Freezing distilled water.

[musicman1956]

I remember years ago on Blue Peter Val Singleton did an experiment where she put pure distilled water in a sealed bottle into a freezer. The water didn't freeze at all even though the temperature was many degrees below 0 degrees Celsius. She gave a reason why which I can't remember. In light of the controversy over the blue peter cat, was she telling the truth or was this another Blue Peter audience manipulation.

[squarebear]

Sure distilled water freezes. In fact, it freezes a lot faster than tap water due to the extra impurites rather than it being pure H2O.

If water has other substances dissolved in it, those substances get in the way of the crystal lattice of the ice as the water freezes. Some extra effort, that is a lower temperature, must be applied to get the extraneous substances out of the way of the water molecules so that the water can freeze.

Since tap water has more of these dissolved substances than distilled water does, it is harder to freeze the tap water.

[kempie]

squarebear - your explanation seems to be about rate of freezing rather than temperature of freezing.

Pure water actually has a freezing point (-42�C) that is different to its melting point (0�C), thus pure water can be supercooled below 0�C without it freezing.

Impurities in water act as nucleation sites from which crystallisation will originate; this heterogeneous nucleation causes the freezing point to raise to that of the melting point i.e. 0�C.

http://en.wikipedia.org/wiki/Nucleation
http://en.wikipedia.org/wiki/Supercooling

[newtron]

By definition, the freezing point of water is the same as melting point. A liquid that is below its freezing point is a super cooled liquid. It is fairly easy to super cool water, but that does not mean that its freezing point is below 0 degrees Celsius. Although it is quite difficult to super cool water to 30 degrees celsius or lower. Squarebear was talking about colligative properties. When a substance is dissolved in a solvent (in this case it is water) the freezing point opf the solvent actually decreases and the boiling point increases. Sorry kempie but what you said about the freezing point of pure water being -42 deg C is just not true.

[newtron]

To elaborate, at 0 deg C, liquid water and ice can coexist in equilibrium. If the temperature is decreasing (energy is being removed from the system), the temperature will remain at 0 deg C until all the water has frozen. If heat is being added to the system, the temperature will remain at 0 deg C untill all the ice has melted. The amount of energy needed to be added to melt the ice is the same amount of energy that is need to leave the system for the liquid water to freeze. In the case of super cooled water, which may be due to the lack of nucleation sites as kempie stated, once an ice crystal forms, the water freezes almost instantaneously. This happens because the system is not in equilibrium. The freezing point/ melting point is the temperature at which both the liquid phase and the solid phase can exist in equilibrium.

[newtron]

In my first post I meant -30 deg C.

[neuron]

kempie, no wonder squarebear went off at a tangent. The bulk of the post can be found verbatim here:

http://www.madsci.org/posts/archives/1999-03/9 21547735.Es.r.html

Plagiarism does have it's disadvantages.

[musicman1956]

Thanks guys. So.. Pure water CAN go below 0 degrees without freezing. Good old Val, I knew she wouldn't lie to me.

[kempie]

My explanation was not to deny the the equilibrium freezing/melting point, it was meant to confirm that liquid water can certainly exist below 0�C.

Water freezing is not simply the reverse of ice melting. Melting is a single step process that occurs at the equilibrium melting point as ice is heated whereas freezing of liquid water on cooling involves ice crystal nucleation and crystal growth that generally is initiated below the melting point for pure water.

Pure water can cool (supercool) below the equilibrium freezing point when there are no nucleation sites for solidification to occur. The limit of this supercooling is the homogeneous freezing point and it is this value which may be vastly lower than the equilibrium freezing point.

[john_jezewski]

There was an accident investigation looking into why a plane had crashed over a mountain. The report concluded that the plane ran into a mist of pure, effectively distilled water, which was at a temperature of -20 degress Celsius. Water does not freeze at zero Celsius if it is pure. It requires impurities to freeze and in this case the propellers were the impurity. The propellers siezed up with ice s they hit the mist of pure water.
John Jezewski www.pheelgood.co.uk

[john_jezewski]

Water will freeze at zero if it contains impurities.

[Yelodoggie]

Yes, it will freeze, but the ice will not hold together very well. So, for instance, if you use distilled water to make ice cubes in a tray, when you go to use the cubes, they will splinter into chips. I expect that the reason is that there are no impurities in the water, and thus, no, or fewer nucleation points to bind the ice.