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We used a freezer this time simply because it was now dark outside, and it was more convenient. We used styrofoam cups instead of pans. We used less water this time because the styrofoam cups were small.All these changes, (except one), while certainly having an effect on how long it takes water to freeze, weren't important for our purposes. We just wanted to illustrate that under the right conditions, hot water will freeze faster than cold water ... and that's what happened! The cup containing hot water began to freeze after about 37 minutes; the cold water began to freeze two minutes later. Apparently cold water freezes faster than hot water under some circumstances, and under other conditions hot water freezes faster. What's the explanation? Hot water that is near the boiling point evaporates a lot faster than cold water. As some of this water evaporates, there is less water left behind to freeze, so it won't take as long. But equally important, evaporation requires energy, which comes from the water. As the hot water is evaporating, it is cooling the water that's left behind. Both of these factors will increase the rate of cooling of the hot water, and under certain circumstances, make it freeze before the cold water does. The special circumstances require that very little of the heat from either sample of water can escape because of conduction. If the heat can escape quickly by conduction, the cooling effect of evaporation won't be enough to make the hot water freeze first. That's why our first experiment with water in pans saw the cold water freeze first ... heat was able to escape through the pans into the ground beneath, and quickly freeze the cold water, which only had to drop 19 degrees to freeze. When we did the experiment again with styrofoam cups, the heat couldn't escape quickly by conduction, since styrofoam is a good insulator. Instead, the extra cooling provided by evaporation caused the hot water to freeze first. Once again, cold water will freeze first if the heat in it can escape by conduction. Hot water will freeze first if there is very little conduction, allowing evaporation to become the more important cooling effect. There are of course other factors to consider. For example, boiling the water removed some of the dissolved gases from that sample, and water will cool faster if it contains less dissolved gas. Also, hot water circulates more rapidly (convection), making it easier for the heat to escape. Nevertheless, conduction and evaporation are the two most important factors that determine freezing time. We first encountered this question when we noticed that skating rinks were often flooded with hot water. The explanation given was that hot water freezes faster, but in that situation, the real reason for using hot water is that it melts the bumps and irregularities on the ice surface, making it smoother. As to whether a thin coat of hot water on ice would really freeze faster than cold water ... we aren't sure. Both conduction and evaporation would have an effect, but which would be the most important factor is unclear, at least to us. Probably both would freeze so fast that it wouldn't make much of a difference anyway. |