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I observe this every day. I heat water by first pouring it in a pan (made of aluminium, not that I think this detail is important). I put the pan on top of a flame, originating from natural gas. After a while, the water starts producing very tiny bubbles along the sides of the pan (I assume the old age of the pan favors nucleation). And then, one second after I turned the stove off, the water in the pan starts evaporating.

This is not a coincidence, I observed it hundreds of times. I don't have a satisfactory explanation, although I thought of some. The phenomenon looks as if as long as the fire was on, a sort of mechanical tension was applied, but I would not have any explanation for that either. So, if I think about differences of temperature, I could suggest that the hottest water is at the bottom of the pan until the heating stops. However, I don't see why it would suddenly diffuse once heating stops.

I also thought about the air surrounding the water, that could be warmer as long as the fire was on, but I dont even know if warmer air would slow down evaporation.

So basically, I have no clue and this goes really counter intuitive to me. I should add that if I decided to let the fire on way longer, then the water would evaporate a little while the fire still on, but the water would be way closer to boiling than my usual recipe. I have not used a thermometer, but I know by taste that my water that evaporate after fire is off, is around 80-85°C when it's done (I know teas and coffees well enough to descriminate such ranges of temperatures).

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What are you observing when you say "the water starts evaporating"? You can't easily see evaporation.

If you mean you start seeing a whitish cloud above the pan, you're directly observing condensation, not evaporation.

So speculation here is:

  • You heat the water via a gas flame
  • The gas flame begins heating the pan and some air nearby
  • As the temperature of the water increases, the vapor pressure and the rate of evaporation both increase. It has been evaporating at a low rate and that rate increases.
  • Water vapor from the evaporation is quickly carried away by the rising, heated air from beneath the pan.

When you turn off the flame, the water maintains its temperature and its rate of evaporation. But now the vapor instead of being carried away by a warm convecting air, encounters room temperature air that is not moving as quickly. The vapor cools to saturation temperature and condenses with the appearance of "steam".

I should add that if I decided to let the fire on way longer, then the water would evaporate a little while the fire still on, but the water would be way closer to boiling than my usual recipe.

As you continue to add heat to the water, the vapor pressure and rate of evaporation continue to increase. More and more vapor is being introduced. But the warm air convecting around the pan is remaining constant. At some point the convection is insufficient to remove all the vapor before it begins to condense.

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  • $\begingroup$ Beautiful! This seems right. And yes, the mist on top of the water can be condensation. The fact it is moving upward made me think it is evaporation. Thank you very much! $\endgroup$ – Exocytosis May 9 at 23:03
  • $\begingroup$ Actually would you have an explanation for why the same phenomenon appears later while the fire is still on? $\endgroup$ – Exocytosis May 9 at 23:11
  • $\begingroup$ Added a bit in the answer. $\endgroup$ – BowlOfRed May 9 at 23:16
  • $\begingroup$ This is brilliant! I love it. I asked many questions but this one was quite personal, thus the enthousiasm. Thank you again. $\endgroup$ – Exocytosis May 9 at 23:18

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