When moderately salty water freezes, you might expect a uniformly mixed salt and ice crystal to form (after all, the salt and water were homogeneously mixed in the first place). But that's not what happens. First, tiny platelets and needles of ice form over the surface of the liquid. The ice crystals incorporate water, but leave the salt behind, so the solution becomes saltier and saltier as the freezing continues. Finally, the flat ice crystals grow together, trapping small pockets of concentrated brine inside. A cloudy, brittle frozen slush forms.
Predicting the volume of that slush isn't easy. The trapped brine drains over time, leaving behind trapped air bubbles that can dramatically lower the overall density. The density of pure ice is about 0.92 g/mL. The bubbles and pores in salty ice can make its density significantly less dense than pure ice (perhaps 0.8-0.9 g/mL) .
If you cool the slush below typical freezer temperatures, the trapped brine eventually becomes completely saturated with salt.
The lowest temperature possible for liquid salt solution is -21.1°C.
At that temperature (called the "eutectic point"), the salt begins to crystallize out of solution (as NaCl·2 H2O) along with the ice until the solution completely freezes.
The frozen solid is a heterogeneous mixture of separate NaCl·2H2O crystals and ice crystals, not a homogeneous mixture of salt and water. If this "eutectic mixture" were completely air-free, it would be slightly denser than pure ice. In practice, the eutectic mixture is likely to be very porous and probably will slightly less dense than pure water ice.
|Phase map for salt water.|
- Austin Kovacs, Sea Ice: CRREL Report 96-7 and 96-11, Cold Regions Research and Engineering Laboratory, U. S. Army (1996).
Author: Fred Senese firstname.lastname@example.org