How does atmospheric pressure affect the boiling point of water?
鈥� Water molecules can always escape from their liquid phase to form vapour. These escapees exert a vapour pressure, which contributes to the overall atmospheric pressure and depends sensitively on temperature. At higher temperatures, water molecules in liquid move faster and are thus more likely to escape into the air. It is the most energetic molecules that escape, lowering the average speed of the remainder 鈥� which is why evaporation cools the remaining liquid.
For water to boil, vapour bubbles must form within the bulk of the liquid. However, these will be squeezed shut by the surrounding atmospheric pressure until the vapour pressure in the bubble matches it. Hence, water boils at the temperature at which these pressures become equal.
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This happens at a lower temperature when the atmospheric pressure is lower 鈥� up a mountain, for example. Conversely, it boils at a higher temperature if the pressure is increased, which is why food can be cooked more quickly in a pressure cooker.
Chris Evans, Earby, Lancashire, UK
鈥� A liquid boils when the atmospheric pressure above its surface equals its vapour pressure. So as the former decreases, the liquid鈥檚 boiling point does too. Thus at the top of Mount Everest, where the pressure is only about 34 kilopascals (compared to 101.3 at sea level), the boiling point of water is only about 71 掳C.
Eventually, when you reach outer space where the pressure is zero, liquid water boils instantly no matter what its temperature. One practical consequence of this is that although you can boil an egg at the top of a mountain, it may not actually cook because the temperature of the boiling water is too low.
Simon Iveson, Chemical Engineering Discipline, Faculty of Engineering and Built Environment, The University of Newcastle, New South Wales, Australia
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This article appeared in print under the headline 鈥淏rewed at altitude鈥