Ice and Salt
Name: Catherine C.
Date: June 2006
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?
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
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
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.
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!
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:
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
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:
I hope that helps!
Todd Clark, Office of Science
US Department of Energy
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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Update: June 2012