Is there a specific pressure that is needed to boil water at room temperature? If there is, what is it? Why does water boil at a low pressure at all?
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Hi and welcome to physics.SE! I'm afraid I can't tell what you're really trying to ask. Please do not only put your question in the title but also use the body of the post to elaborate on it. – ACuriousMind Apr 29 '18 at 15:02
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completed! please tell me if it needs more changing as I don't quite know how this site works. – Alexandra Norris Apr 29 '18 at 15:10
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1Much better! Surprisingly, it seems this question has not been asked before, the most related ones I can find are about evaporation instead of boiling or about whether pasta will cook when boiled at low pressure – ACuriousMind Apr 29 '18 at 15:17
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@ACuriousMind Also Explanation for different boiling points of water on different altitudes – JMac Apr 29 '18 at 15:19
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Even more fun: get the pressure low enough so that you can watch water boil and freeze simultaneously. – PM 2Ring Apr 30 '18 at 12:31
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Since boiling, by definition, occurs when a liquid's vapor pressure reaches ambient pressure, your question is identical to asking what the vapor pressure of water is at room temperature. Here's an example of an online table:
At 23°C, for example, water would boil at a pressure of about 21.1 torr, or about a fortieth of atmospheric pressure.
Chemomechanics
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So does that mean if I'm floating in outer space (without a suit) and I'd cut myself, the wound would boil? Hm, maybe even worse, the blood would be squeezed out of me like toothpaste? – Ruben Verresen Apr 29 '18 at 19:20
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@rubenverresen, when exposed to vacuum, all the water in your body would instantly boil to vapor and you would explode. – niels nielsen Apr 29 '18 at 19:33
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3@nielsnielsen That's a misconception fed to us by Hollywood movies. In fact, you'd perfectly survive floating in outer space, at least for a few minutes, after which you die due to asphyxiation. I guess you'd probably feel a bit bloated, though. – Ruben Verresen Apr 29 '18 at 20:33
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Some fun threads: https://physics.stackexchange.com/questions/26332/how-does-space-affect-the-human-body-no-space-suit-no-space-craft and https://physics.stackexchange.com/questions/11453/direct-exposure-to-the-vacuum-of-space – Ruben Verresen Apr 29 '18 at 20:35
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@rubenverresen, has the experiment been performed? I mean, what's the molar volume difference between liquid water and its vapor? and assuming 15 PSI in the deep body core, versus zero at the skin surface, what contains that differential for a few minutes? Sounds like an experiment for mythbusters to me! but donlt expect me to volunteer... – niels nielsen Apr 29 '18 at 21:08
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@rubenverresen Vacuum freeze drying is a standard industrial process, and it is not that quick. – Bert Barrois Apr 29 '18 at 21:17
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@bertbarrois yes indeed; the vacuum is applied so slowly that the object being freeze-dried has ample time to equilibrate as the water vapor is drawn out of it and scavenged by the vacuum pumps. if one applied that vacuum suddenly, the food being freeze-dried would be torn to shreds. – niels nielsen Apr 30 '18 at 05:20
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It can't be that quick. Even when free energy favors boiling, there is a high heat of vaporization (enthalpy change). The delivery of heat must limit the rate of vaporization. – Bert Barrois Apr 30 '18 at 11:47
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What causes water to boil is not only the ambient temperature but rather the pressure acting upon the water surface. Example, at sea level where atm. Pressure is 14.7 psi, water will start to boil at 212 degrees F. However. At a higher elevation, say in Denver, Co. That is 5,250 feet above sea level, the pressure acting on the water surface is lower and thus, water will boil at a lower temperature.
Gerard De Santis
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