MadSci Network: Chemistry
Query:

Re: Why does salt melt snow and pepper does not?

Date: Tue Mar 20 02:24:49 2001
Posted By: Mike Conrad, Post-doc/Fellow, Microbiology, UNC
Area of science: Chemistry
ID: 983742854.Ch
Message:

Why does salt melt snow and pepper does not?

Because salt dissolves in water. Water with salt dissolved in it freezes at a lower temperature than pure water freezes. When a grain of salt touches snow (or ice), it immediately starts melting the snow. The snow melts and turns into salt water.

Salt comes in grains like pepper, but that's where the similarity ends. Chemically, salt is called sodium chloride, and it is made up of sodium and chlorine atoms. When salt dissolves, the atoms mix with water to make what's called a solution of salt in water. You can see this if you put a teaspoon of salt in a cup of water and stir it up. The salt seems to disappear. That's because the salt dissolves.

Pepper is material made from the ground up dried berries of a pepper plant. If you put a teaspoon of pepper in a cup of water you will see that it doesn't dissolve. The bits of pepper just float around. That's because pepper is a woody plant material made of large molecules that don't dissolve in water. The atoms that make up pepper are all combined together in these large molecules and they just don't dissolve in water. So if you put pepper on snow, it will sit there and nothing will happen. Unless the sun hits it, and then it will warm up and cause melting.

Why does salt make ice melt? Ice starts melting at 32F (0C). Chemists say that the salt "lowers the chemical potential of water". What they mean is that water with salt dissolved in it is more stable than the ice and salt alone. So if you have ice, and you add salt, the ice will melt and dissolve the salt (rather than the ice and salt staying alone by themselves).

Salt can keep water liquid as low as -6F (-21C), but colder than about 14F (-10C) the trick of adding salt to ice doesn't work very well. That's still pretty cold. But sometimes it gets colder than that in Minnesota. When it's colder than that, other chemicals have to be used.

Mike Conrad.


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