Question

Which freezes faster, hot water or cold water?

Asked by: Ben Cobb

 Answer

All things being equal, cold water freezes faster.

It takes time for the energy contained in a hot object to be transferred to a cold object. However, the rate of heat transfer is proportional to the temperature difference between the two objects, so hot water will lose heat faster than cold water. In other words, if you have water at 90 degrees C and water at 10 degrees C and the freezer is at -10 degrees C, the hot water will lose heat five times faster than the cold water; however, the cold water will still win the race. As the hot water cools it's rate of heat transfer will decrease, so it will never catch up to the cold water.

Some people claim that hot water freezes faster because a pot of boiling water can be thrown into the air on a cold winter day, and it freezes in mid air creating a shower of ice crystals. Whereas a pot of cold water thrown into the air comes down as large blobs of water. This happens because the hot water is so close to being steam, that the act of throwing it into the air causes it to break up into tiny droplets. (hot water is less viscous than cold water, listen to the sound it makes when you pour it in the sink) The small water droplets have a large surface area which allows for a great deal of evaporation, this removes heat quickly. And finally, the cooled droplets are so small, that they can be easily frozen by the winter air. All of this happens before the water hits the ground. Cold water is thicker and stickier, it doesn't break up into such small pieces when thrown into the air, so it comes down in large blobs. '

Answered by: Joe Larsen, Ph.D. Chemistry, Rockwell Science Center, Los Angeles, CA

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