Philadelphia style ice cream, manufactured and distributed throughout the Philadelphia region,
for people who live, work, or play in the Delaware Valley.

You probably know that salt is often used in the production of home made ice cream, and that, until the beginning of this century, it was responsible for most manufactured ice creams as well. You may know that it helps to lower the temperature of the ice water bath in which the ice cream mixture is immersed, but, chances are, you're not quite sure why. The following discussion should help to clarify this proceess for you.

When crushed ice combines with equal amounts and water, the water's temperature usually cools to about 32 degrees Fahrenheit, the melting point of ice, and the freezing point of water.

Adding salt to the icy water, however, actually cools the water below 32 degrees fahrenheit, and there are two reasons why this phenomenon occurs.

  1. Common table salt is also known as sodium chloride, a compound containing both sodium and chlorine. When salt dissolves in water, The water dispenses energy (in the form of heat) to break the bonds that hold the salt crystals together and to separate the sodium chloride molecules into sodium and chlorine ions (or charged atoms). As the water loses heat, it gets cooler.

  2. In order for ice to melt, it must get heat from an outside source where the temperature is higher than 32 degrees Fahrenheit. In icy water, the ice can get this heat from the surrounding water. Once the temperature has reached 32 degrees Fahrenheit for the water and the surface of the ice, however, equllibrium is reached, and the ice ceases to melt.

    In this equilibrium state, it is just as likely for individual molecules from the liquid water to unite with the surface of the ice crystal as it is for molecules on the surface of the crystal to cross into the liquid water.

    Dissolving salt into the ice water disrupts this equilibrium, because the separate sodium and chlorine ions, with their electrical charge, attracts molecules from the liquid water, so that the water molecules form a shell around the ions. As more the water moecules connect to the ions, there are fewer water molecules to combine with the ice.

    Water molecules from the ice continue leaving the ice crystal as easily as before, however, to unite with the liquid water, thereby melting the ice. Since melting requires heat, and the heat comes from the water, the temperature of the water drops as it uses heat to melt the ice. As more salt is added, the water temperature will continue to drop as long as the solution still contains solid ice.

    The same phenomenon occurs on snow or ice in the winter. Salt lowers the freezing point of ice, thereby converting the ice on roads and sidewalks into very cold water. Source: "We All Scream for Ice" The Washington Post, Column: PHENOMENA Wednesday, July 9, 1997 ; Page H08