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Old 11-28-2019, 11:49 AM
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Ice or water in a closed, temperature controlled chamber


In a physics, spherical cow universe, say you have a chamber being kept at exactly 0 degrees C at exactly one atmosphere. What would happen if you put in a glass of water? How about a cube of ice?

Would the water stay water? The ice stay ice? Or, would you end up with a slurry of ice and water either way?

I think it's door number three is the right answer, but others here disagree.

Also, Happy Thanksgiving!
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Old 11-28-2019, 11:57 AM
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If the chamber is "kept at exactly 0 C" by virtue of just being really well-insulated, then the water would stay as it is. You need to add heat to melt ice, or remove heat to freeze water, and that heat has to come from or go to somewhere.

If the chamber is kept at that temperature by means of some active refrigeration system with a thermostat, then it would be possible for it to go either way. But depending on how the thermostat system is set up, you probably wouldn't reach an equilibrium with some ice and some liquid. Most likely, you'd either end up stable at all liquid, stable at all solid, or oscillating between the two.
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Old 11-28-2019, 12:10 PM
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Originally Posted by RitterSport View Post
In a physics, spherical cow universe
Can you elaborate on just what "a physics, spherical cow universe" is?

ETA: OK, wiki explained that to me. https://en.wikipedia.org/wiki/Spherical_cow

I'm sorry there's no strong correlation with Gary Larson.

Last edited by Qadgop the Mercotan; 11-28-2019 at 12:13 PM.
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Old 11-28-2019, 12:13 PM
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Can you elaborate on just what "a physics, spherical cow universe" is?
"assume ideal conditions which are not realistic in practice"
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Old 11-28-2019, 12:29 PM
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"assume ideal conditions which are not realistic in practice"
Which is why I failed physics. I couldn't relate these things to real world experience.
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Old 11-28-2019, 12:46 PM
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as always, there's an XKCD For that.

https://imgs.xkcd.com/comics/experiment.png
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Old 11-28-2019, 04:30 PM
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If the chamber is "kept at exactly 0 C" by virtue of just being really well-insulated, then the water would stay as it is. You need to add heat to melt ice, or remove heat to freeze water, and that heat has to come from or go to somewhere.

If the chamber is kept at that temperature by means of some active refrigeration system with a thermostat, then it would be possible for it to go either way. But depending on how the thermostat system is set up, you probably wouldn't reach an equilibrium with some ice and some liquid. Most likely, you'd either end up stable at all liquid, stable at all solid, or oscillating between the two.
I'm probably not thinking clearly because of the turkey overload, but if it were oscillating, wouldn't that imply that it's always a slurry, except for the occasional extremes?
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Old 11-28-2019, 04:41 PM
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as always, there's an XKCD For that.

https://imgs.xkcd.com/comics/experiment.png
Here's a link to the web page for that. You get the hover text plus an excuse to catch up on the latest comics while you're there.
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Old 11-28-2019, 08:19 PM
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As described in t the OP, the answer would be “it depends”. Phase changes are an example of places where things change drastically from one side to the other.

For real world, Mr Faraday did something similar in his Christmas lectures (I think I found on Gutenberg). I think it was “fill metal can with water, put it into ice water bath, can explodes at just below freezing point as it solidifies “

There’s also the “put steam in can or bottle, quick cool, see the can implode when the steam condenses.

I prefer spherical pigs vs cows. Physically perfect bacon.
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Old 11-28-2019, 09:18 PM
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Originally Posted by RitterSport View Post
In a physics, spherical cow universe, say you have a chamber being kept at exactly 0 degrees C at exactly one atmosphere. What would happen if you put in a glass of water? How about a cube of ice?

Would the water stay water? The ice stay ice? Or, would you end up with a slurry of ice and water either way?
!
Happy thanksgiving to you too.

Hope your spherical cow universe extends to Chemistry too.

If so, 100% pure water (even fairly pure water ) will not freeze but get supercooled until about -40C. Water needs nucleation sites to freeze.

Also the melting point of ice is like 0.002 C, so it too will remain frozen because the temperature in this perfect universe is perfectly 0 C.
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Old 11-28-2019, 09:18 PM
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This is an interesting question because the chamber is kept at precisely 0º. I think the thermostat aspects of how that's done need to be ignored for "spherical cow" reasons. Point being, the temperature is kept constant. Instead perhaps think about putting water/ice outside some place that's exactly 0º so no matter what the water/ice does, it doesn't affect the temperature of its surroundings.

