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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, 2017 at 16:50
  • $\begingroup$ I think the speed of sound in water is about 1,484 m/s $\endgroup$
    – JMLCarter
    Mar 30, 2017 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, 2017 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 $\endgroup$
    – Gregoire
    Apr 2, 2017 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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