Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: einsteinium252 on 21/09/2008 23:51:16

Title: What happens to ice when it can't expand?
Post by: einsteinium252 on 21/09/2008 23:51:16

I didn't know exactly where to put this, so I put it in the chemistry section because it is a question concerning matter.

Alright, everyone knows that water expands when it is frozen (about 8% of its original volume), but what happens to water if it can't expand?

Let's say that water is put into a metal sphere that is indestructible and the whole thing is put onto dry ice.  Will the water freeze inside the vessel?  Also, if it doesn't, then at what temperature will it freeze?

Title: What happens to ice when it can't expand?
Post by: DrDick on 22/09/2008 19:36:32

Dry ice temperatures would certainly be enough to freeze the water.

In this instance, the freezing point would change.  As the temperatures cools from 4 °C, the water will try to expand.  Since it can't, the pressure will increase.  Higher pressures require lower temperatures to freeze water, but the difference in temperature is pretty small.  The absolute lowest that the freezing point can go is approximately -20 °C, which is the liquid/Ice(I)/Ice(III) triple point temperature, at a little over 2000 bar(1).  I don't know what pressure would ultimately be achieved, but if you go over that triple point pressure, the freezing point starts rising again.

Dick

(1) Greenwood & Earnshaw, Chemistry of the Elements, 2nd ed., p. 624.

Title: What happens to ice when it can't expand?
Post by: einsteinium252 on 23/09/2008 22:30:30

Thanks Doctor,

I see, it's not that when water freezes it's an unstopable force and it freezes no matter what.  Just that, if water is at the proper pressure, it will freeze at 32 F.

Title: What happens to ice when it can't expand?
Post by: DrDick on 24/09/2008 15:36:28

Actually, at a low enough temperature (below about -20 °C or -4 °F) it WILL freeze at normal or higher pressures.  Of course, if the pressure is low enough, you have the issue of the water preferring to be gaseous.

The freezing point of water stays roughly the same over a fairly broad pressure range.  Keep in mind that it takes roughly 2000 bar (that's 2000x normal atmospheric pressure) to lower the freezing point by 20 °C (36°F).  That's roughly 100 bar per °C.

Title: What happens to ice when it can't expand?
Post by: daveshorts on 24/09/2008 16:20:52

There are high pressure crystal structures of ice, which only occur at the bottom of icecaps, where it is energetically favourable for the molecules to organise themselves slightly differently in order to take up less space.

there is a list of the ice phases at the bottom of this wikipedia article:
http://en.wikipedia.org/wiki/Ice