When seawater freezes, the process naturally removes most of the salt, resulting in ice that is significantly less salty than the original water—essentially becoming fresh water over time.
The Freezing Process and Salt Exclusion
Unlike fresh water which freezes into a solid block of ice, seawater is a complex solution containing various salts, primarily sodium chloride. When seawater begins to freeze, it doesn't form a uniform ice structure that incorporates everything. Instead, the freezing process is selective:
- Water Molecules Align: As the temperature drops to the freezing point of seawater (which is slightly lower than fresh water), water molecules begin to slow down and arrange themselves into a crystal lattice structure.
- Ions Are Rejected: Salt is made of ions (electrically charged particles). These salt ions, such as sodium (Na+) and chloride (Cl-), are too large and do not fit well into the tightly packed crystal structure that water molecules form when they freeze.
- Salt is 'Pushed Out': According to scientific understanding, "As the water molecules lock into fixed positions, the ions are 'pushed out'" of the growing ice crystal. They are forced into pockets or channels within the ice structure or back into the remaining liquid water below.
Over time, particularly in older, multi-year ice, a process called "brine rejection" continues. The trapped pockets of salty water (brine) can drain out of the ice as it warms slightly, consolidates, or is subjected to gravity, further reducing the salt content.
The Result: Fresh Ice and Saline Water
This salt rejection process has two key outcomes:
- Essentially Fresh Ice: The ice that forms contains very little salt compared to the original seawater. While newly formed ice might still contain some trapped brine pockets, older sea ice can be nearly salt-free, making it essentially fresh water.
- More Saline Water: The salt that is pushed out of the ice increases the concentration of salt in the water immediately below the ice. This remaining water becomes richer in ions (more saline) and denser.
This denser, more saline water then sinks, often driving important ocean currents, particularly in polar regions like the Arctic Ocean.
In summary, the inherent nature of water molecules to form a pure crystal structure during freezing physically excludes salt ions, transforming salty seawater into relatively fresh ice.
