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Freezing water

Question:
A bottle of water (retail, plastic, unopened) was sitting in his garage overnight in freezing temperatures (around zero). When he discovered it, it had not even begun to freeze. When he picked it up (agitating it) it decided to start freezing. What caused the phase change, and why did it not freeze on its own.

Answer:

A number of thoughts.

Firstly for the water to freeze it has to lose 334 000 J for every kg of its mass. The rate at which it loses this heat energy will depend on the difference in temperature between it and its surroundings. Therefore if the temperature of the garage was only a little below zero the rate of loss of heat will be small and so the time taken for the water to freeze will be large.

Of course if the temperature rose to zero the freezing would stop and if it was even fractionally above zero any ice formed will be gin to melt though admittedly only slowly.

To test these ideas I suggest you put a bottle of water (no top on) in the freezer and see how long it takes to freeze completely bearing in mind that the temperature in a normal household freezer may be around 15 degrees below zero.

The second major point is this. For any crystals of ice top form there needs to be a nucleus on which the ice can begin to grow. By this I do not mean an atomic nucleus, just a tiny particle of dust or small ice crystal. It is possible to cool pure water below zero without it freezing this is known as supercooling. The liquid is then in an unstable state. Then if you drop in a tiny particle such as a small ice crystal freezing starts immediately.

I feel sure that small vibrations such as the agitation when the bottle as picked up may either have disturbed some potential freezing nuclei from the base of the bottle or even the vibrations travelling trough the liquid may have been sufficient to initiate freezing. In your case it is likely that it was the vibrations that were set up in the water when the bottle was moved that caused freezing to start.

Finally of course plastic is a good insulator!
 
 
 
© Keith Gibbs 2007