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I have a pot of vigorously boiling water on a gas stove. There's some steam, but not alot. When I turn off the gas, the boiling immediately subsides, and a huge waft of steam comes out. This is followed by a steady output of steam that's greater than the amount of steam it was producing while it was actually boiling.

Why is there more steam after boiling than during boiling? Also, what's with the burst of steam when it stops boiling?

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3 Answers 3

up vote 15 down vote accepted

I have read that true steam is clear (transparent) water vapor. According to this theory, the white "steam" you see is really a small cloud of condensed water vapor droplets, a fine mist in effect. So what you are seeing is not more steam, but more condensation and more mist. The speed with which the steam/vapor/mist rises and disperses may also change.

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Let me game out this train of thought, and maybe others can tell me if I'm right: There was in fact a lot of (transparent) steam above the pot when it was boiling. It wasn't condensing because the steady steam output kept the steam's temperature above the condensation point. The moment I turned the gas off, the steam's temperature was allowed to fall below this point, resulting in a big burst of condensate. After this cloud subsided, the water continued to emit steam, but at a much lower rate, insufficient to keep the steam's temperature above the condensation point, thus it was visible. –  SuperElectric Jan 29 '11 at 21:16
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In addition there was hot flue gas from the gas flame. This gas surrounds the pot, and brings extra heat and a kind of shield around the pots rim, thus preventing the steam to condense. –  Georg Jan 29 '11 at 21:32
    
To continue Georg's thought - even after the steam rises beyond the "shield" of extra heat, it has dispersed to much lower density, so even when it hits colder air it is too thin to see when it condenses. –  kharybdis Jan 29 '11 at 21:36
    
To nail the proof, if the pot is covered while it is boiling and the cover is puffing steam out, it stops when the gas is turned off. –  anna v Jan 30 '11 at 5:05
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Hard core of this "problem" is the usual inacuracy of wording of laymen . Steam, vapour, mist, smoke are used often in a wrong way. –  Georg Jan 30 '11 at 13:13

Steam probably dominates the gaseous content forming characteristic bubbles that we see rising in 'boiling' water. 'Bubbling' will decrease the available surface area of liquid water lining the bottom of the container vessel, until the rate of heat input absorbed through the water volume bound, as a mass of matter, in liquid state (a molar mass amount), versus the water which changes state into translucent 'steam', reaches an equilibrium with the rate of heat input over the heat exchange surface area. But I don't think 'steam' is an insulator... too many people have been badly disfigured by super-heated translucent steam scalds.

The clouds of white stuff is water vapor, just like weather 'clouds' can produce rain when they are forced to rise on prevailing air currents across hills, and mountain ranges. The vapor mass loses energy through its expansion at higher altitudes (with lower atmospheric pressure) and condenses/contracts as this energy drop precipitates a phase change back to clearly bounded lquid water. I think this is fluid dynamics.

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The bottom of the pot is the hottest place and the steam is mainly there. But steam is a bad conductor of heat and isolates the bottom from the liquid. The bottom gets very hot. When the gas is switched off the liquid can again make contact with the very hot bottom which produces the burst of steam. This heavy cooking will cool the bottom down so far that you can touch the bottom from underneath without burning your hand (I did it as a child). After some time the bottom warms up again through contact with the warm liquid. Hands off now!

Lesson: the production of steam is not proportional to intensity of the heating.

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Steam at the bottom of the pot doesn't make sense. Wouldn't it rise above due to lower density? –  udiboy1209 Sep 2 '13 at 14:32

protected by Qmechanic Jun 10 at 15:24

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