| When seawater freezes, it undergoes a remarkable natural desalination process — known as brine rejection — that expels much of its salt content. So when that seawater ice melts, the result is almost pure fresh water. It may seem impossible, but it all comes down to what happens at the molecular level when the salty water freezes. As ocean temperatures drop below freezing, water molecules begin forming ice crystals with a highly organized structure. That structure cannot incorporate salt ions, so those ions are largely excluded, pushing the salt out of the ice. Initially, not all of the salt is rejected; some of it is trapped in the ice, forming pockets of concentrated brine. The brine remains in a liquid state because it requires lower temperatures to freeze. So at that stage, the sea ice still has a high salt content, but over time, the ice continues to eject the brine. Thanks to this phenomenon, sea ice contains significantly less salt than the water it came from. In liquid form, ocean water has an average salinity of 35 parts per thousand, while newly formed seawater ice has a salinity of between 12 and 15 parts per thousand. As the ice grows thicker and brine rejection takes place, the salinity decreases significantly: Arctic first-year ice has an average salinity of 4 to 6 parts per thousand, and sea ice four years or older is nearly free of brine. When seawater that has been frozen for years eventually melts, the water released is dramatically fresher than the ocean around it. In these cases, when nearly all the brine is gone, the ice can be fresh enough to provide drinking water when melted — something that's often done during polar expeditions. In his 1911 book, Polar Exploration, the British polar scientist and explorer William Speirs Bruce described how whalers and exploring ships in the Arctic extracted water from pools on the ice, which was often drinkable fresh water. Today, polar expedition members still take an occasional drink from these pools. | 
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