As sea water freezes, dissolved salt is excluded, and the resulting ice has much less salt in it compared to sea water. Repeated recycling between solid and liquid can eliminate virtually all the salt in sea water.
Salt content of different waters
No water is totally free of salt. Sea water has a typical salinity of 3.5%.
Crystal Structures
Ice and salt crystals have very different structures in solid form:
Ice has a hexagonal (six-sided) structure while salt has a cubic (four-sided) structure. It is primarily due to this structural differences that salt and water separate during the freezing process.
Salt content of different waters
No water is totally free of salt. Sea water has a typical salinity of 3.5%.
| Water salinity based on dissolved salts | |||
|---|---|---|---|
| Fresh water | Brackish water | Saline water | Brine |
| < 0.05% | 0.05% – 3% | 3% – 5% | > 5% |
Ice and salt crystals have very different structures in solid form:
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| Crystalline Structure of Ice Source - ps.uci.edu |
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| Crystalline structure of Salt Source - webelements.com |
The Freezing Process
As pockets of sea water start freezing, the crystalline structure of frozen water begins to appear which pushes salt molecules away to produce water pockets rich in salt (aka brine solution). As the salt-free water freezes, the brine solution resists freezing because its high salt content has lowered its freezing temperature.
As ice freezing continues, this brine solution, essentially, leeches out from the water crystals leaving behind water whose salt content is much lower than that of sea water.
Freezing speed, if fast enough, can trap some salt molecules inside ice crystals but these are gradually released as ice crystals reach for their steady state.
Freezing speed, if fast enough, can trap some salt molecules inside ice crystals but these are gradually released as ice crystals reach for their steady state.
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