[nate2612]

I have an interest in super cooled water but am unsure exactly what's happening on a molecular level.

Why does it keep its liquid form?

[Yggdrasil]

The answer has to do with the concept of nucleation.  Briefly, crystal formation requires the growing crystals to break the surface tension of water in order to expand.  However, water's high surface tension makes crystal formation difficult, and as a result, the liquid phase is a  local free energy minimum (metastable state) separated from the solid phase (global free energy minimum) by a somewhat large free energy barrier.

The following wikipedia article and the last paragraph of the post (on superheated liquid which is conceptually similar to supercooled water) may be helpful:

http://en.wikipedia.org/wiki/Nucleation
http://www.chemicalforums.com/index.php?topic=23459.msg89046#msg89046

[nate2612]

thanks.
so let me see if I understand.
The only reason that we are able to supercool water is because it has no reason or want to form ice. If there are no impurities then the molecules have nothing to form on.

So supercooling has nothing to do really with the ways the atoms and molecules are set up?

[Yggdrasil]

Supercooled water wants to form ice (ice will have a lower potential energy), but it is incapable of doing so.  One (rough) analogy you can make is to that of chemical reactions.  For example, it is energetically favorable for hydrogen peroxide to decompose into water and oxygen gas.  But, this happens at a fairly low rate.  However, if you add a catalyst (e.g. manganese dioxide or catalase) the process occurs quite rapidly.  Similarly, supercooled water won't crystallize unless nucleators (catalysts) are present to help water in its liquid to solid transition.

Supercooling does have something to do with the ways the atoms and molecules are set up.  In liquid water, there are fairly strong bonds between water molecules that give water its surface tension.  This surface tension leads to the need for nucleation.

But, yes, the organization of molecules in supercooled water is no different than the organization of molecules in water above 0^oC (aside from the average speed of the molecules).

[nate2612]

So because water has a high surface tension, for ice to form the surface tension must first be broken.

[Yggdrasil]

Yes.

[nate2612]

Thank you very much

[Polleke]

Just to help you in general the following website:
http://www.lsbu.ac.uk/water/chaplin.html
This website is really great and helped me a lot allready. The writer is also a "famous" chemist who is specialised in "water chemistry".



This website is really great if you have any questions about water.
(however I dont know if the website tells about supercooling, maybe here : http://www.lsbu.ac.uk/water/phase.html you can find something)