Saltwater doesn't just look, smell and taste different from pure water. The sodium chloride – salt – in saltwater affects certain chemical reactions including its freezing point.
TL;DR (Too Long; Didn't Read)
Pure water freezes at 32 degrees Fahrenheit, while a salt solution may not freeze until it reaches minus 6 degrees Fahrenheit because salt disrupts the movement of molecules entering and leaving the solid.
Freezing Point of Water
The freezing point of water is the temperature at which it changes from a liquid into a solid. Pure or distilled water freezes at 32 degrees Fahrenheit (zero degrees Celsius). This is the same as the melting point when water goes from solid ice to liquid water. However, the freezing point of water may be lower if the water contains foreign matter that could trigger freezing point depression. Under some conditions, water may not freeze until it reaches a temperature of minus 40 to minus 42 degrees Fahrenheit. This is because water needs a seed crystal or nucleus – a tiny particle – to create a crystal structure around. If water is pristine, it can hold its liquid state until it reaches the temperature at which the crystalline structure forms.
Freezing Point of Salt Solution
Pure water freezes when water molecules of hydrogen and oxygen bond together to form a crystalline ice structure. When salt is added, it is more difficult for the molecules to bond. Saltwater has a much lower freezing temperature. The greater the level of salt, the lower the freezing point gets. A salt solution at the point of saturation – the point at which it's not possible to dissolve any more salt in the liquid – reaches the freezing point at minus 6 degrees Fahrenheit (minus 21.1 degrees Celsius). During the freezing process, the salt remains behind in the liquid. When you begin with water that isn't saturated with salt, the remaining water becomes saturated as it freezes. For example, if the water begins freezing at minus 10 degrees Celsius, more water freezes as the temperature drops until the last of the water freezes at minus 21.1 degrees Celsius. While pure water freezes at one exact temperature, saltwater that is not saturated freezes across a range of temperatures. Because frozen saltwater contains little salt, it can be melted down to use as drinking water.
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Density of Water
Another difference between pure water and saltwater relates to density or how tight a substance is put together. Saltwater becomes denser as it drops toward its freezing point. Pure water is at its most dense at 39.2 degrees Fahrenheit, which is much higher than its freezing point.