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Name: Catherine C.
Status: student
Grade: 9-12
Location: PA
Country: N/A
Date: June 2006

Question:
In the winter, the borough uses road salt so we may drive on the roads when it snows. I understand that the salt does not actually "melt" the ice but what is the salt really doing to the ice?



Replies:
Hi Catherine.

This is a great question. I grew up in PA, too...I remember the winter they ran out of salt and my town completely shut down.

You are right, salt does not actually "melt" the ice, but rather, it lowers the freezing point (makes it so it needs to be a lower temperature for the salt/water mixture to freeze).

This is called a "colligative property".

So, what is actually going on in this system? Well, what is really cool is that the salt molecules are not reacting with the water. In fact, colligative properties are based not on reactions or what you are adding, just how much stuff you are adding. For example, we could dump lots of sugar on the ice, and we would get the same effect (it would just be a lot more expensive!)

Colligative properties are based on entropy. Entropy is basically how disordered a system is. If you add more molecules of stuff to a system, it becomes more disordered. (Analogously, let us say you have a classroom of kindergartners. They run around a lot..and there is a lot of disorder (entropy in the classroom) Now, imagine turning off the heater in the classroom in winter, everyone starts to huddle together, and be still (this is sort of like molecules in a solid). Now, if you do not want them to stay still, you either have to add energy (increase the temperature), or add molecules, or in this case, children to the classroom. It is harder to get the greater number of children to stay still. This is an increase in the entropy.

When things are frozen, they have little disorder, or entropy. When we add salt, we are increasing the number of molecules, which increases the entropy, and makes it harder for the water molecules to slow down and assemble into a solid.

We can actually calculate how many degrees the salt will effect the freezing point of water by using the following equation:

Delta T = - RM(T^2)m/Delta H

This says that the change in temperature is equal to the Gas constant (R) times the molar mass of your solvent(M), in this case your solvent is water, times the regular freezing temperature (T) squared, times the molality of the solute (this is the molality of the salt--kg salt/L solution) divided by the delta H fusion (the amount of enthalpy required for the phase change of liquid water to ice).

Hope this helps.

--Michelle Weinberger


Catherine,

The salt does in fact melt the ice! When salt is dissolved in water and then is frozen, it takes a lower temperature to freeze the solution. A sodium chloride solution will freeze at -10 C, which is 10 degrees colder than the freezing point of pure water. Sodium chloride (NaCl) provides two ions for every mole of salt dissolved. Road salt, or rock salt, is calcium chloride (CaCl2) and provides three ions for every mole of salt dissolved. The more ions present, the lower the freezing point of the solution. Different salts are used throughout the country depending on what temperature range is expected.

When the salt is spread onto the road, the water is already frozen. Salt, however, is hygroscopic (NOT hyDRoscopic, as many wrongly term). This means that it will actually absorb water out of the air and the surrounding environment to, in essence, dissolve itself. When this happens, it lets off a small amount of heat. Salt is in a very rigid crystal form and it takes energy to get the ions into this very organized form. When the salt dissolves, the crystal lattice breaks and releases its energy in the form of heat. This heat is actually melting the ice, which dissolves the salt more and lets off more heat, hence making salt a very effective chemical to melt ice on the road and keep it melted!

Matt Voss


Unless there is some atypical definition applied to the term "melt", I would disagree with the statement that "salt does not actually melt the ice." Salt, or more commonly now calcium chloride and urea, lower the freezing point of water by several degrees. In addition, the salts in some formulations give off heat when dissolving in the water formed upon melting the ice. This heat produces an additional mechanism for melting ice/snow. The following web site provides and overview of the performance of various ice-melting formulations:

http://www.peterschemical.com/Break%20the%20Ice.htm

Vince Calder


Catherine,

Thanks for your question!

Adding salt to water creates a salt water solution that has a higher boiling point and a lower freezing point than pure water. Boiling point and freezing point are colligative properties (see http://www.infoplease.com/ce6/sci/A0812887.html).

So, placing salt on the roads lowers the temperature at which the salt/water mixture will freeze.

Here are some other questions an answers on "Ask a Scientist" that you may also find helpful:

http://www.newton.dep.anl.gov/webpages/askasci/chem03/chem03336.htm

http://www.newton.dep.anl.gov/webpages/askasci/chem03/chem03079.htm

I hope that helps!

Regards,

Todd Clark, Office of Science
US Department of Energy


Catherine,

Do a search within this forum for a colligative property called "freezing point depression". It will describe how a solution (composed of a solvent and solute) will freeze at a lower temperature than the parent solvent. Thus, since a solution of water and salt will freeze at a lower temperature, adding salt essentially prevents the snow from freezing at 0 degrees C, thereby causing the ice-salt solution to melt.

Greg (Roberto Gregorius)



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