Anyway, my first thought is that it would stay in the state it was put into the chamber. So if you put water in, it stays water, if put ice in, it stays ice. I don't have much to back that up, but I guess the question is can the latent heat be released/absorbed when the chamber is kept at 0º? I don't think it can. It seems there's a thermal "nudge" factor that's necessary to start the freezing/melting process. So at 0.0000...1º the ice would start to melt, or at -0.0000...1º the water would start to freeze, but at exactly 0º no phase change happens.
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Old 11-29-2019, 12:10 AM
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At 0.00001º C, the melting has already completely finished. The temperature won't budge above 0 until all the melting is done.
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Old 11-29-2019, 09:44 AM
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I think your thought experiment should say the chamber walls are perfectly adiabatic so we can forget quibbling about what exact temperature you should specify.

The ratio of ice, water, water vapor and air should stay pretty nearly the same. But there are energy differences inside your chamber because of the geometry of the interfaces and the interfacial surface energies therein. All of the interfaces are going to include all of the phases and chemical species diffusing back and forth, though if the local energies in the system aren't changing, the diffusion rates are the same in each direction.

I think the local energies are going to change for a while to dissipate locally higher interfacial tension energies. For example, if there are any sharp corners on the ice, they will round themselves off, and ice will grow elsewhere on the block. I also think anyplace there are separate blobs of any species, these will coalesce. All of this will minimize the surface energies. As that process exponentially decays over the thousands, millions, and billions of years, the system will stop evolving.
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Old 11-29-2019, 11:21 AM
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While it's true that the system will evolve in that way, I'm skeptical that any of the processes would have timescales as long as thousands of years. I mean, in an open system, ice can sublimate away entire over the course of days or weeks.
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Old 11-29-2019, 11:30 AM
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I thought that the triple point of water was 0.01°C. Surely, at 0°C, the ice would remain as ice. Perhaps, over time, the interaction between negatively charged oxygen and positive hydrogens might, as Napier says above, cause 'rounding' of corners until the ice settled into a perfect sphere (assuming no gravity)

Last edited by bob++; 11-29-2019 at 11:31 AM.
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Old 11-29-2019, 12:15 PM
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Originally Posted by RitterSport View Post
...say you have a chamber being kept at exactly 0 degrees C at exactly one atmosphere. What would happen if you put in a glass of water?
It will freeze. Because the melting point of pure water is not 0 °C.

The last time NIST measured it (in 1995), they determined the melting point of pure water is 0.000089 °C ± 0.000010 °C.
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Old 11-29-2019, 12:22 PM
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I thought that the triple point of water was 0.01°C.
It is. But it doesn't have anything to do with the melting point of water. The triple point of water can only occur when the pressure is 611.657 pascals, which is less than 1% of standard atmospheric pressure.
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Old 11-29-2019, 12:23 PM
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I thought that the triple point of water was 0.01°C. Surely, at 0°C, the ice would remain as ice. Perhaps, over time, the interaction between negatively charged oxygen and positive hydrogens might, as Napier says above, cause 'rounding' of corners until the ice settled into a perfect sphere (assuming no gravity)
A triple point is specified by temp and pressure. Well, partial pressure which isn't all that easy to get your head around if you haven't studied this. In the case of water it is 0.00603659 atm. So conditions have to be juuuust a bit right.
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Old 11-29-2019, 04:33 PM
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While it's true that the system will evolve in that way, I'm skeptical that any of the processes would have timescales as long as thousands of years. I mean, in an open system, ice can sublimate away entire over the course of days or weeks.
Yeah, but in an open system the driving potential is most of the vapor pressure of water at those conditions. In the OP's closed system the driving potential is a very tiny fraction of that, especially in the limit as the sharpest curvatures round themselves off.


On another note, the OP doesn't say whether the chamber has exclusively water in it, or was there air in there too? That is, is this a special chamber where we pump a hard vacuum before introducing the water and ice, and were they purified so as not to have dissolved air in them?

Note the ice point and the triple point are two (slightly) different temperatures.
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Old 11-29-2019, 05:17 PM
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Yeah, but in an open system the driving potential is most of the vapor pressure of water at those conditions. In the OP's closed system the driving potential is a very tiny fraction of that, especially in the limit as the sharpest curvatures round themselves off.


On another note, the OP doesn't say whether the chamber has exclusively water in it, or was there air in there too? That is, is this a special chamber where we pump a hard vacuum before introducing the water and ice, and were they purified so as not to have dissolved air in them?

Note the ice point and the triple point are two (slightly) different temperatures.
If you pump a hard vacuum, then we'd have to change the temperature. I do specify that it's exactly at one atmosphere.

Crafter_Man, good catch, but let's assume that the chamber is exactly at the freezing/melting point of water at whatever the exact pressure is.

