Fresh water will melt if the air around is
warmer than 32 degrees Fahrenheit. If the air
is cooler than 32 degrees (like it is in your
freezer), it won't melt. This temperature (32° F),
is called the melting point of the water.
The warmer the air is, the faster it melts.
When you add salt to the water, it lowers
the melting point of the water: the more salt
you add, the lower the melting point becomes. For
example, some really salty water will have a
melting temperature of 20°F, instead of 32°F. This
means that an ice cube made of salty water will
melt at lower temperature than fresh water.
Now, imagine that If you take a salt water ice
cube and a fresh water ice cube out of the freezer
at the same time, and put them on the counter in
your kitchen, which has a temperature of 70°F. The
temperature of your kitchen is 38 degrees higher
(70-32=38) than the melting point of the fresh
water, and 50 degrees higher (70-20=50) than the
melting point of the salt water ice cube. So, the
salt water ice cube melts faster!
As written this question is nonsensical: water is
already liquid, and therefore incapable of
melting. You could be interested in
freezing(/melting) point depression, as discussed
here, answers #2 and 3
here, and (sort of)
here, or in ice which is being melted in
fresh water vs. a solution, as in
here and answer #1
Salt water has a lower freezing temperature
than fresh water. Because it freezes at a lower
temperature, it will melt more readily when
above that temperature than fresh water would.
What I don't know is exactly why salt water has a
lower freezing temperature than fresh water. My
guess is that it has to do with the water
molecules forming cage-like structures around the
salt ions, which prevents the water molecules
themselves from forming crystals.
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