Naked Science Forum
General Science => General Science => Topic started by: jacquot on 06/01/2010 18:17:32
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My weather station is currently covered in snow and it's reporting that the outside temperature, well away from the nearest building, is 0°C. In various other places in my garden, icicles have formed, which gives me the impression that the air temperature is actually lower than 0. That got me to wondering about what temperature ice can cool to. When plain water's heated to 100°, it changes form by turning to vapour, and can't, as I understand it, at sea level and normal atmospheric pressure, ever exceed a temperature of 100°C. When water's cooled to 0°C, it changes form into ice, but I don't know whether the resulting ice's temperature can decrease beyond 0. If, for example, the air temperature is -20°C, will the temperature of ice exposed to that environment continue to decrease, and will its temperature eventually match that of the air? Perhaps the weather station, covered or partly covered in snow, will continue to inaccurately report the temperature unless I clear snow from it.
[PLEASE BE SURE TO PHRASE YOUR THREAD TITLES AS QUESTIONS, WHICH IS THE FORUM POLICY. THANKS. CHRIS]
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The points at which water changes to ice or boils to form water vapor are called phase changes. As you correctly state, the temperatures at which these occur depend on pressure. Once water is a solid, you can continue to cool it to nearly absolute zero without consequence. Once it is a gas, you can continue to heat it (almost) without limit.* There are cases where liquid water can be cooled below its expected freezing temperature or heated above its boiling temperature. This usually involves very quickly cooling/heating it and making sure there are no impurities in the water. The water is said to be supercooled or superheated. If you disturb the water by dropping something into it or smacking the container, it can spontaneously freeze or boil over. (The particularly dangerous example of this is superheating water for tea in the microwave, then having it boil over instantly when you put the teabag in.)
Supercooling: Superheating:
The weather station might be reporting 0oC because the snow covering it is being melted by the sunlight and the melting happens at 0oC, or the snow might be insulating it from becoming colder, or it might actually be 0oC outside...
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* I say "almost" without limit because you can never reach absolute zero because of quantum mechanical effects, but you can get really close. Also, if you heat something too much, I suspect the molecules of the gas will have so much energy that they'd probably start undergoing nuclear reactions (breaking apart or fusing when they smash into each other) or become a plasma instead of a gas...
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Right, so ice will continue to cool. Those super-cooling experiments are worth repeating next time a few of my young nephews and nieces are around. Scientific magic! Think I'll leave the super-heating ones, though. Thanks for the input. Cool, really.
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It’s fairly easy to cool purified water to 20 °C
I'm sure it is! [;D]
Erm, you might want to take another squint at those numbers.
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"The temperature of 0 °C is the melting point of ice, and technically not the freezing point of water. "
I thought it was technically the temperature where they were at equilibrium.
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Colder ice becomes harder ice. The recent Mars polar explorer encountered ice that was quite cold and therefore quite hard. This can be observed at home. Take an ice cube out of the freezer. Wait several minutes. Take a second ice cube out the freezer. Hit the cubes with a hammer. The ice cube that has been out for a while mushes. The ice cube just taken out of the freezer shatters. Anyone that has ice climbed knows this to be true. On cold days the ice is brittle and more difficult to climb as it shatters more easily.