Physics Help Forum the relationship between he boiling point and the vapour pressure

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 Aug 16th 2018, 10:46 AM #1 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 27 the relationship between he boiling point and the vapour pressure As I understand it, boiling takes place when the saturated vapour pressure equals to the atmospheric pressure. But, why does the vapour pressure need to be equal to the atmospheric pressure for water bubbles to form? According to this video, the atmospheric pressure is cancelled by the vapour pressure at boiling, which allows for the bubbles to form (due to the zero total pressure). https://www.youtube.com/watch?v=ffBusZO-TO0 But, I think the atmospheric pressure is the same. So, if we consider the atmospheric pressure to be 1atm at a time, the surface of the water feels 1atm pressure every time (I mean that atmospheric pressure is 1atm as same as the before, but the vapour pressure and the air pressure is acting as a partial pressure according to Dalton's partial pressure concept, it doesn't affect the total atmospheric pressure.) So what is the real reason?
 Aug 17th 2018, 01:05 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,011 I would perhaps take issue with the video slightly. I would suggest that bubbles form when the saturated vapour pressure is greater than the atmospheric pressure. The boiling point (when the when the saturated vapour pressure is equals the atmospheric pressure) is more a theoretical point we pass through between not boiling and boiling. It not really a practical feature we will notice every day when we prepare our morning cuppa. If one were to very slowly and carefully raise the temperature to 100°C, then reduce the energy input until there is just enough energy to exactly match the heat lost to the surroundings (so there is no excess energy available to boil the water), then the water will sit there at the boiling point, without bubbling. topsquark, oz93666 and osalselaka like this. __________________ ~\o/~
Aug 17th 2018, 02:29 AM   #3
Senior Member

Join Date: Apr 2017
Posts: 506
This is the crux of your question ....

 Originally Posted by osalselaka But, why does the vapour pressure need to be equal to the atmospheric pressure for water bubbles to form??
It has to be greater in a bubble ...in the video the beaker is sitting on the hot plate with 10cm of water sitting in it , the bubbles form at the bottom , rise , expanding and burst the surface .

The pressure at the bottom of the beaker is atmospheric plus the height of the water , another 1% in pressure , so the temperature of these bubbles is higher than 100C , they rise , expand and transmit part of their heat to the water, then only when they break the surface are they at 100C and atmospheric pressure .

 Aug 19th 2018, 05:33 PM #4 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 27 Ok...if we consider a situation which is very closed to the boiling point ( such like 99C) due to the opened container all the water vapor will pass to the out of the container by the diffusion, meanwhile how there is a vapor pressure building on the surface of the water... Also, as I mentioned in my original post will the atmospheric pressure change even when the vapor is gaining out of the liquid or will the vapor pressure acts as a dolton's partial pressure while the atmospheric pressure is constant every time? Last edited by osalselaka; Aug 19th 2018 at 05:43 PM.

 Tags boiling, point, pressure, relationship, vapour

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