cube and a bubble of steam. And the answer would probably be the same (not sure of the latent heat in steam will cancel out the ice crystals, but on the other hand, water will spontaneously form into steam and ice anyways, so the answer would probably be the same).

[Virex]

An interesting question in this case is: What role does the surface energy play. Intuitively, the system should be able to lose entropy by minimizing the water-ice interface, since at the interface, water molecules are constrained in their motion. The system should be able to minimize the surface of the ice crystal given enough time because a dynamic equilibrium exists between the water and the ice. So, given enough time, the ice should become a sphere. But in doing so, it releases surface energy, which adds heat to the system. That heat would cause some of the ice to melt. So I'd say that if you were to open the box after an infinite amount of time, after opening, you'd find a spherical ice crystal that weighs slightly less than the amount of ice you put in.

[Capntastic]

It takes energy to form into ice, so it would not do so, assuming a perfectly insulated container.

[Virex]

It takes energy to form into ice, so it would not do so, assuming a perfectly insulated container.

At 0 degrees forming ice doesn't cost any energy? 0 degrees Celsius is, by definition, the point at which the formation of ice goes from endothermic (above 0 C) to exothermic (below 0 C).

[kaijyuu]

Forming ice releases energy. Melting it costs extra energy than what it would normally take to raise the temperature.



This is why it's really easy to get 0 degree water. Just get some crushed ice and mix.

[Virex]

Screw me, you're right. At 0 degrees, the free Gibbs energies of the two phases are equal, but melting is still exothermic.


My point still stands though, the energy of fusion could and should be provided by a reduction in the surface energy, given enough time.