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Name: James
Status: other
Grade: other
Location: MD
Country: N/A
Date: January 2007

Question:
We put tap water in a glass jar, sealed the jar and applied vacuum to approximately 28". The water boiled rapidly for about 50 to 60 seconds, then it slowed to a stop. That same jar of water will no longer boil at 28" of vacuum. If we replace the water with fresh water the same thing occurs, it boils the first time but then will not boil after that. What change occurs to the properties of the water (which remains in liquid form) that prevents it from boiling the second time?



Replies:
Two things: the dissolved gases that were in the water escape, and the water gets colder, so that its temperature is below the boiling point at 28" (I presume you mean inches of mercury?) vacuum. That temperature is about 37 C, which is probably warmer than your tap water to begin with. Most likely, then, the boiling you observe is actually outgassing. Either way, you should notice that the "spent" water you remove from the vacuum chamber is cooler than when it went in.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming James,

What you are seeing is not really water boiling. Rather, gases trapped within the water sample is outgassing. The bubbles you see is not water turning into water vapor, but air that is escaping from the liquid water. The boiling point of water at 29.5" (atmospheric pressure at sea-level) is 100 degC. Reducing the pressure down to 28" will minimally change the boiling point of the water to approximately 99 degC. Thus, unless you happen to be heating the water to this temperature, the water is not really boiling.

Greg (Roberto Gregorius)


The information you have provided is incomplete. Did it "boil" just by pulling a vacuum or did you have to apply heat? Did you continue to pump on it to maintain the 28" of mercury vacuum?

If you were not applying heat and you were maintaining vacuum I would suggest that it never boiled but that you were pulling dissolved air out of the water. Once all of it had come out, the "boiling" stopped and could not be repeated.

To find out if this is the case:

A

1 bring it back to atmospheric pressure in air
2 stir or shake it vigorously in air for a few minutes
3 pump to 28" of mercury vacuum & observe the results

B

1 seal it off at 28" of mercury vacuum
2 stir or shake it vigorously under vacuum for a few minutes
3 pump to 28" of mercury vacuum & observe the results

If it "boils" again in experiment A but not in experiment B then it is likely you are not boiling (vaporizing the water) but pulling dissolved air out of the water.

Greg Bradburn


James-

Only two things can be happening.

1) dissolved air can be out-gassing. This is irreversible, only happens once. Just like what you observed.

2) the water, by boiling, will be cooling down. This is reversible.

In an hour or so, if you close off the vacuum port but let in no air, the water will have warmed back up to room temperature, and you could show boiling again by opening the vacuum again.

When water is heated on the stove, the air bubbles leaving are usually much smaller than the water vapor-bubbles of boiling. Think about it: by reducing the heating rate, the boiling bubbles would get a little smaller. And by reducing atmospheric pressure, the dissolved-air out-gassing bubbles would get bigger. The perceptible difference between out-gassing and boiling would be reduced, conceivably even obliterated or reversed.

When water is cooled by vacuum-evaporation (boiling at room temperature), it gets cooler than room temperature, but probably not as cold as freezing. Room temp. is 20-25C, freezing is 0C, so the ambient is only 20C warmer than the boiled water. Probably less. And the jar is glass not metal, so it has poor thermal conductivity. So the heat-flow from room air into the beaker will be a lot slower than it is for a pot over high heat on the stove.

To cause repetition of the dissolved-air out-gassing, one would re-saturate the water with air. Close off the vacuum line, insert a tube to the bottom (maybe with many small holes like the tip of a fish-tank bubbler), and pump air through it for some time. Not sure, I would start with an hour, but less might be enough. Opening the lid and waiting overnight might be enough too. Stirring would have some rate in between. The water will re-saturate with air, and appear to boil when you again evacuate it. If you have not dissolved as much air as it had before, the bubbling will seem less.

I'd look up the vapor pressure of water, if I were you. Get a table. There is a little graph at Wikipedia: Vapor_Pressure:

http://en.wikipedia.org/wiki/Vapor_pressure, halfway down the page.

You could use a more precise pressure-gauge too. 29.9"Hg -28"Hg =1.9"Hg absolute pressure. 2"Hg absolute pressure is more than the vapor pressure of water at room temperature. I think your vacuum pressure might not be low enough to force actual boiling at room temperature. Maybe all you have seen is the out-gassing stage that always happens before boiling, and then the action stopped because you did not have enough vacuum to do the next stage. Tell you what, put a hot-plate under that beaker (set too low to boil a water drop), or a couple of big light-bulbs beside it, to warm it gently.

If you warm your water up, at some temp (~50C?) it will boil continuously, (and your vacuum pump will have to cope with a lot of re-liquefied water in its exhaust.) You can learn your real vacuum pressure from that temperature the vapor-pressure table.

Jim Swenson


It is difficult to analyze exactly what is going on without knowing more of the detailed experimental conditions. For example the "capacity" of your pump -- not just the final pressure, but what volume per unit of time the pump can remove volatile components. Without that information, I would suspect that the initial "boiling" you see is not actually boiling, in the sense of removing converting liquid water to water vapor. Rather, in the absence of more details, I would suspect that what you are observing is the removal of the components of air dissolved in the water. These would include mainly carbon dioxide and oxygen. This would be consistent your observation that there is no "boiling" the second time you carried out the evaporation process.

You could test this hypothesis by taking the water that has had the air removed and shaking it vigorously to re-dissolve the components of air. If just shaking the water, from which the components of air have been removed, and then allowing those components to redissolve -- and then you see the "boiling" reoccur -- you could be pretty much on the right track that the "boiling" is not the evaporation of water itself, but more the de-gassing of the water.

You could then confirm this observation by boiling the water at its normal boiling point -- which would remove the atmospheric components -- then re-do the boiling experiment. If you do not let this water stay round long, you would not expect to see the "boiling" of dissolved gases.

You could further confirm this hypothesis by freezing the water a couple of times before carrying out the "boiling" experiment. Gases are not very soluble in ice, so you would expect that a couple of freeze/thaw cycles before the "boiling" would remove the dissolved gases and you would not expect to see any bubbles that you are calling "boiling".

These kinds of experiments are how science works. Observation: Test(1) -- Hypothesis(1) -- Test(2) -- Hypothesis(2) -- and so on.

Vince Calder



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