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A friend and I were having a discussion and we came to this question we couldn't answer: When boiling water in the classic way, you heat it and create convection due to the hot water moving to the surface and the cold water going down to get heated.

But what if you boil the water by reducing pressure under $63 {\rm hPa}$? Will there still be a movement due to the propagation of the pressure reducing?

We can go further, imagine a sphere of water floating in the middle of a gravity-less room. If you change the pressure to less than $63 {\rm hPa}$, will it be a boiling sphere, with its whole surface boiling?

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  • $\begingroup$ What do you think? Why is there convection when you heat the water? What is the difference when you reduce the pressure? $\endgroup$ – sammy gerbil Mar 30 '17 at 16:50
  • $\begingroup$ I think the speed of sound in water is about 1,484 m/s $\endgroup$ – JMLCarter Mar 30 '17 at 16:52
  • $\begingroup$ If "the classic way" means in a pan sitting on a stove/cooker/grille, then you are applying heat mostly to the bottom of the pan, and all of the evaporative cooling is happening at the top surface. $\endgroup$ – Solomon Slow Mar 30 '17 at 17:52
  • $\begingroup$ @sammygerbil when lowering pressure the water boils at less than 100°C. Like at half our atmosphere pressure (500 hPa), water boils near 85°C $\endgroup$ – Grégoire Fruleux Apr 2 '17 at 2:05
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You can take a look at the videos of people doing this experiment. Logically there should be convection, since you cool the surface of the water due to evaporation, and this water goes down to be replaced by the warmer water from the bulk. However it is hard to judge, because there is insane boiling due to the dissolved gases leaving the liquid in the form of bubbles, which continues effectively until the water freezes.

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