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Consider water as an example. I m really confused when does boiling takes place. When it's vapour pressure equals atmospheric pressure or when it's saturation atmospheric pressure equals atmospheric pressure.

Wait this is even more confusing. Either of two doesn't make sense. Consider this image

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Here the water isn't covered by lid so there won't be any vapour pressure, as all the vapours will escape. This is the case where vapour pressure will exert vapour pressure.

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But in the first case irrespective of the vessel covered by a lid, boiling takes place. Neither vapour pressure equals atmospheric pressure nor saturation vapour pressure equals atmospheric pressure.

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  • $\begingroup$ Related: chemistry.stackexchange.com/questions/38653/… $\endgroup$
    – Ivan Neretin
    Commented Oct 12, 2018 at 8:11
  • $\begingroup$ Saturated atmospheric pressure is a phrase meaning that the vapor pressure is equal to 1 atmosphere. So it isn't just "atmospheric pressure" but a pressure of exactly one atmosphere. // For a open pot boiling imagine a thin layer above the liquid which is the pure vapor of the liquid. The vapor then diffuses into the atmosphere. $\endgroup$
    – MaxW
    Commented Oct 12, 2018 at 8:14
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    $\begingroup$ @IvanNeretin reading your post makes me even more confused. Firstly why are bubbles forming at the bottom of the liquid. Secondly, as you wrote that the bubbles that are formed at the bottom contains pure water vapour so why does boiling takes place when vapour pressure equals atmospheric pressure, because at the bottom there will also be and additional pressure of $h\rho g$ which would prevent the formation of bubble $\endgroup$
    – user66707
    Commented Oct 12, 2018 at 8:41
  • $\begingroup$ $\rho gh$ will be there all right; I simply ignored it. $\endgroup$
    – Ivan Neretin
    Commented Oct 12, 2018 at 9:34
  • $\begingroup$ The red heat of the burner makes the bottom of the pot a little hotter and causes the highest vapor pressure. If "bubbles" developed at the very top surface of the liquid, they might be invisible. $\endgroup$
    – James Gaidis
    Commented Oct 12, 2018 at 13:04

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Water boils because the vapor pressure IN THE LIQUD reaches a level where the molecules can overcome the external pressure of the surrounding atmosphere and form bubbles. This level can be reached either by raising the temperature (the molecules gets more energy) or lowering the surrounding pressure (like in the mountains). Actually, the pressure a water molecule has to overcome to turn into vapor, is the sum of the pressure from the water above plus the atmosphere. That is why water in hydrothermal vents can be several hundred degrees without boiling. It makes absolutely no difference whether the atmospheric pressure comes from a bone dry gas or from pure water vapor.

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