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Sean Clayden
2006-Nov-26, 09:17 PM
I've heard of super cooled rain and the amazing footage of the big freeze. But, how is it possible for water to remain as a liquid below freezing point ?

01101001
2006-Nov-26, 09:41 PM
But, how is it possible for water to remain as a liquid below freezing point ?

It becomes supercooled (http://en.wikipedia.org/wiki/Supercooling).

Why would it not be possible for something to remain a liquid below its freezing point?


A liquid below its freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs.

Sean Clayden
2006-Nov-26, 10:08 PM
It becomes supercooled (http://en.wikipedia.org/wiki/Supercooling).

Why would it not be possible for something to remain a liquid below its freezing point?

I thought the question would speak for itself. If a liquid had a freezing point, how would it be possible for a liquid to remain a liquid below is's freezing point ?

grant hutchison
2006-Nov-26, 10:20 PM
Water releases energy when it freezes, but it loses entropy: so the transition from water to ice is favoured by the change in heat, but unfavoured by the change in entropy. The combined effect of the heat change and the entropy change is called the Gibbs free energy, and the freezing point is defined as the temperature at which the Gibbs free energy is zero: that is, when there's no net thermodynamic advantage to either the liquid or the solid phase. That doesn't mean the water has to freeze at that temperature: it can remain poised in a thermodynamically unstable state if there are no nucleation sites to trigger freezing. Hence supercooling.
We've had a discussion about supercooling before over on this thread (http://www.bautforum.com/showthread.php?t=38596).

Supercooled rain ("freezing drizzle") is an exciting phenomenon. Years ago I worked in Thunder Bay, Ontario, and got caught out in freezing drizzle a couple of times. Rain would splat on the windscreen of the car with a sort of clicking, crunching undertone, and the splat would immediately freeze on the windscreen so that the wiper blades just skipped over it. Within seconds of this splat-splat-splat process beginning, the windscreen would be opaque as a privacy window and the road surface was like a skating rink. Not a great combination for driving.

Grant Hutchison

01101001
2006-Nov-26, 11:25 PM
I thought the question would speak for itself.

Oops. Sorry. I thought the Wikipedia article (http://en.wikipedia.org/wiki/Supercooling) would speak for itself.


If a liquid had a freezing point, how would it be possible for a liquid to remain a liquid below is's freezing point ?

Your definition of freezing point (http://en.wikipedia.org/wiki/Melting_point) is quite broken.


The melting point of water at 1 atmosphere of pressure is 0 C (32 F, 273.15 K), this is also known as the ice point. In the presence of nucleating substances the freezing point of water is the same as the melting point, but in the absence of nucleators water can supercool to −42 C (−43.6 F, 231 K) before freezing.

danscope
2006-Nov-27, 01:46 AM
Water releases energy when it freezes, but it loses entropy: so the transition from water to ice is favoured by the change in heat, but unfavoured by the change in entropy. The combined effect of the heat change and the entropy change is called the Gibbs free energy, and the freezing point is defined as the temperature at which the Gibbs free energy is zero: that is, when there's no net thermodynamic advantage to either the liquid or the solid phase. That doesn't mean the water has to freeze at that temperature: it can remain poised in a thermodynamically unstable state if there are no nucleation sites to trigger freezing. Hence supercooling.
We've had a discussion about supercooling before over on this thread (http://www.bautforum.com/showthread.php?t=38596).

Supercooled rain ("freezing drizzle") is an exciting phenomenon. Years ago I worked in Thunder Bay, Ontario, and got caught out in freezing drizzle a couple of times. Rain would splat on the windscreen of the car with a sort of clicking, crunching undertone, and the splat would immediately freeze on the windscreen so that the wiper blades just skipped over it. Within seconds of this splat-splat-splat process beginning, the windscreen would be opaque as a privacy window and the road surface was like a skating rink. Not a great combination for driving.

Grant Hutchison

Hi, I have been in that stuff. I've seen cars that were standing still in the center of a crowned road all of a sudden start sliding sideways and slam into the curb!!! Slicker than socks on a waterbug!! And believe you me, that's pretty slick. NEVER go out in that stuff. Live to see another day,Please!!!
Dan

Ivan Viehoff
2006-Nov-27, 01:53 PM
It's like children tidying their bedrooms. Gibbs says, (thermodynamically), it ought to happen. But nothing actually happens (kinetically), unless you make it happen, or the conditions are so extreme that resistance is useless.

Good thing too, or I'd be a pile of ash and this place would look like Mars.

Nicolas
2006-Nov-27, 03:07 PM
However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs.

And how low is that for water in standard atmospheric pressure (101325 Pa)?

OK, -42C , if I'd only read Wiki before asking :).

Sean Clayden
2006-Nov-27, 03:42 PM
Thanks for your help.......

Thought that a small droplet of water falling several hundred feet of freezing air would be enough to freeze it. Under normal circumstances it would be. However, supercooled rain needs a culmination of events which makes it more fascinating.

Understand that water can stay soluble at freezing point in a controlled environment or in large volumes.

sarongsong
2006-Nov-27, 03:51 PM
Fascinating micro property of water:
Scientists discover water is sticky on a small scale... PHYSORG (http://physorg.com/news66320589.html)

Lord Jubjub
2006-Nov-28, 01:12 AM
The other extreme is also dangerous. If you fill a clean mug full of water, put it in the microwave and heat it past the boiling point, it will stay liquid until you disturb it.

At that point, you should not have your hand on the mug.

grant hutchison
2006-Nov-28, 01:39 AM
Yeah, same problem with an absence of nucleation sites: this time preventing the easy formation of bubbles.
When you boil a kettle it turns over convectively and the agitation seems to be enough to allow bubbles to form as soon as the boiling point is reached. Microwaved water heats more uniformly and doesn't roll over. So it can superheat a little above the boiling point and then evolve vapour very rapidly when disturbed or exposed to nucleants.
So it's possible to have a mug boil over suddenly in your hand as you remove it from the microwave. The relevant About Urban Legends and Folklore (http://urbanlegends.about.com/library/weekly/aa011900a.htm) page gives a fair summary of the situation, and links (on its second page) to a video (http://urbanlegends.about.com/gi/dynamic/offsite.htm?site=http://www.flurl.com/item/Superheated%5FWater%5Fu%5F194398) of the phenomenon in action.

Grant Hutchison

JohnD
2006-Nov-28, 03:13 PM
I find that well written SF stories can help understanding of this kind if concept. Hal Clement's "Close to Critical", about a high gravity world with a methane atmosphere, deals well with these.

John

Romanus
2006-Nov-28, 11:56 PM
My understanding of the matter is that pure water can be supercooled *or* superheated.

Something I've seen often myself is to put water in the freezer, pull it out an hour or two later as a clear liquid, only for it to spontaneously turn into opaque slush as soon as its disturbed. You can literally see the fine crystals thread bloom through the water like cotton balls and mesh together into a mass; it takes no more than three or four seconds.

Try it! :)

Lord Jubjub
2006-Nov-30, 02:17 AM
A third 'super' situation is supersaturation. Most substances have a concentration limit to how much they can dissolve in other substances. This limit depends on the nature of the two substances and the heat. Heat will allow for greater dissolution. If two substances are heated, dissolved to their concentration limits at that temperature and then cooled without disturbance, they will remain dissolved until disturbed.

To bring this back to the OT, icy fog can form this way.