To those who thought that the pressure of an unsealed bottle would result in the water not freezing I would like to offer this explanation.
It is true that pressure has a great deal of an impact on what phase matter is in. Extremely low pressure causes liquids to boil rapidly with almost no change in temperature at all. Extreme high pressure can force gases to become liquids like the liquid hydrogen and liquid oxygen used for rocket fuel. Cooking instructions will tell you lower temperatures for boiling water if you are at higher altitudes.
However all of these examples I listed are results of extreme pressure changes. The pressure inside of one of those water bottles would not have that noticable an effect on whether the water would freeze or not.
Also, a bottle of water is a fairly expandable container so any pressure "built up" as a result of the ice expanding and causing an increase in the pressure of the fluids in the bottle can be released by the expansion of that bottle.
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