I asked my chemistry teacher this a while back but didn't get a clear answer. I've returned to the question a few times in my mind, but keep forgetting to ask. I've forgotten much of the chemistry vernacular since then, so bear with me. You know how in intro chem classes one exercise you'll do is calculate the energy to take a quantity of h2o from solid to liquid to gas, and how you have to calculate the energy for the phase changes between solid and liquid then liquid and gas? I recall it being stated along the lines that for the quantity to phase from liquid to gas, the entire thing must be 100C (with perhaps some caveats). At least by the calculations we did, that is suggested to be true.

Anyways, my question is "when I'm making coffee in the morning and I'm bringing water to a boil, once the first bubble appears, does that mean that the entire pot is 100C?" My teacher answered "yes, well, actually no" but she never clarified so I was left not understanding.

The utility reason I ask this question is because I want to know at what point a pot of water can be said to have first reached 100C.