Salt, Sidewalks, and Ice Cream
Name: Erle W.
We recently made some homemade ice cream which required the addition of rock
salt to chopped ice surrounding the canister. The addition perature of the ice to
facilitate the process. However, a question arose as to why we put salt on our
sidewalks or driveways during the winter to melt snow and ice. Based on what happened
to the temperature to our rock salt/ice mixture, should not one expect our driveways
and sidewalks to freeze upon the application of rock salt?
No, what happens when you sprinkle salt on the driveway and/or sidewalk is that it lowers
the freezing point of the ice and snow so that at say 28F the water/salt mixture is liquid,
whereas, at the same temperature pure ice would be a solid, being below its "normal" melting
point of 32F.
The salt did not lower the temperature of the ice. However, as the salt dissolves in the
film of water on the ice, it causes the ice to melt -- an energy consuming process -- thus,
the observed falling temperature of the ice/salt/water system. The salt/water solution
produced by application of salt to ice has a lower freezing point than water alone. Within
limits, the more salt that dissolves, the greater the amount of ice melted and the lower
the freezing temperature of the resulting solution.
The addition of salt to the ice/water mix did not lower the temperature of the ice.
There are three ingredients in the tub surrounding the ice cream container. They are:
The ice. The ice provides the cooling to freeze the ice cream. Since the freezing point of
the ice cream is below the freezing point of water the ice must also be at a temperature below
the freezing point of water. The starting temperature of the ice is the coldest point in the
time/temperature profile of the vat.
The water. The ice "drives" the freezing of the ice cream but it does not make good thermal
contact with the container holding the ice cream. A heat transfer fluid is needed that will
make contact with the entire surface of the ice cream container AND the surface area of the
ice. Water is used for this.
The salt. The source of the water is usually from the melting of the ice. Since the ice
starts at a temperature well below the freezing point of water and the ice cream needs to be
below the freezing point of water to set up something has to be done to keep the water as a
liquid even when it is very cold -- i.e., below its normal freezing point. Dissolving salt
in the water serves this purpose. It lowers the freezing point of the water so that it can
be very cold and remain a liquid, providing good thermal contact with both the ice and the
ice cream container. In fact, you do not add water to the ice cream maker, you add ice and
salt. The salt slowly is dissolved in the melting ice water, lowering its freezing point so
that the water can drop in temperature to the ice temperature without freezing. The same thing
happens when you put salt on a sidewalk covered with ice. The surface of the ice has a thin
layer of water which begins to dissolve the salt. The salt water has a lower freezing point
and stays liquid at very low temperatures.
When a small amount of salt (solute) is dissolved in the ice/water mixture (solvent),
the freezing point of the resulting solution will be lower than that of the solvent.
For example, solutions of salt in water may freeze at temperatures as low as -21 degrees
Centigrade, 21 degrees below the freezing point of water (0 degrees Centigrade).
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Update: June 2012