I’m a lunchbucket kind of guy, and there is a curious thing that happens every time I clean my thermoses. I run some hot water in the bottle, cover the mouth of the bottle with the palm of my hand, and give the bottle a shake. Immediately the pressure inside the bottle forces my hand to break the seal, and air gushes out. Where did this pressure originate? Trapped gas in hot water? It does not happen with cold tap water, only hot tap water. Granted, this is not really a great cosmic question, but it is one that has puzzled me for a few decades. Thank you for enlightening me!
Finnie the Pooh
There’s your answer, right there. Hot water gives off water vapor at a fairly high rate, which increases the pressure. Try this experiment. Squeeze an empty plastic soda bottle til it collapses. Fill it about a quarter full with hot water, then cap it. Watch what happens.
When you shake the thermos, the hot water heats the remaining air in the bottle quickly, causing it to expand and raise the pressure.
If you “burp” the thermos, and continue shaking, it shouldn’t really keep happening since the air should quickly come up to the temperature of the water and not expand further.
-Jeff
I thought the question was going to be
“How do it know?”
The response you got from zlexiss is correct Finnie, pay no mind to Q.E.D.'s assertation that “hot water gives off water vapor at a fairly high rate, which increases the pressure.”
What’s an “assertation”?
zlexiss’ assertion is correct. The hot water heats the air.
Assertation do to be a word. 
You know, I never even thought about the air IN the bottle! Of corse it will expand when heated with the hot water. Wait 'till I tell the guys at work about this. Hell, they’ll just scowl and shake their heads like always when I try to astound them with my scientific knowledge.
:smack:
When they fix the smileys, hopefully that will make more sense.
My cursor says that smiley is smack! :dubious: cool thread 
Whateva!!!
Slight hijack: As a fellow Ontarian, where the hell is Kaministiquia?
About 20 km north and west of good old Thunder Bay. Up in the Finn woods and hills.
Actually, I believe Q.E.D. had it right.
Remember gas laws from your chemistry or physics classes? One, sometimes called Gay-Lussac’s law, states that pressure and temperature are proportional–they vary directly if volume is held constant. If the temperature doubles, then so will the pressure. But the temperature has to be in Kelvins. For our purposes, assume room temperature is 25 degrees Celsius and your hot water is 90 degress Celsius, which is really hot (water boils at 100 degrees Celsius, you remember). In Kelvins, this is a change from 298 to 363, which would result in multiplying the pressure by a factor of 1.22–a 22% increase in volume would result from the heating of the air inside the bottle from room temperature to so hot it would burn your hand.
On the other hand, the vapor pressure of water at 25 degrees Celsius is 24 mm Hg (3.2 kPa). At 90 degrees Celsius it is 526 mm Hg (70 kPa), according to my handy CRC Handbook of Chemistry and Physics, 48th edition, page D-110. This increase in pressure is added to the air pressure already trapped in the bottle, which we assume to be 760 mm Hg (101.3 kPa). So the pressure has more than doubled due to the hot water changing into vapor when you shake it. This is a much larger effect than the one due to expansion of air when it is heated.
Chemie, now you have it wrong too. In your first example, the pessure has doubled by a factor of 1.22. In your second example, the pressure is now multiplied by a number of more than 2. However, in both examples you are doing the same exact thing. Do you think the pressure will be different based which perspective you view things?
Where are you going to get 90 deg. C water to fill a thermos and then hold you hand over the mouth?
Somebody took a wrong turn somewhere.
Try your calculations with 128 deg.F = ~53 degC.
I would just like to add that the reverse is also true. Cold water shaken in a thin walled plastic bottle will cause the bottle to partially collapse.
Let me try this again.
Finnie has a thermos into which he runs hot tap water. Then he covers it with his hand and shakes, but pressure always builds up and causes air to burst out. We are arguing whether the heating of the air in the bottle or the changing of water into vapor causes the build up of pressure.
Of course, both of these things contribute. I would like to show that the contribution from the vapor pressure of water is larger than from the heating of the air.
First, the calculation about the change in pressure due to heating the air. I’ll use a cooler temperature as suggested by spingears, but my reference table from this location doesn’t include that exact temperature. I’m using 60 degrees Celsius (333 Kelvins). As before, if we start with room temperature of 25 degrees Celsius (298 Kelvins) and assume a constant volume, then we see that the pressure will be multiplied by a factor of 1.12. This means the pressure only increases by 12% due to the heating of the air in the thermos alone. The pressure would increase from 101 kPa to 113 kPa, if this were the only effect occurring.
Now, on to the calculation for the pressure produced by the water vapor. As I said before, the vapor pressure of water at 25 degrees Celsius is 24 mm Hg (3.2 kPa). At 60 degrees Celsius, the vapor pressure is 149.4 mm Hg (19.9 kPa). There is a difference of 16.7 kPa. So the pressure would go from 101 kPa to 118 kPa if this were the only effect occurring.
Having done the calculation at a cooler temperature, I see that the vapor pressure change is not as dramatic at this temperature, but the calculations do seem to support my claim that the larger effect is from vapor pressure. It is a small difference, though. I don’t have the correct references, but if the calculation is redone with spingears temperature of 53 degrees Celsius, it is possible that the pressure increase due to the heating of air would be larger than the pressure increase due to the vapor pressure of water.
Of course, since both are occurring in the same bottle, and considering Dalton’s Law of Partial Pressures, the pressure is additive and there would be an increase from 101 kPa to 130 kPa, which is enough to cause the effect Finnie noticed. There may also be some contribution from the shaking and soap, if that is used at this point in the thermos-cleaning process.
So, it’s a close call at these temperatures, and I don’t think you can tell without calculating specifically.
In the interests of scientific research and impartial observation, I tried a variation on this experiment.
I added nothing to the vessel, and put my hand over its mouth, then shook it forcefully. Sure enough, there was a pressure buildup that forced my hand away, followed by a hissing expulsion of gasses. And several expletives, not reproduced here.
Of course, I should add that the vessel used was my mother-in-law…
The word Thermos® is a brand name. The generic term is vacuum bottle.
If you put chilled water into a bottle and shake it, the air cools down, creating a suction effect - I think it very unlikely that this effect is caused by condensation of vapour, but rather by simple thermal contraction of the air.
Perhaps we should carry out an experiment to determine whether the pressure arising from the addition of hot water is due to vapour or simple thermal expansion; how about adding something warm that doesn’t evaporate, such as heated sand to a bottle, shaking it and seeing whether the increase in pressure is comparable to when hot water is used?