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.
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!