When a pot of water starts boiling, does that mean that the top surface of the water was heated repeatedly by convection? Meaning that it was initially at the bottom of the pot, then it rose up to the top when heated, then circulated back down and then up again until it reached 100C? (Is that what actually happens or is it just a way to explain how boiling happens?). Also, is there another way to explain why hot water/air rises other than "it became lighter because of it's lower density"?
[Physics] How exactly does convection happen when heating water? Also why does hot water/air rise
convectiontemperature
Related Solutions
To figure out why this happens, you need to think about what boiling is, and how it works.
As you would know, the water in the pot boils because its temperature was raised above the boiling point by the flame. This required a net transfer of heat from the flame, through the pot, to the water in the pot. Why did the heat flow in this direction? Because the flame is hotter than the water in the pot, even when the water starts boiling ($T_{flame} > T_{boil}$)
Now, think about the water in the bottle. The only way for it to get heat is through the water in the pot. As long as the temperature of the water in the pot, $T_{pot}$, is less than $T_{boil}$, it is still liquid, and it transfers some heat to the water in the bottle. The water in the pot boils off at $T_{boil}$, and can no longer transfer heat as efficiently to the water in the bottle.
This effectively means that the water in the bottle is restricted to a maximum temperature of slightly less than $T_{boil}$, and that is why it never boils.
Another way to think of this is, there must be a temperature difference for a heat transfer to take place. Since the maximum possible temperature of the pot water is $T_{boil}$, the temperature of the bottle water can never exceed this.
EDIT: Another factor to consider is the low conductivity of glass, which means a high temperature difference is required to let a small heat flux through.
Both A and B are slightly wrong. The 'boiling point' of water is the temperature at which steam and liquid exist at equilibrium, and the roiling boil of a pot of water on the stove indicates a lack of equilibrium. Each steam bubble, expanding as it rises from the bottom of the pot, is accumulating vapor from the surrounding liquid (not staying a constant volume).
So, A is wrong to think that there is an equilibrium-temperature indication in the boiling pot. A single molecule can become vapor only at the water surface, or by doing work against surface tension and water pressure by expanding the diameter of a bubble. If the work is done leaving uncondensed water vapor, it must have been hotter than 'the boiling point'.
And, if B is naiive in thinking that the temperature outside the
pot is important in determining the temperature inside.
The evaporation of water
is a heat sink more than capable of cooling the metal, it might just
be that higher outer temperature turns a boil with four streams of bubbles into a similar boil with eight streams of bubbles. More
heat doesn't guarantee higher temperature, just higher heat flow.
As for 'significantly higher' temperature of the bubbles, that calls for judgment. The observation of small bubbles expanding as they rise, means there is significance, because it's observable.
Best Answer
The idea that water heated at the bottom of a pot rises to the top and returns to the bottom by convection is correct. The convection happens because hot water has a lower density and thus rises up to the surface where it cools and sinks down to the bottom again. When water starts boiling there is the additional effect of the formation of water vapor bubbles that have a much stronger buoyancy than hot water. Thus these water vapor bubbles by their fast movement to the top where they leave the liquid also produce a strong upward water current leading to an increases convection in the pot.