While pure water freezes at 0°C (32°F), salt
water needs to be colder before it freezes and
so it usually takes longer to freeze. The more
salt in the water, the lower the freezing point.
Very salty water freezes at around -21 °C, or
about -6 °F. What is interesting is that this
effect is used all over the place for practical
reasons. Often, salt is put on roads to melt
ice. If there's a lot of ice, you need a lot of
So then the question is why does salt water
need to be colder to freeze than fresh water?
Basically, ice is a crystal, an almost
perfect array of pure water molecules with
almost no salt in it. To make that out of
pure water requires limiting the ways the water
molecules move around. To make that out of salt
water requires BOTH limiting the ways the water
molecules move around AND limiting the ways the
salt can move around (it's stuck in the liquid,
or in separate crystals), which is harder to
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