Why Does Adding Salt to Ice Make It Colder?
Salt does not make ice colder, but it does lower the melting point of water. This causes ice cubes to melt, and it prevents salt water from freezing at the same temperature at which fresh water freezes. By lowering the freezing point, the ice turns back into the liquid state.
Humans use this property of salt for a variety of applications. Salt is used in the production of ice cream and other frozen dairy products. In order for ice cream to freeze, it must be chilled lower than 32 degrees Fahrenheit. As ice does not get colder than this through conventional means, salt is added to the ice to keep it liquid. The resulting salt-water solution can be chilled much lower than 32 degrees, which allows for the proper cooling of the cream.
In the winter, ice- and snow-covered roads can be treated by spreading salt. As the salt combines with the ice, it melts the ice, which allows the salt to form a solution with the melted water. This melted, salty water further melts more ice. This principle only works to a certain point. At temperatures below 15 degrees Fahrenheit, the salt does not bond well with the water.