Ask A Scientist Chemistry Archive

name         Erle W.
status       other
age          30s

Question -   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?
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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.

Vince Calder
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Erle,

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.

Regards,
ProfHoff 703
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Erle,

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.

Greg Bradburn
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Erle,

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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