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When we boil water, the glass lid automatically gets foggy. I believe this is due to vapor turning to water again at the lid. Why does that happen? Because the lid is at a lower temperature? I am not sure.

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You are right, it is because the lid is at a lower temperature. The same goes for all surfaces that fog. The gas in immediate contact will cool down to the temperature of the surface. If that surface is colder than the dew point of the gas, the air can no longer hold the vapour and the water will condense on the surface.

The same principle applies when you enter a humid room from the (cold) outside with glasses on. At first, your glasses are colder than the dew point of the air, leading to fogging. After you spent some time in the room, the glass will have the same temperature as the air in the room, and fogging no longer occurs.

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  • $\begingroup$ Thanks a lot! By dew point, do you mean the same as condensation temperature? (Similar to boiling point ~100 degrees)? $\endgroup$
    – user137288
    Jan 27, 2021 at 19:32
  • $\begingroup$ If not the same, is it overall more or less than boiling point? $\endgroup$
    – user137288
    Jan 27, 2021 at 20:03
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    $\begingroup$ I haven't heard the term condensation temperature, but it sounds like it's the same. The dew point is the temperature at which the relative humidity is 100%. If some amount of air is cooled below this point (without removing water in some way), it can no longer hold the water vapour and the water will start to form tiny droplets, the fog. $\endgroup$
    – noah
    Jan 27, 2021 at 20:12
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    $\begingroup$ Oh no, it's always below the boiling point and will depend on the water content of the air. Above the boiling point the water is gaseous anyway, so it doesn't need to be mixed with the air to stay in the gas phase. $\endgroup$
    – noah
    Jan 27, 2021 at 20:29
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    $\begingroup$ It means that the water vapour is not "dissolved" in the air, the effect of the air around it is mostly just to exchange heat energy with the vapour. There are small corrections, but the results would be mainly the same if O$_2$, N$_2$, etc. were removed. Therefore the droplets forming on a surface come directly out of the gas phase (except if the droplets are already floating in the air as steam). $\endgroup$
    – noah
    Jan 27, 2021 at 21:31

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