# How to melt ice?

## Main Question or Discussion Point

Question: What is the best way to melt ice?

## Answers and Replies

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I am working on this project where we have to melt some ice in about 2 mins. The experiment will be taken place indoors, at room temperature. We have to melt as much ice as possible in that time. No salt or any chemicals can be used. Conduction or convection is all we can do. Any suggestions for materials or a way to design this.

mgb_phys
Homework Helper
If you can't use chemical means then it is simply a matter of getting as much heat into the ice as quickly as possible.
A couple of factors to consider
Ice is a poor conductor of heat so just heating the outside of a large block isn't a good plan.
Water has a very high heat capcity so heating the melted water will waste a lot of energy.

Danger
Gold Member
If it's allowed, break the ice into as many small pieces as you can to maximize the surface area in contact with the air.

Melting temperature also depends on the pressure: increase of pressure will decrease melting point. If ice is at T=0, then any increase of pressure will cause some ice to melt instantly (until melting uses so much heat that T reaches new melting temperature). The higher the pressure, the larger part of the ice will melt.

Obviously low pressure and high temperature, however you achieve that.

Can you think of ways to lower pressure and simultaneously increase temperature? If you can then any method will be only using convection and conduction.

Of course as Danger said if you could atomise or at least break it down very quickly before you start, that would be good but I doubt you can do that easily. Get a lump hammer and crush it all to hell then stick in a bell jar, which gradually but obviously very slowly sucks the air out and heat said bell jar.

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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.

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.
Ok why would increasing pressure work better than decreasing pressure and heating? Not sure I get that?

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.

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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.
I think that only works for cool ice though, if you heat it and lower pressure, I'm not sure but I think it may well work better.

You do understand why ice is less dense than water at certain temperatures?

I don't know for sure which is better either given two minutes. But I guess if you can decrease pressure and heat it fast enough, it may well work out better.

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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.

Take the ice, break it up onto some sort of container. Get something that's very heavy and place it on top of the ice chunks, then rotate the container while keeping the heavy item on the ice stationary.

I see something like this
{ || } <--- the thing on top of the ice
[container]

I haven't done this, but since you can't use chemicals, maybe you can use the friction and pressure that gravity exerts on the "thing on top of the ice" to melt the ice.

I'm not sure HOW much ice you're using, which is why I suggest breaking them up into bits.

Definitely break it up if you can. Put it in a vertical tube open at both ends so that will generate a cold draft downward. This will draw warmer air into the top and down across the ice. It would be good if the ice were on a screen of some sort so the water could drain away and the air could flow around each piece of ice. But as the pieces get small they will fall through a screen.

Andy Resnick
I'm surprised nobody simply suggested pour as much boiling water as you can into the ice. If that's not allowed, then pee on it. Or is hot water considered a chemical?

After smashing up the ice.

If you're not allowed to physically crush the ice, heat it on a wire rack. Allow the ice to sink into the rack and the water to drop off.

But crushing is better.

Thanks alot of the comments once again. Heating is not allowed. You can only heat it up using the light in the room which is useless. Also the ice cannot be crushed before the experiment, it must be crushed by some means during the experiment. The vertical tube is a great idea. But in terms of the melting water, get rid of it or let the ice sit in it?

it is possible to generate some frictional heat during the experiment? Is it allowed to bring some tools or materials for the test? If it is allowed, bring a meat tenderiser to crush and apply pressure to the ice.

I think, drain the water away so you don't use some of your heat to evaporate the water. How 'bout building a heat sink into the ice container.

Use a thawing plate. Big block of metal (copper would be good, aluminum is cheaper). Maybe cut some grooves into it for drainage if you're feeling particularly creative.

How much ice is being provided?

Ok why would increasing pressure work better than decreasing pressure and heating? Not sure I get that?
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.

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Probably what I'd do is crush the ice, just as much as possible. If you can somehow reflect the light & focus it as well, that might help. Maybe even the classic black cloth might work... then again, I'm not really sure what the rules are. It seems you can't use pressure but you're allowed to crush it? This I don't really understand. Using pressure will melt the ice, its just like if you step on ice in the winter time, it changes to water & you slip & fall. If you're crushing the ice, I'd leave it sit in the water, because it will melt faster (the classic water is warmer than ice, particles move around speed up melting of ice process).

Are you allowed to breathe on it?

LURCH