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Good question! When salt is added to ice, two things happen. Some of the ice melts and the whole mixture gets colder, at least until it absorbs heat from its surroundings. The first effect is used for de-icing, the second effect is used to get the low temperatures for ice cream making.

There is a bit more to the ice cream story. Most of ice's heat-removing capability comes from its so-called latent heat of fusion. It takes about 80 calories to change 1 gram of pure ice at 0 degrees C to water at 0 degrees C. (That same amount of heat would raise the temperature of the 1 gram of water from 0 C to 80 C !) When the ice cream mixture is put in contact with the ice, the heat absorbed by the ice goes into raising the ice's temperature until the freezing point of the ice is reached. Thereafter, all of the heat absorbed by the ice/water goes into melting ice; the temperature is effectively locked at the melting point of the ice until it is all melted. Thus, the temperature of the ice cream mixture can in general be lowered only to this melting-point temperature. Typically, the freezing point of the ice cream mixture is a few degrees below 0 C; so one needs to lower the melting point of the ice to below the ice cream's freezing point. This is accomplished by the addition of salt to the ice. (When Fahrenheit devised his temperature scale, his 0 degree point was the coldest temperature he could obtain using an ice-salt mixture.)


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