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Came across this claim here that "The liquid water is at exactly the boiling point temperature. The water vapor (steam) can be hotter. You can have a thermometer immersed in the water which will show the boiling point, then pull it up out of the water and the temperature rises."

Why wouldn't the temperature of steam be the boiling point?

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2 Answers 2

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There is a saying that "Paper never refused ink", the same goes for the web. At 1 atm pressure, if you boil water, and water vapor exist at equilibrium and the vapor pressure of water is also 1 atm at 100 centigrade. The temperature of water and steam will be the same. Having a different temperature of steam than bulk water defies the concept of equilibrium temperature. The quora author appears to be confused between the concept of latent heat and temperature.

As the graph shows, from Hyperphysics, as long we have water, the temperature of bulk water cannot exceed 100 centigrade. Although heat is being added its temperature is not increasing. However, one can independently heat steam from another heat source and its temperature can be way higher than boiling water.

enter image description here

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    $\begingroup$ It is hard to be at equilibrium in the real world. When you boil water in a surrounding that is at room temperature, you are never at thermal equilibrium. There will be some superheated liquid and some supercooled steam (in need of nuclei), and probably also some liquid below the boiling point (near the sides of the pot) and some steam above boiling point (near the bottom of the pot). It would be different if the temperature of the air is near the boiling point of water, or the entire room is filled with steam (like an autoclave). $\endgroup$
    – Karsten
    Commented Jan 6, 2023 at 15:39
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    $\begingroup$ Right Karsten, I agree but chemists live in an ideal world. $\endgroup$
    – ACR
    Commented Jan 6, 2023 at 15:58
  • $\begingroup$ Your answer appears to be saying something different from the other posted answer here. Could you have a look at it? $\endgroup$
    – xasthor
    Commented Jan 7, 2023 at 6:54
  • $\begingroup$ @xasthor, The other answer is talking about microscopic temperature. I do not have any proof or studies handy which show that a water vapor bubble inside the bulk of water is at higher temperature than bulk of water. What one can guarantee is that the bulk temperature of water cannot exceed 100 C during boiling at 1 atm. I don't think the bubble temperature is any higher than 100 C during boiling. $\endgroup$
    – ACR
    Commented Jan 7, 2023 at 16:15
  • $\begingroup$ Now there are certain bubble (or better cavitation) phenomena where the temperature inside the bubble in water is 5000 C or more!! That is another world. Check cavitation and sonoluminescence but it has nothing to do with your query about normal boiling process. $\endgroup$
    – ACR
    Commented Jan 7, 2023 at 16:16
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In everyday life the term boiling generally means the nucleation and growth of bubbles of steam. Typically in a saucepan or a kettle the water is heated from below and the bubbles of steam form on the bottom of pan, or on the element at the bottom of a kettle, then grow, break free and rise to the surface.

The reason for the steam being hotter than 100°C is that there is an energy barrier to nucleation and to nucleate a bubble of steam we have to superheat the steam. So if you were able to measure the temperature inside a bubble of steam as it formed at the bottom of the pan you would indeed measure a temperature above 100°C.

However I am unconvinced that the temperature would rise as you pull the thermometer upwards from the bottom of the pan. The upper surface is being rapidly cooled by evaporation and I would expect the bubbles of steam to cool as they rise and approach a temperature equal to the water surrounding them as they reach the surface i.e. 100°C. Hence I would expect a bubble of steam to start out hotter than 100°C at the bottom of the pan but to have cooled to 100°C but the time it reached the surface.

As for the temperature of the surrounding water, I would expect convection currents to provide rapid enough mixing that there was little or no temperature gradient in the water. However I have to concede that I have never done the experiment.

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  • $\begingroup$ Hm, so the heat energy does not first go towards converting all the water to steam before further increasing the temperature of the system? What if I was to boil water in the absence of gravity, where the steam didn't need to form a bubble and rise, would then the temperature of steam be 100? $\endgroup$
    – xasthor
    Commented Jan 6, 2023 at 12:58
  • $\begingroup$ @xasthor Formation of a bubble has an energy barrier whether the bubble is forming in a gravitational field or not. Have a look at my answer to Why does soda fizz when it meets ice? for more on this. That question is about a CO₂ bubble forming in soda rather than a stem bubble forming in water, but the principle is the same. $\endgroup$ Commented Jan 6, 2023 at 15:02
  • $\begingroup$ @xasthor The issue is that heat is transferred from the bottom only. A microwave in outer space might help with your model. $\endgroup$
    – Karsten
    Commented Jan 6, 2023 at 15:34
  • $\begingroup$ @JohnRennie Okay. So why is it taught that after the temperature hits 100°, the heat goes towards phase change until the last drop, after which it goes again towards increasing the temperature of the steam. In this case, it appears that heat is simultaneously going towards superheating the steam as well as phase change. Is it a good approximation of what's happening? $\endgroup$
    – xasthor
    Commented Jan 6, 2023 at 15:48
  • $\begingroup$ @xasthor we tend to simplify things when we first teach students about phase changes and we don't mention superheating (or super cooling). In real life we almost always get some degree of heating (or cooling) past the transition temperature before the phase change occurs. $\endgroup$ Commented Jan 6, 2023 at 16:46

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