When boiling water on a stove, will the temperature of the steam vary significantly with the temperature of the burner?
Person A's argument: So, once individual water molecules reach 100C/212F, they become vapor. The water molecules in the pot are <100C; the water molecules in the air are >100C. Generally, the only way to heat up the water vapor to significantly more than 100C would be to trap the water vapor. In a big kitchen, the water vapor rises rather quickly and gets sufficiently far away from the burner. Within the first couple seconds that the molecule becomes vapor, the vapor may still be close enough to the burner to become slightly more than 100C (101C?), but generally, no matter what the temperature of the burner, the water molecules will escape at 100C and won't reach a temperature significantly above 100C, given a large room.
Person B's argument:
With a hotter burner, the water in the pot is hotter and as a result the water molecules that become steam – and bubble up from the bottom of the pot – transfer less heat to the surrounding water on their way to the top of the pot and leave as hotter steam.
Or do persons A and B just have a poor grasp of physics?
Best Answer
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.