Mr. Wizard, help me out!

November 22, 2005 8:06 PM

Physics 101: I'm cooking soup on the stove and have the heat on about medium. The soup is bubbling but with a minumum amount of steam coming off the top. Then I turn the heat off, and loads of steam pours out. Why? If the soup is getting more heat when over a flame, why wouldn't it steam more? Trying to wrap my head around this one...Bill Nye, where are you?
posted by zardoz to Science & Nature (17 answers total)

Might have something to do with the hot air flowing around the pot when the flame is on, and suppressing the steam* formation over the pot. When you turn the flame off, then you've got regular room temp air wafting over the pot, providing more of a temperature differential and promoting the steam production.

But that's just my guess.

* not really steam, just water vapor condensing in air; steam is invisible and far hotter than what you're seeing
posted by intermod at 8:14 PM on November 22, 2005

Also, take a hot cup of coffee outside on a cold day. See more steam?
posted by intermod at 8:15 PM on November 22, 2005

My guess is that air transmits heat more quickly than the boiling water, and also dissipates heat faster. You turn off the heat and the burner stops perpetuating the warm air bubble above the pot and there is hence a greater temperature contrast, meaning more steam. Disclaimer I got a D in physics.
posted by moift at 8:15 PM on November 22, 2005

On postview: sup wild guess buddy :highfive:
posted by moift at 8:16 PM on November 22, 2005

weird. i've never heard of such a thing, nor seen it before in my life.

i suppose, though, one possible explanation could be this:
- the flame heats the air around the sides of the pot
- the warm air rises via convection to the top of the pot as long as the flame is on
- when you turn the flame off, the flame-heated air stops rising and relatively cold air from the surroundings replaces it
- steam is formed (as always) when water vapor contacts the colder air

but i'm of the opinion that something else is going on here that we don't know about.

do you have any more information?
posted by sergeant sandwich at 8:16 PM on November 22, 2005

Response by poster: Well, let's see. It's a bit cold in my kitchen as it's not really heated, maybe about 60 degress F. The air is really dry right now. I seem to remember seeing this phenomenon during the summer when it was really hot and humid. And it's not just soup, same happens when I boil water.
posted by zardoz at 8:28 PM on November 22, 2005

I've seen the same thing. I've always assumed it was that until the heat source was removed the steam was too hot to be visible. That is, it dissipates before condensing into visible water vapor.
posted by justkevin at 8:32 PM on November 22, 2005

Related?
posted by StickyCarpet at 8:35 PM on November 22, 2005

does it happen if you use a big flat saucepan? that would test the "air around the sides" theory. (well - not completely - but i would think the bigger the diameter, the less air you get around the edges)
posted by sergeant sandwich at 8:45 PM on November 22, 2005

I'm thinking it's the same reason your breath is visible in the cold winter air and not in the warm summer air, right? The air around the soup pot is going to cool down much faster than the soup itself.

Plus what justkevin justsaid.
posted by trip and a half at 8:45 PM on November 22, 2005

I see the same thing whenever I heat liquids, so you're not crazy.
posted by sdis at 8:59 PM on November 22, 2005

Best answer: When the soup is bubbling, the bubbling is the steam—you don't see it because the steam (and the air directly above the pot) is hot enough to prevent condensation. When you turn off the heat, the air above the pot cools, and the soup is no longer being heated at its boiling point. Thus, whatever water evaporates has just barely enough heat energy to get away from the pot; when it cools a little bit, it is more stable for the steam to condense, forming visible water vapor (really, tiny droplets). I apologize if I have not described this very well; email me if you like and I will gladly elaborate.

[Even when the temperature of the soup is slightly below boiling, there are still some molecules with a high enough velocity at that temperature to evaporate. This is illustrated by the Maxwell-Boltzmann energy distribution curve.]
posted by jenovus at 10:05 PM on November 22, 2005

Best answer: The soup is bubbling but with a minumum amount of steam coming off the top. Then I turn the heat off, and loads of steam pours out.

That's your mistake. The amount of steam escaping the pan doesn't change. When you turn off the heat, the air around and above the pan cools very rapidly, relative to the steam. Once that happens, the steam condenses much more quickly, so you see more steam. There was just as much, if not more, steam present before, but it was condensing over a much more widely dispersed volume, further from the pan, when the heat was turned on. When the heat is suddenly turned off, the steam condenses, and is therefore visible, much closer to the pan and in a much smaller volume of ambient air.

Strictly speaking, what you're seeing isn't steam. Steam is water in gaseous form. When it cools, condenses and becomes visible it's tiny droplets of hot water suspended in air.
posted by normy at 12:56 AM on November 23, 2005

What jenovus and normy said. Water vapor isn't steam—when it's hotter more of the water is turning to vapor and staying a vapor while, in contrast, when you turn the heat down the temperature lowers and the vapor partly condenses and you see steam.
posted by Ethereal Bligh at 1:28 AM on November 23, 2005

(Oh, I see that normy and I disagree. I should look up the definition of "steam". But I'm pretty sure that steam is what you can see...which isn't water in gaseous state, it's liquid suspended in the air, like clouds and rain.)
posted by Ethereal Bligh at 1:31 AM on November 23, 2005

EB - I'm working from thermodynamics classes more than ten years ago, so feel free to correct me, but in mechanical engineering "steam" is used way above condensation temperatures/pressures. For example, water is very much not an ideal gas (PV = nRT doesn't work), so physical data is usually referenced from experimentally determined published data in "steam tables".
posted by normy at 2:55 AM on November 23, 2005

Oh, well I'm just going on everyday language. "Steam" has always been what you can point at. "Hey, look at that steam over there." But water vapor is, ideally, invisible. We agree on what's happening, though.
posted by Ethereal Bligh at 3:15 AM on November 23, 2005