I recently read that evaporation takes place on the surface of the liquid and boiling takes place throughout the liquid. The part about evaporation seemed legit as it is observed in nature that water dries off gradually surface by surface. But the part about boiling seems a bit counterintuitive as boiling too seems to occur on the surface as water, when it boils, evaporates from the surface. How can this be justified?


Boiling consists on the formation of vapour bubbles within the liquid. If, at some point within the liquid, the temperature is high enough to produce a local change of phase (vaporisation), it can produce a bubble. If the pressure inside the bubble is kept high enough to prevent it's collapse, it will reach the surface of the liquid container (because it has a lower density and therefore it experiences an upwards buoyancy force). That is when the vapour appears in the surface and it looks like evaporation just happened in the surface.

The difference between evaporation and boiling, which are two forms of vaporisation, is that evaporation can happen in the surface at lower temperatures, while boiling requires a the high temperature needed to achieve a hight pressure to form bubbles inside the liquid.

To conclude, I'd say that boiling and evaporation are different by definition, since they are driven by the same phenomenon, which is a phase transition from liquid to gas.

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