Why does regular water freeze faster than salt water?

Why does regular water freeze faster than salt water? — CD, Crown Point, IN

When salt dissolves in water, its individual sodium positive ions and chlorine negative ions are carried about by the water molecules. Each of these ions is wrapped in a solvation shell of water molecules. These solvation shells and the salt ions themselves interfere with the water’s ability to crystallize into ice. The ice crystals that form when salt water freezes rarely include the salt ions so the water molecules must abandon the salt ions in order to crystallize. Because of the attraction between the salt ions and the water molecules, and because of the loss of randomness that comes with forming pure ice crystals in the midst of salty water, you must lower the temperature of salt water below the freezing temperature of pure water before that salt water will begin to freeze into ice. When ice does begin to form, it will be relatively pure water crystals and the remaining water will become increasingly saltier. If you’re ever lost in the winter without a supply of fresh water, look for sea ice—even though it forms from salt water, it contains very little salt.

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