MadSci Network: Chemistry

Re: Why does sodium chloride melt ice the fastest?

Area: Chemistry
Posted By: Mike Weibel, Dept. of Chemistry, Univeristy of Utah
Date: Sun Feb 2 15:40:33 1997

Hi Sarah.

Sorry for the delay!

Here's my best response to your question:

All the substances you mentioned (rock salt, table salt, calcium chloride, rubbing alcohol, cat litter) dissolve in water except for cat litter. I'll deal with the water soluble chemicals, and then at the end talk about why the cat litter won't work very well (to melt ice).

Most of the time, when you dissolve a chemical (solute) in a solvent, you end up with a lower freezing mixture. This behavior is in many chemistry texts under "Freezing Point Depression" (called a "colligative" property). The reason for this is that for the solute (that which is being dissolved) to dissolve in the solvent, the solvent molecules have to interact with the solute molecules. We know that salts (like NaCl) are ionic, and hence are very polar too (a positive end and a negative end). Water is polar too...the oxygen atom is more "electronegative" than the hydrogen atoms, which means that it pulls the electrons in the water bonds closer to itself. There is an adage in chemistry which goes "like dissolves like", meaning that polar molecules dissolve in polar solvents. This means that really polar things (like salts) dissolve really well in water, which in turn means that the water molecules "wrap 'em up" really well.

So if we have something dissolved in water, it means that the water molecules are hanging on to it. Things become a gas when they escape from the forces holding them together into a liquid (or solid). When you boil something, you add enough heat to make the molecules in it stretch and bend and go crazy until they can pull free. Lots of molecules break free during boiling! Anyhow, the dissolved stuff makes it harder for the water molecules to get free because not only are there attractive forces between water molecules (called "hydrogen bonds"), but there are these forces between the dissolved molecules and the water molecules. This is important because at 0 degrees Centigrade, the "vapor pressures" (the amount of molecules escaping from the mixture) of both ice and water are the same (you will have to trust me on this one, but it is true). Since you have added something to the water, the solution has a lower vapor pressure than the ice (which is pure water) at this temperature (where the ice and pure water would coexist (equilibrium between the two). Ice cannot form under this condition. At colder temperatures, the vapor pressure of ice drops more rapidly than that of water, until you get the vapor pressures of the mixture equal to the vapor pressure of the ice, and you freeze your solution...

Ionic things are held tightest by the water, so they lower the freezing point the most. Sugar and rubbing alcohol aren't very polar, so they probably wouldn't lower the freezing point much.

Now that you >>>hopefully 8^) <<< understand this much, let's talk about melting stuff. The finer the material is, the more surface area it has. This means that finely ground table salt has more surface area than rock salt. When you dump the material on the ice, it interacts with the solid ice. The material can chemically react with the ice, which will melt the ice if it dissolves (by the freezing point depression points I made above...the mixture now freezes at a lower point, making it melt!). This dissolved ice makes a better interface with the ice/material, so that more ice is dissolved.

I expect that either NaCl or CaCl2 would work equally well if both were the same size particles (is your CaCl2 in larger bits than the NaCl table salt?). This is because the ions in the salts have strong interactions with many water molecules. The other materials wouldn't have such strong interactions, and then you would need more to melt the same amount of ice.

Now, for the kitty doesn't dissolve in water, so it won't lower the freezing point!

Make Sense?

I hope that this was a complete answer for you, so that you understand these processes better!

If you have any questions, please feel free to contact me.

Mike Weibel

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