Is there anything that can be added to water (that is safe) that will lower its freezing temperature? This would be useful on a winter, overnight camping trip.
There's nothing practical and safe to add to water that will lower its freezing point very much. Salt water freezes at a lower temp that plain water, so a little salt will help, well, a little. Better to add powdered Gatorade or something similar, and the freezing point will drop a bit. Alcohol will lower the freezing point some too. But you use enough, say Jack Daniels, to lower the freezing point much and you've got a mind-altering substance. I use a few more practical ways in winter. 1) heat water up before bed, sleep with it in a bottle with a tight lid. 2) store water bottles upside down so ice forms on the bottom instead of freezing the lid on. 3) leave an inch or so of space in a water bottle during the day so the sloshing around slows the freezing process. 4) bury a bottle of water in a foot of snow and the snow insulates the water keeping it from freezing.