Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 4 down vote favorite
3

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?

share|improve this question

1 Answer

up vote 7 down vote

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.

share|improve this answer
1  
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
1  
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
1  
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

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.