Does hot water really freeze quicker than water at room temperature? — MH, Dallas, TX
In most cases the answer is no. All things being equal, the room temperature water will have a head start and will freeze first. The hot water must first cool down to room temperature and then it will simply follow the behavior of the room temperature water. However, in the special case where the water is held in an insulated container that’s open at the top, it’s possible for hot water to freeze faster. That’s because evaporation of water molecules from the exposed surface of the hot water makes an important contribution to the cooling process in that case and a significant fraction of the water molecules will have left the container by the time it reaches room temperature. Since it takes less time to freeze a smaller quantity of water, the container of hot water can freeze before the container of room temperature water. However, there will be less ice in the container that was once filled with hot water.
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.