[iScience]

when the atmospheric pressure = vapor pressure, why does gas start to form on the bottom on the liquid? what makes this happen?
are nucleation sites required for boiling to occur or can gases just start forming anywhere in the liquid?

[Corribus]

Formation of bubbles is energetically unfavorable, because there's a surface energy required for it to happen.  A nucleation site reduces this energy cost.  At the boiling point, transition from liquid to gas becomes spontaneous (favored by negative Gibbs energy), and so there is generally energy available to make up for the cost of bubble formation.  The number of nucleation sites also increases with increased temperature.  Otherwise, evaporation happens primarily from the surface of liquid.

Boiling is a complex process, far more complex than introductory chemistry courses would have you believe. 

[Borek]

Add to that fact that at the bottom liquid is usually slightly hotter, as that's where it is heated.

[curiouscat]

Add to that fact that at the bottom liquid is usually slightly hotter, as that's where it is heated.

Add to that, forced circulation versus natural convection etc. Yes, boiling is complex. Predicting boiling heat flux accurately has occupied engineers for decades.