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Most natural ice is made of fresh water, because freezing water locks its molecules into a crystal structure that leaves no room for the dissolved salt. Even ocean ice becomes "fresh" water as it freezes, since the freezing water both excludes the salt from its structure, and since the freezing temperature for salt water is much lower than for fresh water. Fresh water freezes at around 0 degrees Celsius, but adding any dissolved substance causes the freezing temperature of the solution to become lower than 0°C. At or around 0°C, salt water will exclude the salt from its crystal structure as it slowly freezes, raising the salinity of the surrounding water and creating fresh water ice. Below 0°C, ice will allow more and more salt to be fixed inside its crystal. The freezing temperature will vary as the salt saturation raises, and when the water reaches full saturation so no more salt can be dissolved, the freezing temperature will be around -21.1 degrees Celsius. Therefore, it is possible to create salt water ice cubes, but it requires freezing temperatures below the capability of normal freezers. It is possible to freeze salt water with liquid nitrogen or oxygen, both of which are very cold.
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