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Ordinary water freezes at 0° C (32° F). When you add salt to
water, it lowers the water's freezing temperature-it has to get colder
than 0° C to freeze.
How much colder depends on how much salt is mixed in with the water.
The salt you sprinkle on the ice cube lowers its freezing temperature
and, since the ice cube can't get any colder than it already is, it starts
to melt. A little pool of water forms on top of the ice cube and the
string sinks into it. As the ice cube melts, it dilutes the salt/water
mixture in the little pool; the freezing point starts to go back up again.
The ice refreezes, trapping the string. As soon as the ice cube hardens,
you can raise it by lifting the string. All this happens very quickly,
of course.
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| | Salt is useful for clearing ice from sidewalks and roads. But have you ever noticed that it doesn't work when temperatures get very, very cold?
That's because if it's cold enough, the water will stay frozen. Even though the salt lowers the freezing point of water, the air temperature is so low that ice doesn't melt. | |
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