Sea ice over the North Pole in January, 2008.
Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 0 degrees Celsius (32 degrees Fahrenheit), but seawater freezes at about -1.9 degrees Celsius (28.4 degrees Fahrenheit) because of the salt in it. When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water.
At least 15 percent of the ocean is covered by sea ice some part of the year. On average, sea ice covers almost about 25 million square kilometers (10 million square miles) of the Earth.
Sea water becomes more and more dense as it becomes colder, right down to its freezing point. Fresh water, on the other hand, is most dense while still at 4 degrees Celsius (39.2 degrees Fahrenheit), well above the freezing point. The average temperature of all ocean water is about 3.5 degrees Celsius (38.3 degrees Fahrenheit).
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