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).

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?

  • $\begingroup$ Related: chemistry.stackexchange.com/questions/38653/… $\endgroup$ – Ivan Neretin Aug 16 '18 at 5:10
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    $\begingroup$ @ Ivan Neretin : I read your answer at the linked page and I could understand the logic of forming the bubbles at the boiling point. But I am still arise the doubt that will the water surface feel 1atm or zero atm while the both pressures are same? Thanks $\endgroup$ – Osal Selaka Aug 16 '18 at 18:24
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    $\begingroup$ The bubbles form under the liquid surface where the liquid pressure is also 1 atm. For a bubble to form under the liquid surface, the pressure inside the bubble must be high enough to push the liquid away. The force balance on the bubble interface with the liquid below the surface tells us that the pressure inside the bubble, namley the vapor pressure, must match the atmospheric pressure in the liquid. $\endgroup$ – Chet Miller Aug 17 '18 at 0:33