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I'd like to keep a container of potable water in my car for when I get stuck, but in the winter it gets cold enough for bottled water to freeze.

I can't put salt in it, because then it won't be drinkable anymore (or at least, will make my condition worse if I do drink it), and I can't mix it with alcohol in case I get the car unstuck and have to drive.

What else could be added to water to sufficiently lower its freezing point without making it undrinkable?

(Or to keep it in line with physics rather than chemistry, how do I calculate the amount of pressure the water need to be under to prevent it from freezing at a given temperature, say 15-20o F?)

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    $\begingroup$ Hmm... this seems like a chemistry question to me (and therefore quite possibly off-topic for this site). Thoughts from anyone else? $\endgroup$ – David Z Jan 2 '11 at 4:41
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    $\begingroup$ Okay, but everything has to do with physics. Is there a survival.SE? :) $\endgroup$ – wsc Jan 2 '11 at 7:10
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    $\begingroup$ @Robert: sometimes, but if you're talking about altering the melting/freezing point by changing the composition of the solution, I would classify that under chemistry. (If the question were about altering the melting/freezing point by changing the pressure, that would definitely be thermodynamics.) $\endgroup$ – David Z Jan 2 '11 at 7:15
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    $\begingroup$ Chemistry is just an applied physics -- I think it should stay. $\endgroup$ – user68 Jan 2 '11 at 11:36
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    $\begingroup$ Everything is applied physics with such a point of view. If there is a physicist here who has detailed knowledge about solutions and antifreezes then yes, he might get an answer. But it is more likely to find such a person in a chemistry related forum/messageboard. $\endgroup$ – Cem Jan 2 '11 at 13:58
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It may not be the answer you are looking for but I recommend you get a thermos or a well insulated flask. These are what mountaineers use and you do not have to change the chemical composition of water this way.

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