Saltwater has a much lower freezing point (the freezing point is the temperature where something freezes) than freshwater does. And the more salt there is in it, the lower the freezing point gets. So in order to know the exact temperature that it’s going to freeze, you have to know just how salty it is. For saltwater that’s as saturated as it can possibly get (i.e. there’s no way to dissolve any more salt in it no matter how hard you tried), the freezing point is -21.1 degrees Celsius. This is when the saltwater is 23.3% salt (by weight).
p.s. As the water starts to freeze, the salt gets left in the liquid. So if you start out with water that isn't saturated with salt, as it freezes the leftover water will get saturated. So if the water starts to freeze at, for example, -10°C, more will freeze as it's cooled further until finally the last bit will freeze at -21.1°C. Thus for un-saturated saltwater the freezing happens over a range of temperatures, not all at one exact temperature, unlike pure water. Mike W.
(republished on 07/25/06)
(published on 03/04/07)