This is a trivial little question but the phenom puzzles me. If you heat up
a pan of water, you usually don’t see a vapor cloud until immediately after
you turn off the heat (we’re using a gas stove, so the heat removal is
immediate), even if you haven’t brought the water to a boil. Clearly water
molecules are escaping from the liquid both before and after the heat is
removed. Yet only after the heat is removed does one see fog formation. Now
its easy to say that the heat prevents the cloud from forming. My problem
with this is that the heat source is beneath the pan, and it doesn’t seem
possible that the air temp above the pan could be so suddenly cooled that a
fog could form. After all, the formation of the fog is almost immediate
after the heat is removed.
Any ideas?
David Frank (david_fr…@brown.edu) wrote:
> This is a trivial little question but the phenom puzzles me. If you heat up
> a pan of water, you usually don’t see a vapor cloud until immediately after
> you turn off the heat (we’re using a gas stove, so the heat removal is
> immediate), even if you haven’t brought the water to a boil. Clearly water
> molecules are escaping from the liquid both before and after the heat is
> removed. Yet only after the heat is removed does one see fog formation. Now
> its easy to say that the heat prevents the cloud from forming. My problem
> with this is that the heat source is beneath the pan, and it doesn’t seem
> possible that the air temp above the pan could be so suddenly cooled that a
> fog could form. After all, the formation of the fog is almost immediate
> after the heat is removed.
> Any ideas?
If you’re using a gas burner, you’re going to be getting a lot of hot air
flowing up and around your pan as you are heating the water. This hot air
will prevent the water vapor from condensing into a "cloud" (note that water
vapor is invisible, it’s only when tiny liquid water droplets form that you
get a "cloud"). Once the flame is removed, the rising column of hot air
fades and the vapor quickly cools, forming the steam you see rising from
your pot of boiling water.
-Austin Appleby
In article <david_frank-140494224…@tonto-slip12.cis.brown.edu> david_fr…@brown.edu (David Frank) writes:
|From: david_fr…@brown.edu (David Frank)
|Newsgroups: sci.chem
|Followup-To: sci.chem
|Date: 15 Apr 1994 02:54:44 GMT
|Organization: Brown
|Lines: 13
|NNTP-Posting-Host: tonto-slip12.cis.brown.edu
|
|This is a trivial little question but the phenom puzzles me. If you heat up
|a pan of water, you usually don’t see a vapor cloud until immediately after
|you turn off the heat (we’re using a gas stove, so the heat removal is
|immediate), even if you haven’t brought the water to a boil. Clearly water
|
|Any ideas?
Ya, I had the same question once, and soon got the reason.
Exactly, you can’t see ‘vapor cloud’ but only can see *water
cloud* on the pan, vapor is not visible because it is a
kind of gas.
When the pan is heated the surrounding air on it is very
hot due to burning flame, then vapor cannot be condensed
to water. But when you turn off the heater, hot air will
not flow up any more. Then the vapor will go up and touch
cold (i.e. room temp.) air and will be cooled to make
condensed water particle. That’s the reason I think.
Hideo Konami
Inst. for Chem. Reaction Science, Tohoku univ, Sendai,
Japan
In article <david_frank-140494224…@tonto-slip12.cis.brown.edu> david_fr…@brown.edu (David Frank) writes:
>This is a trivial little question but the phenom puzzles me. If you heat up
>a pan of water, you usually don’t see a vapor cloud until immediately after
>you turn off the heat (we’re using a gas stove, so the heat removal is
>immediate), even if you haven’t brought the water to a boil. Clearly water
>molecules are escaping from the liquid both before and after the heat is
>removed. Yet only after the heat is removed does one see fog formation. Now
>its easy to say that the heat prevents the cloud from forming. My problem
>with this is that the heat source is beneath the pan, and it doesn’t seem
>possible that the air temp above the pan could be so suddenly cooled that a
>fog could form. After all, the formation of the fog is almost immediate
>after the heat is removed.
>Any ideas?
At a guess…
Depending upon the configuration of your burner and pan you will have a
sheath of hot exhaust from the burner and convected heated air passing over
and around the pan and preventing the surronding cooler air from coming in
contact with the hot moist air coming off the water surface. When you turn
off the burner the heat reduction is very sudden with gas as you noted, so
this sheath of air dissappears rather abruptly giving the appearance of the
fog.
Try repeating the experiment on an electric stove with the heat turned up
really high using the same pan.
Rob Hay H…@lincoln.cri.nz
New Zealand Institute for Crop & Food Research +64 3 325 6400 (ph)
Private Bag 4704 ___ +64 3 325 2074 (fx)
Christchurch ___ |
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