Napier, I'm not quite sure what you're saying. Does the end point matter whether you start with a cup of water or a cup of ice? If you started with a slurry that's already at 0 degrees C (+/- the Crafter_Man factor)?
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Old 11-29-2019, 05:52 PM
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The H2O can be held at 0 deg C while adding or removing heat, and this will affect the relative amount of liquid or solid. The OP is not clear on the conditions. Liquid water has 80 calories per gram more heat than ice. If you start with 100% liquid and remove 80 calories per gram, at equilibrium, it will be 100% ice while remaining at 0 deg C. If you start with 100% liquid and remove no heat, equilibrium will still be 100% liquid. If you remove some heat, but less than 80 calories per gram, it will become a mixture of liquid and ice.

In the case of a having a mixture, the ice would probably end up as a big crystal rather than slush.

I am ignoring the additional complication that the H2O is in a glass that is inside a chamber of gas, since OP asks for spherical cow type assumptions.
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Old 11-29-2019, 09:43 PM
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The weight of the water/ice on itself has not been addressed. If the pressure at the surface of the water/ice is exactly one atmosphere, the pressure at the bottom of the container will be higher, resulting in a lower freezing point at the bottom. Back of the envelope calculations put the freezing point at a depth of 10 cm of water to be roughly .00075 deg C lower than at the surface.


Consider this experiment. Take equal masses of water and ice at approximately 0 deg C and mix them in a cylindrical flask to a depth of 10 cm, then put the flask into a perfectly insulated isolation chamber made by Spherical Cow Inc. After a period of days,weeks, or months, I would suppose that you would wind up with a layer of nearly solid ice in the (roughly) top half and mostly water in the bottom half, due to the pressure gradient.



If you read Crafter_Man's link from the NIST, they took hydrostatic pressure into account in determining the ice point. I take that to mean the ice point is the temperature at which the top layer of water will freeze, but the underlying water will not. So if we perform the OP's experiment at this temperature, I think all of the ice cube will melt except a thin layer. Starting with a glass of water, it will likely supercool and stay liquid, but if you drop a snowflake into it as a nucleation site, the top layer will freeze.
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Old 11-29-2019, 10:06 PM
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If you read Crafter_Man's link from the NIST, they took hydrostatic pressure into account in determining the ice point. I take that to mean the ice point is the temperature at which the top layer of water will freeze, but the underlying water will not. So if we perform the OP's experiment at this temperature, I think all of the ice cube will melt except a thin layer. Starting with a glass of water, it will likely supercool and stay liquid, but if you drop a snowflake into it as a nucleation site, the top layer will freeze.
Btw that link also says to mix up the (ultra-pure) shaved ice and water into a homogeneous slurry, and to keep adding ice + pumping water out of the bottom precisely so you do not get a layer of water on the bottom. So the (real) ice-point measurement is not quite the same as melting ice cubes in a sealed box.
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Old 11-30-2019, 04:37 AM
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The weight of the water/ice on itself has not been addressed. ....
Assume we're doing this in space. Or, whatever you need to answer the question I'm trying to get to:

If you hold a block of ice or a glass of water at exactly the melting/freezing point, will it stay at its current state, transition to the other, or form a slurry?

The reason I think it will be a slurry is that, through some random movements, you'll always have ice crystals breaking apart to become water and water molecules sticking together to become ice. At the exact temperature, neither is preferred, so both will exist however you started.

I disagree with Chronos's statement about oscillation because I don't see why all of the molecules would go one way or the other. However, he's a lot smarter than I am when it comes to the sciences, so I'm happy to be shown the error of my ways.
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Old 11-30-2019, 10:22 AM
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The oscillation, if it happened, would be due to the operation of the thermostat. Let's say that it's set up to add heat if the temperature is ever below -0.001º, and remove heat if the temperature is ever above 0.001º. In that case, the thermostat wouldn't switch until after all the water had changed phase. If you started off with a mix of liquid and solid, and the chamber was gradually gaining heat, the temperature would stay constant at 0 until all the ice melted, then it would increase until it reached the thermostat's high set point, and then the chamber would start losing heat, and so it'd go back down to 0 and then start re-freezing, and once it'd completely re-frozen, it'd start decreasing the temperature again until it reached the low set point, and so on.
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Old 11-30-2019, 05:02 PM
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If you pump a hard vacuum, then we'd have to change the temperature. I do specify that it's exactly at one atmosphere.

Crafter_Man, good catch, but let's assume that the chamber is exactly at the freezing/melting point of water at whatever the exact pressure is.

Napier, I'm not quite sure what you're saying. Does the end point matter whether you start with a cup of water or a cup of ice? If you started with a slurry that's already at 0 degrees C (+/- the Crafter_Man factor)?
Oops. I missed the point that you said it was at one atmosphere. That means you have air in there too.

