I was thinking about how one can create water vapor, for example, at room temperature by creating a full vacuum (0 psia) environment - such as placing the water in a bowl under a bell jar, and using a vacuum pump to suck out the air. BUT!
Paradoxically, don’t we define pressure by the resultant force of all molecules colliding with the walls of a vessel? So, if I have a vapor at 0psia in a bell jar at room temp (72 F), I would still have molecules colliding with the walls because I am not at absolute zero - where all molecular motion stops. Therefore, do we really have 0psia when T<> absolute zero (273 C)??? And if so, then is our definition of pressure correct?
This question just hit me out of the blue…next time, I’ll duck!
You cannot get to 0 PSI when there is water vapor present. The vacuum pump will pump out the vapor and cause more water to boil, or sublime, until there’s no water left. Even then limits on the efficiency of the vacuum pump, and outgassing from the container walls will make it hard to get the pressure below 10[sup]-13[/sup] atmospheres, much less Zero.
Hmm, Squinx: Your answer sounds reasonable, but in college we did a lab to find the triple point of water. I forget how we did it, exactly. But, it did involve a vacuum pump, and we did observe all three phases of water co-existing in equilibrium for a subtstantial amount of time.
(A) I guess Squinx would argue, perhaps, the vapor sucked off by the vacuum pump was at a slow enough rate as to sustain this for a substantial length of time?
(B) Granted, maybe we weren’t at 0.0000 psia, but I assume we were close enough…and yet when would you stop generating vapor? In short, how much vapor could one generate at/near 0.0000 psia…until the very existence of the accumulated vapor would defeat the very vacuum conditions which sustains it?
Jinx
Me thinks Squink’s answer covers my next question: Just how much water vapor could one generate under full vacuum before you no longer have a vacuum! I mean, the more vapor generated, the more the pressure will build - until boiling is surpressed
There is no particular reason why you need low pressure to get water vapor. As you have no doubt noticed, it’s plenty easy to get water vapor at standard room temperature. In fact, (I won’t even pretend to have remembered this from high school chemistry) the vapor pressure of water depends only TEMPERATURE. (http://dbhs.wvusd.k12.ca.us/GasLaw/Vapor-Pressure-Data.html). Lowering the ambient pressure does not actually increase the partial pressure due to water vapor. However “boiling point” is defined as the point that the vapor pressure of the liquid exceeds the vapor pressure of the external pressure. (http://www.ilpi.com/msds/ref/boilingpoint.html).
So at lower pressures, the water may boil at lower temperatures. But the amount of pressure due to water vapor (in an ideal system) remains the same.
If you put a beaker of water in a bell jar, pump out all the air then seal the bell jar, you will get the equilibrium vapor pressure of water at ambient temperature. The number in torr is about the same as the temp in degrees centigrade near room temp.
If you put a beaker of water in a bell jar then seal the bell jar without any pumping, you will get the equilibrium vapor pressure of water at ambient temperature. The number in torr is about the same as the temp in degrees centigrade near room temp. The inert gas makes no difference at all in the vapor pressure of the liquid to first order approximation. Calculated exactly, inert gas pressure acts as a permeable pistion and the liquid’s vapor pressure is very slightly increased.
If you put a beaker of water in a bell jar, pump out all the air then continue to dynamically pump, the water will continue vaporizing from the mass thereby cooling it. Pumping water under dynamic vacuum will cool it to freezing, then the solid ice will sublime (re freeze drying or lyophilization).
Note that the normal boiling point of a liquid is the temp at which its vapor pressure equals atmospheric pressure, 760 torr. However, its boiling point in general is that temp where its vapor pressure equals the applied pressure. Since water at room temp has a vapor pressure of about 20 torr, pumping it to below 20 torr wll set it boiling.
>> May the moderator be with me and close/delete this double post!
No, he had to lock the other thread where I posted thinking he’d lock this one. And having been logged out for the umpteenth time I feel like sodomising someone but I am not sure if I should sodomise a hamster or a moderator.
