521. Why do people put salt on icy sidewalks in the winter?
Whenever a molecule dissolves in water, the water molecules bind to that molecule and surround it, forming a shell of water molecules around the impurity. Salt water is filled with these tiny balls of water, each one surrounding a single salt ion (either a sodium positive ion or a chlorine negative ion). These little water balls can't crystallize into ice because ice can't fit a sodium ion or a chlorine ion into its orderly structure. As a result, the presence of salt in the water makes it harder for the water to crystallize into ice. The water has to exclude the salt from the crystals that form as it freezes and this difficult process requires that the salt water be cooled below the freezing temperature of pure water before it will freeze. The more salt the water contains, the lower the temperature at which that salt water will freeze. This effect even works when you just sprinkle salt on ice. As long as the temperature of the ice isn't too cold, the salt will begin to dissolve in the water molecules of the ice and ice's crystalline structure will begin to break down. The result will be a puddle of cold salty water. That's why people use salt to melt the ice on sidewalks. But if the ice is too cold, the salt will remain separate and the ice will stay pure ice. That's why salting only works when the temperature isn't too far below freezing.