It's almost impossible to answer every question during a Please Explain segment, and today's discussion of salt left us wondering about one question in particular—Elliott from New York asked: Why do you put salt in an ice cream maker to keep the ice from melting…then put salt on the sidewalk to make ice melt?
I did a little research and found the explanation. Salt lowers the freezing point of water, which is normally 32°F (0°C). When you spread sodium chloride on a sidewalk, the freezing point becomes about 15°F (-9°C).
When you make ice cream, the ice needs to stay below a freezing temperature for a long enough time to allow the milk or cream to freeze. So you add salt to the ice in order to keep its temperature well below freezing, even after it has melted.
Here's a simple recipe for homemade ice cream that requires no special equipment:
1/2 cup milk
1/2 teaspoon vanilla
1 tablespoon sugar
4 cups crushed ice
4 tablespoons salt
1 quart size zip-top bag
1 gallon size zip-top bag
Mix together the milk, vanilla, and sugar, the pour into the small bag and seal, making sure there's as little air in the bag as possible. Put the small bag inside the large bag and add the ice, then add the salt. Seal the bag with as little air inside as possible. Wrap the bag in the towel and shake and massage it for 10-15 minutes. The ice will melt but will remain below freezing, and the milk will turn into ice cream!