How Everything Works
How Everything Works How Everything Works

Question 716

If you have a glass of water that is real cold but not frozen, can the addition of one normal ice cube make it all freeze? Can I do this in the kitchen? - D
The answer to both questions is yes. If you begin with very pure, dust-free water in a very clean glass, you should be able to supercool it below its normal freezing temperature of 32° F (0° C). That's because water has difficulty forming the initial seed crystals upon which ice can grow. If you then add an ice crystal to the supercooled water, it should begin to freeze rapidly. While I have never done this myself, it shouldn't be too hard. You should probably use distillated and filtered water and a brand new glass that you've cleaned thoroughly. Cover the water to keep out dust. Cool it carefully through 32° F in the freezer and then add a tiny ice chip. The water should begin to crystallize around that ice chip. A simpler example of this sudden freezing phenomenon is a heat pack—one containing sodium acetate. At room temperature, it contains a supercooled solution of sodium acetate that is unable to freeze spontaneously. When you press a button in the pack, you trigger the crystal formation and the whole pack freezes in seconds. The crystallization process releases enough thermal energy to keep the pack hot for hours. Incidentally, ski resorts regularly seed the water they use to make artificial snow with molecules that initiate crystal growth to avoid forming supercooled liquid. Doing so greatly enhances the amount of snow they make.

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