I think the end point depends on how much water and how much ice you start with. If you keep your walls adiabatic, the total energy in the closed system can't change, so to a pretty close approximation the ratio of ice to water can't change.
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Old 12-01-2019, 07:59 AM
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The chamber is being kept at exactly 0 degrees (or the freezing/melting point) by some magic mechanism. Not sure if that clears things up.
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Old 12-01-2019, 08:24 AM
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Adding magic to a science question always does the exact opposite from clearing things up. We had questions that needed to be answered if the original question could be answered. And now, you're not only not answering those questions, you're declaring that they cannot be answered. And thus, the original question can't be answered, either.

But I'll take a stab at it. If the temperature is being held constant by magic, then all of the water spontaneously turns into cotton candy.
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Old 12-01-2019, 08:47 AM
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Ok, let me try one more time. Maybe the answer is, this is unknowable.

If I put a block of ice or a glass of water in a chamber being kept at exactly 0 degrees, will the ice melt or the water freeze or something in between? Or, since no chamber can be kept at that temperature perfectly in real life, the answer is, depends on the chamber? Is 0 degrees a discontinuity where the state of the water is in principle unknown?
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Old 12-01-2019, 08:53 AM
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You don't need magic, just make the chamber out of a black body material, then plunge the chamber into a gigantic ice bath.
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Old 12-01-2019, 09:37 AM
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Quote:
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If I put a block of ice or a glass of water in a chamber being kept at exactly 0 degrees, will the ice melt or the water freeze or something in between?
You need to state your assumptions.

Are you assuming water melts/freezes at precisely 0 °C?
Are you assuming ever gas molecule inside the chamber is precisely 0 °C?
Are you assuming the chamber walls are precisely 0 °C?
Are you assuming no air convection currents, and that the air is perfectly still?
Are you assuming the isotopic composition of the water is VSMOW, or something else?
Are you assuming the liquid water has no vapor pressure?
Are you assuming the liquid water has no convection currents, the water is perfectly still, and has no internal pressure gradient?
Are you assuming the block of ice has no vapor pressure, and no internal pressure gradient?
Are you assuming there's no such thing as supercooled liquid water?

And so on, and so on...

If you make enough assumptions, you can create a simple, mathematical model where the liquid water would get closer and closer to 0 °C without actually "getting" there. Same goes for the ice. But it's an uninteresting model, and not really worthy of an in-depth discussion.
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Old 12-01-2019, 11:05 AM
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And the answer is what I said in the first reply: What happens to the ice depends on how you're keeping the temperature there.
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Old 12-01-2019, 11:19 AM
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I think the OP wants the basic physic interpretation, not to be further confused. More information other than temperature and pressure needs to be specified to determine the relative amounts of solid and liquid. Liquid water at 0 deg C and atmospheric pressure has a certain density and specific internal energy. Ice at 0 deg C and atmospheric pressure also has a particular density and specific internal energy, but those are different than the values for liquid water. When you have a mixture of liquid and solid at 0 deg C and atmosphere pressure, you need to know the total amount of internal energy (or some other thermodynamic information) to figure out the relative amounts of liquid and solid.

That is why you need to know how this is being maintained at this temperature and pressure. If no heat is being added or removed (or, equivalently, volume is held constant), the relative amounts of liquid and solid will not change. If you start with 100% liquid and increase the volume a little (i.e., remove some heat) while maintaining the temperature and pressure, you will get some ice to form.

If you have a 50-50 mixture of liquid and ice, and maintain the temperature and pressure while adding or removing no heat, it will remain 50-50. Since the molecules are constantly moving around, which molecule is part of a liquid or solid phase will change with time. That is the basic thermodynamics interpretation. Beyond that you have to worry about super cooling and crystal formation related to surface energy effects. With time, I think fairly stable crystals will form.
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Old 12-01-2019, 05:51 PM
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I think the OP wants the basic physic interpretation, not to be further confused. More information other than temperature and pressure needs to be specified to determine the relative amounts of solid and liquid. Liquid water at 0 deg C and atmospheric pressure has a certain density and specific internal energy. Ice at 0 deg C and atmospheric pressure also has a particular density and specific internal energy, but those are different than the values for liquid water. When you have a mixture of liquid and solid at 0 deg C and atmosphere pressure, you need to know the total amount of internal energy (or some other thermodynamic information) to figure out the relative amounts of liquid and solid.

That is why you need to know how this is being maintained at this temperature and pressure. If no heat is being added or removed (or, equivalently, volume is held constant), the relative amounts of liquid and solid will not change. If you start with 100% liquid and increase the volume a little (i.e., remove some heat) while maintaining the temperature and pressure, you will get some ice to form.

If you have a 50-50 mixture of liquid and ice, and maintain the temperature and pressure while adding or removing no heat, it will remain 50-50. Since the molecules are constantly moving around, which molecule is part of a liquid or solid phase will change with time. That is the basic thermodynamics interpretation. Beyond that you have to worry about super cooling and crystal formation related to surface energy effects. With time, I think fairly stable crystals will form.
I think this is what I'm looking for. Thanks!
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