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Q & A: Pressure vs. Boiling Point

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Most recent answer: 10/22/2007
Do you know the value of the boiling point at 10e-6...10e-7 torr. I work on a vacuum chamber but i have a problem with water. I can see it on a mass spectrogram at 10 times over nitrogen so i wonder if it condense at 10e-6 torr.i can t find data at this pressure. thank you very much.
- dam (age 24)
paris france
Dam -

I don't know what the exact value you're looking for is, but I think I can answer your question. An easy way to think of this is to look at the "phase diagram" for water. This is basically just a graph showing the water's state of matter at different temperatures and pressures. The lines between the states show the boiling points, melting points and sublimation points:

As you can see, as the pressure falls, so does the boiling point. Just by looking at the graph, you can see that the boiling point is around room temperature at about 0.1 atm (or about 75 torr). At pressures much lower than this, such as the one you mentioned, the boiling temperature will be significantly below zero degrees Celsius. So it seems unlikely that, at room temperature, your problem is condensation.

Further, you may have noticed on the graph that below a certain pressure (the "critical point"), it can not exist in the liquid phase at all. It changes directly from a gas into a solid, or vice-versa. For water, this pressure is about 0.006 atm (or about 4.5 torr). So at the pressures you're talking about, it would not be possible for the gas to "condense", persay.


(published on 10/22/2007)

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