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?)