Chemistry Question #4067

Hannah, a 10 year old female from Austin asks on January 7, 2008,

Why does salt melt ice?

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The answer

Barry Shell answered on January 8, 2008

It all has to do with reducing the amount of water molecules available for freezing. Ice forms from liquid water as you lower the temperature of the water to 0 degrees C (32 degrees F) and lower. You have to picture the liquid water as billions of trillions of molecules all rumbling around with energy. Freezing happens because as you cool the water, the "rumbling energy" decreases to the point where the water molecules become nearly motionless, locked into a solid which we call ice. When water freezes, individual water molecules have to slow down enough and also they have to "get the feeling" to "lock together" into a solid form. This solid is actually a regular pattern of molecules called a crystal, an ice crystal.

Now at any given moment, even at 0 degrees or lower, all of the water molecules have different amounts of energy--some have more, some less. It's random. The ones with less energy are the ones that are going to slow down enough to attach to the crystal and solidify or freeze into ice. Scientists say that there's a fraction of molecules that are ready to freeze at any given instant. Obviously, the colder it is, the bigger this fraction is, but at any time no matter how cold it is, there's always some that want to freeze and some that don't.

When you put some salt into the picture, things change. Basically the fraction of water available for freezing changes. Here's why. Salt is made of sodium and chlorine atoms. So when it gets mixed with water it dissolves into the water contributing sodium and chlorine atoms to the mix of water molecules. Now if you think about it for a minute, you will see that by adding salt, you are reducing the fraction of water molecules in the total, because some of that total is taken up by sodium and chlorine atoms.

Let's say you originally had 100 water molecules, and of those 100, 50 were ready to turn from water into ice at some moment. So that means the fraction ready to freeze is one half, 50/100, 50 out of 100 molecules. Now if you were to put some salt in that same mix, instead of there being 100 water molecules, maybe there would be only 80 water molecules and 10 sodium and 10 chlorine atoms, just to keep everything adding up to 100. So if the fraction that are ready to freeze is still one half, you only get 1/2 of 80 or 40 water molecules ready to freeze at that instant. So as you can see you are reducing the amount of water molecules available for freezing. That means that they are staying as water and not becoming ice. If you have reduced it from 50 molecules to 40 molecules that is a change of 10/50 or 1/5 or 20%. So you have made the water 20% less likely to form ice.

There are other forces involved, too, but the main reason salt melts ice is because of the reduction in the fraction of water molecules in solution available for freezing at any moment in time. For this reason it does not really matter what kind of salt you use, or for that matter any kind of other molecule you use. For instance, you can put alcohol in with the water and it will prevent the water from freezing. This is what is done with windshield washer fluid. Also people add anti-freeze, which is the chemical ethylene glycol to the water in car cooling systems (the radiator) to keep it from freezing. It all works on the same principle: reducing the fraction of water molecules available for freezing at any given temperature and time.

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