|MLA Citation:||Bloomfield, Louis A. "Question 567"|
How Everything Works 19 Jun 2018. 19 Jun 2018 <http://howeverythingworks.org/print1.php?QNum=567>.
There is another interesting effect that occurs when freezing hot water. If you boil water, you will drive most of the dissolved gases out of it. You see these gases emerge as bubbles on the sides of a pot as the water heats up when you put the pot on the stove. If you freeze boiled water, it will probably freeze slightly faster than unboiled water. That's because the dissolved gases also come out of solution during the freezing process and these gases form bubbles in the ice. These bubbles slow the flow of heat through the ice and delay the freezing of its center. Thus, while room temperature water will freeze quicker than hot water, previously boiled water that's now at room temperature will freeze even quicker than normal room temperature water. The boiled water will also form clearer ice cubes—they won't have any bubbles in them.
Finally, John Newell points out an interesting practical reason why hot water may sometimes freeze faster than cooler water in a household refrigerator—the temperatures of those refrigerators fluctuate because their thermostats have hysteresis. Once it has stopped operating, a refrigerator's compressor won't turn on again until the refrigerator temperature drifts upward significantly. If you put cool water in the refrigerator's freezing compartment, it may be quite a while before the compressor turns on and the refrigerator begins to pump heat out of the freezing compartment. But if you put hot water in the compartment, you may raise the temperature of the refrigerator enough to start the compressor, thus accelerating the freezing of the water.