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Whenever I wash my thermos, I put hot water and then some soap in; then I seal the one end with my hand or use the lid. After shaking it up, if I slowly remove the lid or my hand, it expels a little air. Why is that? Does it have something to do with increased surface area of soapy water? Or is it the fact that the air is heated by the water, even though the water must surely cool slightly?

Qmechanic
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Peter Rankin
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    I am so glad somebody asked this question. I have observed the same thing. – M. Enns Feb 25 '21 at 15:53
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    FWIW: In my on experience, you don't need the soap. Shaking a container in which, just moments earlier, some clean, almost-but-not-quite boiling water was sealed will increase the pressure in the container. – Solomon Slow Feb 25 '21 at 17:11
  • It's true the effect works fine without soap. I have also managed to measure that if you shake the container with cold water the pressure drops which supports Gert's answer. – M. Enns Mar 18 '21 at 15:23

2 Answers2

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When you pour the hot water in, the air inside the thermos is still quite cold (ambient temperature, approx.)

But then when you shake it up the cold air is heated by the hot liquid. Gases expand considerably when heated, approximately acc. the Ideal Gas Law:

$$pV=nRT$$

This causes a modest (and harmless) pressure increase in the flask, which is what you experience.

Gert
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    You can also observe the opposite effect when trying to open a fridge again right after closing it. – noah Feb 25 '21 at 16:42
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    @noah Why "right after closing it"? Shouldn't the air pressure difference become larger if you give the air in the fridge more time to cool down? Is it because fridges are too leaky to allow pressure differentials to persist? – Will Feb 26 '21 at 00:16
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    @Will i have a standard freezer top fridge bottom combo and the way i was able to replicate the effect, at least without leaving the door open for a significant length of time, was to open and shut both the freezer and fridge door at the same time. After that, there was a very significant increase in the force required to open the door again. The reason is that most modern combo units have the freezer and fridge compartments connected, so if you just try it with the fridge door alone it really doesn't work. but the most likely reason is indeed that small leaks quickly end the differential – eps Feb 26 '21 at 00:51
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    @Will yeah that's right. The cooling down will create a low pressure inside, but this is then slowly equalized by air leaking back in (which is cooled down again, maintaining a little bit of pressure difference). – noah Feb 26 '21 at 07:34
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    Funnily enough, a (very slightly) leaky freezer seems to be a feature not a bug. You'd never get the door open otherwise!! – josh Feb 26 '21 at 09:49
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    It's not harmless if you open the thermos tilted upside-down and your face and neck get covered in half a liter near-boiling coffee. – Pål GD Feb 26 '21 at 17:25
  • I'd posit that the mere act of shaking the container imparts a bit of energy to the contents, thus increasing the temperature, and therefore the pressure, of the contents. – Self Evident Feb 26 '21 at 18:23
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    @SelfEvident The increase in temperature you can impart that way is really small. See Joule's experiment. – Gert Feb 26 '21 at 19:23
  • @PålGD There's no cure for idiocy, I'm afraid... – Gert Feb 26 '21 at 19:23
  • @Gert, it might be small, but neither irrelevantly, nor imperceptibly so. I just did it: I filled up a room temperature water bottle with room temperature water, closed it, shook it up, and opened it. As it opened, there was a very small "poof" sound as the air pressure equalized. If I didn't shake the bottle, there was no sound. Sure, not a rigorous study, and probably tainted a bit by confirmation bias or general ignorance, but I usually just all that... – Self Evident Feb 26 '21 at 20:14
  • ", but I usually just ignore all that..." is what I meant to say. :) – Self Evident Feb 26 '21 at 20:27
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    @SelfEvident The shaking simply increases the rate of heat transfer from the hot liquid to the air. Try this: fill your thermos half full with boiling water. Then stopper and allow to stand without shaking for 1/2 hour, then open. You'll get the same "poof" but has taken a little longer to get there. – Gert Feb 26 '21 at 21:59
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    @SelfEvident To measure the effect of 'cold shaking' just measure temperature before and after. – Gert Feb 26 '21 at 22:01
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    Great answer. I'd add one more bit to address OP's comment about the water should cool down if it heats the air. You implicitly mention it by saying how gases expand considerably, but they might not make the connection that liquids don't, which is why it doesn't just even out. – Kat Feb 28 '21 at 17:49
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There is another effect here which is significant, as follows.

Warm water wants to evaporate, but in a flask-shaped container, the evaporation can take place only at the free surface of the water in the flask. Furthermore, as soon as the boundary layer of air right next to the warm water becomes saturated with vapor, the diffusion of water vapor into the air slows down greatly.

If you close the container and shake it vigorously with soap added, tiny air bubbles get mixed into the warm water, producing a huge surface area available for evaporation to occur across. The bubbles expand as they get loaded with vapor and the pressure inside the container jumps up suddenly.

niels nielsen
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