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It's a common school experiment to make water boil at room temperature by putting it in a container and reducing pressure.

I am wondering if it is possible to make a sealed sphere containing water where the pressure is carefully set to room temperature so that it boils "forever".

Obviously it can't boil forever, because it would eventually all turn to vapor. What I am thinking though is that minor fluctuations in room temperature will cause it to cycle between boiling and condensation without the need for any power.

Is this line of thinking correct? Has anyone attempted to make something like this? I think it would be a really cool desk ornament, like a little box of cycling boil-condense water.

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  • $\begingroup$ Why, you can make such a container, but it would not look cool at all. In a relatively tiny enclosed space, vapor will almost instantly get saturated via surface evaporation alone, without any visible boiling. $\endgroup$ – Ivan Neretin Nov 23 '16 at 18:47
  • $\begingroup$ @IvanNeretin Why would it get saturated so fast? Assuming it's a 6x6x6 inch cube, is there no way to make it take a few minutes to get saturated? Is there a formula to figure this out? $\endgroup$ – popctrl Nov 23 '16 at 18:58
  • $\begingroup$ By way of comparison, concerning "minor fluctuations in room temperature will cause it to cycle between boiling and condensation" (however using dichloromethane or similar instead of water): en.wikipedia.org/wiki/Drinking_bird $\endgroup$ – Loong♦ Nov 23 '16 at 19:02
  • $\begingroup$ Well, let's put it this way: maybe I shouldn't have said "instantly", for it wouldn't really get saturated that fast. But then, water wouldn't heat that fast either. $\endgroup$ – Ivan Neretin Nov 23 '16 at 19:04
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    $\begingroup$ Yes. You make a container, boil the water in it so it flushed away the air from it and than seal it off. In such case even tiny warming will cause the water boil. However it is easier said than done as vapor pressure of water is rather low at room temperature, so surprisingly high vacuum is created when the water in the container cools down. $\endgroup$ – permeakra Nov 23 '16 at 19:27
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You can buy a hand-heat powered boiler. But something must provide energy to keep the bubbles forming, and remove it to condense the gas back to liquid, or it would all be in the vapor state. In this toy, the heat of your palm provides the input energy, and ambient air cools the vapor in a condenser.

BTW, this demo won't work if the room temperature is exceedingly high, because the liquid (probably an ethanol-water mix) won't condense and the vapor pressure becomes too high for bubbles to form in the remaining liquid.

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Water boils if the vapour pressure of the liquid phase is larger than the total pressure in the gas phase (of a forming bubble). In a closed vessel, an emerging vapour bubble finds that as it grows, so is the total pressure. Which makes it shrink again.

Unless there is a large temperature gradient, this cannot work.

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