When sodium chloride is added to the water, a solution is formed. The forming of the solution interferes with the orderly arranging of the particles in the crystal. This results is a lower freezing point. You may want to mix up the ice cream ahead and omit this part of the procedure from your lab. Sodium chloride is added to the ice to lower the freezing point of the ice.
Large crystals dissolve more slowly than small crystals.
This allows time for the ice cream to freeze more evenly. When sodium chloride is placed on the highway or steps, the freezing point is lowered and the ice melts. Sodium chloride is used for three reasons. First, some solids such as sugar do not dissolve in ice water as well as salt. Only advanced students would need to know this concept.