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?

up vote 6 down vote accepted

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:

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At 23°C, for example, water would boil at a pressure of about 21.1 torr, or about a fortieth of atmospheric pressure.

  • 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 at 19:20
  • @rubenverresen, when exposed to vacuum, all the water in your body would instantly boil to vapor and you would explode. – niels nielsen Apr 29 at 19:33
  • 2
    @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 at 20:33
  • @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 at 21:08

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.

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