After the long Thanksgiving holiday (my favorite, by the way) I have two carcasses that need to be reduced to broth. I toss them into a kettle, with some other stock vegetables and spices. I boil it considerably. Then I bring it back to a simmer for a long while. A few noodles and many spices later, I have a soup. I let it simmer. When I turn off the heat, the conotion begins to steam.
My question is: Why does my soup steam only once I turn off the heat?
I may not have this exactly correct - it’s been a while since I’ve taken any physics classes - but I think this is what is happening:
When you simmer a liquid on your stove, the liquid at the bottom of the pot is hotter than the liquid near the surface, due to the heat coming from the burner. The liquid at the bottom of the pot is at or near boiling, and steam bubbles begin to rise to the cooler liquid near the surface of the pot.
As they rise, they cool and collapse from the air pressure above the surface of the liquid. When you remove the source of heat, the entire temperature of the liquid begins to reach equilibrium, allowing the steam bubbles to rise to the surface, and escape as the visible steam that you see.
So how’d the soup come out?
Dire, with all due respect (and recognition that I am an English teacher, not a rocket-scientist), I disagree.
I think that the reason Or’n’ry only sees steam after he turns off the heat is this:
While the heat is on, and the soup is boiling, steam molecules are escaping very rapidly from the surface of the soup… they dissipate rapidly over the boiling liquid (too rapidly to gather enough in one area to be seen). After the heat is turned off, the boiling slows, the soup cools a bit, and the steam molecules escape more slowly… allowing them time to accumulate in sufficient numbers to be visible!
I think Astroboy14 is closer to the truth. The soup can’t be hotter than 100C, so any heat added to the soup will be removed as steam. Reduce (or eliminate) heat input, and you should reduce steam production.
But keep in mind that steam is invisible. What you see as “steam” is tiny droplets of liquid water. So steam needs some time to cool down and condense into liquid drops and become visible. When you turn off the heat, this condensation happens a lot closer to the pot.
At least that’s my theory. I’ve got to admit I’ve never noticed this phenomenon myself.
Thanks for the support, scr4!
And perhaps you have hit on the actual crux of the matter: as the soup cools, enough steam molecules can accrue near the surface of the soup to condense… and thus become easily visible.
BTW: I make soup fairly often, and if you look closely, you can see steam… just not very much…
My WAG: I assume you’re using a gas stove as an electric stove doesn’t turn off quickly. My guess is that the hot, burning gasses rise and dissipate the water vapor. When the gas is turned off, this convection stops and the condensing water vapor can be seen.
Hoo boy, are your High School science teachers going to be mad!
Steam is invisible. What Oscar is seing is water vapor.
Just put a plate above the soup when you are cooking it, youll see water vapor form on the plate, thus proving its steaming. If you shine a strong light on it Im sure you can see the steam.