I’m helping on a lesson plan for grades 3-12 where students make ice cream. Adding salt to the ice makes the ice colder. I’m having trouble explaining why we put salt on the roads to melt ice, but in making ice cream the salt actually lowers the temperature of the ice. — N
These two observations—that salt melts ice and that salt makes ice colder—are actually consistent with one another. When you add salt to ice, you make a relatively ordered mixture—pure crystalline ice and pure crystalline salt. This orderly arrangement is looked on unfavorably by nature; given a chance, nature tends to maximize randomness. There is a much more disorderly arrangement available—salt water—and nature tends toward disorderly arrangements. When you put the salt and ice together, nature’s tendency toward randomness begins to drive the system to rearrange. The ice begins to melt so that the salt can dissolve in it. Although the melting of ice requires energy, the randomness this melting and dissolving produces makes this process take place. The energy needed to melt the ice is extracted from the remaining ice and that ice gets colder. When you’re making ice cream, some of the energy needed to melt the ice also comes from the ice cream mix, so that it gets colder, too. If there is enough salt around, the ice will melt completely to form very cold salt water—the desired result with salt on a slippery sidewalk. The salt water remains liquid well below the normal freezing temperature of water because forming ice crystals would require the salt and water to separate from one another—an orderly and therefore unlikely event. In short, nature’s trend toward disorder causes salt to melt ice, even though that melting lowers the temperatures of everything involved well below the freezing temperature of pure water.