Yes! Salt does not fit well chemically into ice, so
as ice forms the salt is left behind in the liquid. When you freeze it
slowly, you start off by getting just a few very tiny ice crystals. The
first tiny crystals form out of almost pure freshwater, since the salt
sticks much better in the liquid than in the ice. This pushes more salt
into the unfrozen part. As it freezes further, the crystals get bigger
and bigger, but continue to be made out of almost pure freshwater
(since water gets frozen onto the crystal just a little bit at a time).
So eventually, you get a big crystal of almost pure freshwater ice
surrounded by some extremely concentrated saltwater. Lowering the
temperature further pulls even more water out of the liquid into the
ice, leaving the salt outside as separate salt crystals. If the ice
forms very quickly, however, the salt can get trapped in pockets in the
You probably know that salt water freezes at lower
temperature than pure water. That's why putting salt on ice can melt
it. The reason for this effect is also that the salt sticks well in
liquid water but very poorly in ice crystals. The salt in the liquid
makes it more stable, so it's harder to freeze.
separation of salt and water in freezing is especially important for
the study of cryogenics. (Cryogenics is when biologists or doctors
freeze living cells to be thawed and used later.) In cryogenics, you
try to freeze cells that are surrounded by what is basically saltwater.
(Cells need to be surrounded by saltwater to survive.) But if you
freeze it too slowly, the water outside the cells forms freshwater
crystals, leaving the unfrozen water with a lot of salt in it. This
super-high salt concentration can actually poison the cells, killing
them before they are even frozen.
(republished on 07/25/06)