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I have noticed this several times. When I am boiling water, a few seconds before its boiling point, vapours are formed as usual. But if I turn the gas off before boiling, the moment it turns off, I see a lot of vapours being formed all of a sudden from the hot water for a second or two. Can anyone tell me why this happens?

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What you are seeing is not actually vapor - vapor is invisible. The mist seen above boiling water, commonly but inaccurately called vapor, is actually made of tiny droplets of liquid water, formed when the vapor cools down and condenses.

While the stove is on, the constant influx of vapor from the boiling water keeps the air above it hot, so condensation is minimal and there is little visible mist. When the gas is turned off, boiling stops, the air above the water cools down, and the vapor it contains suddenly condenses, creating a large plume of mist.

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    $\begingroup$ Great answer. I didn't even know what OP was talking about until you explained it. Kudos! $\endgroup$ Dec 21, 2018 at 16:22
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    $\begingroup$ ...and the hot combustion gases from the burner are also hiding the water vapor until the stove is off. $\endgroup$ Dec 21, 2018 at 20:53
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    $\begingroup$ That's a great explanation. This also cleared my misconceptions about vapour. Thanks! $\endgroup$
    – user217702
    Dec 22, 2018 at 6:09
  • $\begingroup$ @user217702 If you consider this to be the right answer you should mark it as such. $\endgroup$ Dec 23, 2018 at 13:16
  • $\begingroup$ John Rennie's answer complements this one by considering the water vapour generated by combustion. I have not noticed the effect described by the OP with my electric burner. Curious. $\endgroup$
    – xxyzzy
    Dec 26, 2018 at 13:35
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Without seeing your experiment we can only speculate, but my guess is that this is due to the convection currents generated by the combustion of the gas.

When the gas is burning there is a large volume of hot carbon dioxide and water vapour generated by the combustion, and this flows upwards and around the pan. This has two effects. Firstly it keeps the temperatures high around and above the pan, so it hinders condensation of the water vapour. Secondly the flow rapidly carries away and condensed water droplets that do form. As soon as you turn off the gas these two effects cease so there is more rapid formation of condensed water droplets.

You might be interested in reading Amount of Steam Generated using Gas burner and Induction cooker as I think this is related to your question.

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    $\begingroup$ I've also observed the described phenomenon on my stove which is electric and thus doesn't have an combusting gasses. This seems to imply that there is at least something else at play. $\endgroup$ Dec 22, 2018 at 14:31
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Hotter steam has a diffraction index closer to air than steam which is cooler. As the steam cools the droplets get larger, increasing the diffraction making it appear like there is more, when in fact there is less.

Put another way, hot steam scatters light less than cool steam.

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