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Author Topic: Why does the most vapour come off water just before it boils?  (Read 3901 times)

Allen

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Allen asked the Naked Scientists:
   
When I heat water on the stove there is a little steam given off. However, if I turn off the heat source just before boiling, I see a big increase in vapor. Why is that?

What do you think?


 

Offline lightarrow

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Why does the most vapour come off water just before it boils?
« Reply #1 on: 12/08/2009 11:44:32 »
Allen asked the Naked Scientists:
   
When I heat water on the stove there is a little steam given off. However, if I turn off the heat source just before boiling, I see a big increase in vapor. Why is that?

What do you think?
It's the same question I asked several times in different forums, without a definitive answer.
My idea is that the phenomenon is related to the convection currents: when you turn off the heat source, in some way you stop the production of those currents and so the bottom of the container becomes hotter, which is still hot, heats at higher temperatures the water close to it (because that water cannot move up and cool down anylonger), releasing more vapour.
« Last Edit: 13/08/2009 20:43:49 by lightarrow »
 

paul.fr

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Why does the most vapour come off water just before it boils?
« Reply #2 on: 13/08/2009 19:34:45 »
I noticed this topic and thought about it whilst on a bike ride. I hope my stray thoughts are understandable.

As lightarrow says it's about convection currents. When you have the kettle on you cookers ring, and that ring is turned on, it's not only heating your kettle but the air around and above it. That heated air has the capacity to hold more water vapour than the cooler air when you turn off or move the kettle away from the heat source that is your cooker ring....possibly.
 

Offline Edster

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Why does the most vapour come off water just before it boils?
« Reply #3 on: 20/08/2009 22:10:45 »
Usually if you keep on going you see some saturated steam just on the point of boiling and this quickly reduces as much more steam is being formed and leaving the liquid surface.  Keeping the heat input to just a fraction over 100c would form saturated ( condensing due to cooling) visible steam as it wouldn't be pushed out by more  steam from the vigorously boiling water, whose steam bubbles are likely to be several degrees above 100c and thus invisible.
« Last Edit: 20/08/2009 22:13:02 by Edster »
 

Offline Madidus_Scientia

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Why does the most vapour come off water just before it boils?
« Reply #4 on: 21/08/2009 12:32:00 »
Well the 'steam' that you're seeing isn't actually steam, steam is invisible. It's fog, steam which has cooled and condensed into small liquid water droplets, not gas. So maybe when the water is boiling the steam is hotter for longer and doesn't condense until it's dispersed away from the pot, whereas just before it boils it condenses to fog almost immediately.
 

Offline lightarrow

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Why does the most vapour come off water just before it boils?
« Reply #5 on: 21/08/2009 12:36:34 »
Well the 'steam' that you're seeing isn't actually steam, steam is invisible. It's fog, steam which has cooled and condensed into small liquid water droplets, not gas. So maybe when the water is boiling the steam is hotter for longer and doesn't condense until it's dispersed away from the pot, whereas just before it boils it condenses to fog almost immediately.
I think you're right. When we switch off the flame the steam condenses into the fog.
 

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Why does the most vapour come off water just before it boils?
« Reply #5 on: 21/08/2009 12:36:34 »

 

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