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HK911
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Question: What is the best way to melt ice?
Lojzek said:No, pressure should be increased to melt ice. But I calculated this effect is to weak to achive impressing results: about 5*10^-5 of the mass of ice will melt for each bar of pressure increase.
This metod would make sense only if we were limited to very short times.
TeTeC said:I cannot show that increasing pressure will work better than decreasing pressure, but I can explain why increasing pressure leads to melting :
http://www.bae.uky.edu/~snokes/BAE549thermo/gasesv1.jpg"
You can see on this diagram that when you are bellow the triple point temperature and around the 1 kPa, increasing the pressure will lead to melting. But it only works for water.
As I've said, I don't know which way is the fastest.
Schrodinger's Dog said:Ok why would increasing pressure work better than decreasing pressure and heating? Not sure I get that?
HK911 said:Thank you very much for the comments. Changing the pressure would be best, but unfortunately, we can't use any external source. The only external sources we can use are the room temperature and gravity for instance.
billiards said:Water (H2O) is unusual in that the solid phase is less dense than the liquid phase, hence ice floats on water - a rare physical property which actually is essential for life as we know it*.
This relates to the fact that under typical surface conditions (i.e. what might be considered normal pressures and temperatures at the surface of a planet) H2O has a negative Clapeyron slope - that basically means that the melting temperature goes down as the pressure is increased. Consider this: imagine you had a body of water all at the same temperature, from experience we know that the ice will form at the surface of the water (I guess we have to be careful here - maybe that happens because the air above the surface is colder, but I believe in laboratories we can control the conditions and can convince ourselves that it is not because of the colder air at the surface); now where is the pressure the greatest? Of course, the pressure is greatest at the bottom of the water because it has a load of water on top of it, but the ice doesn't form at the bottom - the ice forms at the top where the pressure is lowest. So does this not mean that water has a higher freezing point at lower pressures?
That's my understanding anyway.
*If ice was denser than water then the oceans would have completely frozen, because there wouldn't have been an insulating cap of ice at the top to stop all the water from getting very cold (and then freezing solid), life needs liquid water, ergo, if the oceans were frozen we wouldn't have life.
HK911 said:Above is a few of the frequently asked questions. If we used the plates, would it be best to place it in some sort of container? We were thinking of designing a box made from acrylic with holes in the bottom and top to all air to flow. Should be do that are just let it sit in the air with no casing around it?
Uh, ok, you're right, but I don't remember claiming to prove anything, or even hinting that "reducing and heating would (not) be better than increasing pressure" ... whatever .Schrodinger's Dog said:Yeah I know that but it doesn't prove that reducing and heating would be better than increasing pressure at all.
No they wouldn't, even if the ice formed in the deep it would float up to the surface by buoyancy. You can't be still be claiming that the melting temp of ice is higher at higher pressures, I would agree if we were talking about ice III, V, or VII, but we're talking about everyday "normal" ice here: ice I, which if you look at the http://www.homepages.ucl.ac.uk/~ucfbanf/water_ice.htm" for water ice you will see has a negative Clapeyron slope. As I mentioned earlier this is important for life, because it means that under great ice bodies, where the pressure is high, the ice can melt more easily providing underwater reservoirs which may harbour life (chemoautotrophic life, probably) - c.f. Europa. It's also an important mechanism for the movement of glaciers, the water at the bottom of a glacier pushes the ice upwards reducing friction with the ground accomodating "basal sliding".Bodies of water would also freeze from the bottom up making life difficult to get started in the first place.