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Q & A: De-Salting Saltwater - The ’Cool’ Way

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Most recent answer: 10/22/2007
Q:
Can you separate salt from saltwater by freezing
- Zachary Fuller (age 9)
Asbury, Hampton
A:
Zachary -

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 ice.


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.


The 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.


-Tamara

(published on 10/22/2007)

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