Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Allen on 11/08/2009 19:30:02
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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?
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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?
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.
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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.
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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.
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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.
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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.