Plain water

Pure water freezes at 0 degrees Celsius or -32 Farenheit. When things freeze, or become solid from a liquid state, the usually form a crystal structure at the atomic level. In other words the atoms of water line up to form a regular pattern when it freezes. Like snowflakes.

Latent heat of Salt makes it to melt faster

Pure water freezes at 0 degrees Celsius or -32 Farenheit. When things freeze, or become solid from a liquid state, the usually form a crystal structure at the atomic level. In other words the atoms of water line up to form a regular pattern when it freezes. Like snowflakes.

Now if you add stuff to water, like salt, it makes it harder for water to form a crystal structure. This is because there is stuff in the way. What this means is that it has to get a little bit colder for impure water to freeze then pure water.

Salt is a good choice for the impurity because it dissolves well in water and is relatively cheap.

So when salt is added to wet snow, the part that isn't frozen yet will have a harder time freezing.

A better explanation of this is on Wikipedia.
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