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Topic: vapor pressure and boiling  (Read 287 times)

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Offline Damiano0707

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vapor pressure and boiling
« on: April 11, 2019, 05:43:53 PM »
Hi
I have a doubt about heating a liquid in a closed container. Firstly, we need to say a liquid boil when its vapor pressure equals external pressure. If we heat a liquid in a closed container, no boiling will occur. It's impossible to realize total vacuum so the pressure in the container is always bigger than the vapor pressure because the pressure in the container is the sum of the pressure of the initial air pressure and the vapor pressure. This should be right. But what if we consider total vacuum is possible? In this case the pressure in the container (the "external pressure") would be the vapor pressur itself and the boiling must occur. Is it right?
I hope my English wasn't so bad

Offline Corribus

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Re: vapor pressure and boiling
« Reply #1 on: April 11, 2019, 06:38:37 PM »
If you replaced the air with vacuum, then yes the water would start to boil. Be careful doing this though: https://what-if.xkcd.com/6/
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: vapor pressure and boiling
« Reply #2 on: April 12, 2019, 10:01:07 AM »
[...] If we heat a liquid in a closed container, no boiling will occur. [...]

You made several implicit assumptions that aren't necessarily right.
  • Vapour pressure isn't always at equilibrium with the liquid. If the liquid boils, the vapour pressure can rise quickly, but without boiling it takes its time.
  • Heat can escape the container, and then boiling and condensation transfer heat from the source to the walls. Very common in boilers.
More generally, equilibrium is seducing because there are some theories for it, but real processes are often away from equilibrium.

Offline Damiano0707

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Re: vapor pressure and boiling
« Reply #3 on: April 12, 2019, 02:02:49 PM »
I don't think boiling in a closed container will raise vapor pressure because while molecules pass from liquid to vapor, the same amount of molecules pass from vapor to liquid. So boilng will occur withount an end because vapor pressure would equal container's pressure (that is vapor pressure itself). Boiling will happen at any temperature without any temperature change. Naturally, conditions are strictly ideal as I said because if not absolute vacuum is not possible and the pressure of the container would be always begger than vapor pressure.
I read what the previous user linked me. The site said what happen when there is absolute vacuum. The pressure is zero so the liquid starts boiling but what I'm asking is what happen when the pressure in the container reaches the vapor pressure.
Thanks a lot for the replies

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