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I put nearly boiling water in a Thermos flask (trade name for a vaccuum flask for home use). Later when I unscrew the stopper, steam and droplets of hot water are forcefully sprayed out, proving that the pressure inside the flask is higher than ambient pressure.

But how can that possibly be? When I poured the water in, all the air above the water but inside the flask was at atmospheric pressure (if the heat of the boiling water I was pouring in had raised any of it above ambient pressure, it would instantly have escaped through the opening at the top). Heat energy escapes from the flask while the stopper is on, as proved by the fact that the stopper feels very warm while it is screwed down.

So how on earth can the pressure inside a cooling flask rise from atmospheric, when I screwed the stopper down, to above atmospheric when I unscrewed it?

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  • $\begingroup$ Maybe that's the reason for the warnings not to put boiling water into the thermos flask? If you won't let the water cool down enough, it will continue to boil inside the closed flask for a while and build up pressure. This is just a guess, though... $\endgroup$
    – Wojciech Morawiec
    Commented Mar 16, 2017 at 22:10
  • $\begingroup$ But once I stop pouring in water, no more energy goes into the system, so there is nothing to build up pressure. Pressure can only build up if there is heat being added, or the volume is being reduced. $\endgroup$
    – Stephen F
    Commented Mar 16, 2017 at 22:47
  • $\begingroup$ What do you think happens to cool air inside the closed container when it gets heated by the boiling water? Adding hot water is adding heat to the flask. $\endgroup$
    – sammy gerbil
    Commented Mar 16, 2017 at 22:49
  • $\begingroup$ Yes, sammy gerbil, you are quite right. I did not see that. I didn't realise that the air needs longer to reach the same temperature as the water than the 2 or 3 seconds I gave it before screwing down the stopper. $\endgroup$
    – Stephen F
    Commented Mar 16, 2017 at 23:59
  • $\begingroup$ I just tried cleaning a flask with boiling water. Half filled it, sealed it, and shook it. The pressure increase overcame the seal and sprayed boiling water all over my hand. This supports the "you heated up the cold air" answers - more cold air - more pressure increase. I do not recommend repeating this experiment. I have a nasty burn. $\endgroup$
    – Andy Newman
    Commented Dec 19, 2020 at 14:07

3 Answers 3

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When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of thermal equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.

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    $\begingroup$ Aah, thank you very very much, Mr Hashemi! I blush that I did not see that long ago. The key is that I close the stopper about 2 or 3 seconds after pouring in the water, and that does not give the air time to reach temperature equilibrium with the water. Its temperature will continue to rise after I trap it by screwing the stopper down, so contrary to what I thought its pressure IS rising. I suppose this could cause the flask to explode, though I've saved boiling water like this for years. $\endgroup$
    – Stephen F
    Commented Mar 16, 2017 at 23:57
  • $\begingroup$ Wait just a bit longer and allow the steam from the water to push more air out before you close it. $\endgroup$
    – David Schwartz
    Commented Mar 17, 2017 at 10:42
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The air in the flask was cooler than the water when you closed the lid.

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  • $\begingroup$ Right on the nail, JMLCarter! Thank you for your fast and crisp response. I confess I did not fully understand your answer until Alireza Hashemi gave his fuller explanation below, but now I see you are quite right. $\endgroup$
    – Stephen F
    Commented Mar 17, 2017 at 0:02
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Yea, air inside and flask was cold, but what about equilibrium vapor pressure? If you have air humidity below 100% in that too, and even if u put 25°C water inside it, sooner or later it will build up pressure due to that. If you put in it boiling water it will be sooner. Also i would say that Boiling water is like Carbonated water, but reaching 100% humidity below boiling is very slow process (like 4x slower each 10°C)
Source: Equilibrium vapor pressure, Pressure of water

What i wanted to say: If you put no air, or air with 100% humidity and same temperature as water - there is no way to create more pressure.

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  • $\begingroup$ Wow, that's interesting! $\endgroup$
    – Stephen F
    Commented Mar 28, 2017 at 10:39

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