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fringeview -> RE: Can anyone tell me... (Get ready for Science!) (4/13/2013 12:59:42 PM)
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Motion of the liquid water wouldn't appreciably displace gas from the pot – the volume of the liquid isn't changing, hence no increase in pressure to force gas out. The heated sides may have an effect in some cases, but I suspect the effect is largely caused by other factors.
In the interest of Saturday morning science/procrastination, I boiled some water in a teapot on low heat with a thermometer placed against the external side of the pot a bit above the water level. The temperature reading when the pot began to whistle was still below the boiling point, which would preclude heat transfer from the pot to the fluid, but the increase in whistle intensity was still present when removing the pot from heat and sloshing the liquid around.
So, what might be causing it? At the boiling point, the water is able to change phase from a liquid to a gas, but the surface tension and viscosity of the liquid can slow the process down a bit. There's also a limited surface area for exchange of gas between the boiling, two-phase fluid in the bottom of the pot and the gas in the top. Jostling the pot while removing it from the heat probably disrupts some of the surface tension retarding bubble expansion, allowing gas bubbles to expand and rise more rapidly, and the sloshing surface opens up more area at the fluid interface for gas to escape into the top of the pot. The end result is a sudden, but brief, increase in the rate of gas released into the top of the pot, which in turn raises the pressure and forces gas out of the pot at a higher rate, increasing the whistle intensity.
At any rate, that explanation – and the cup of Lupicia rhubarb and strawberry tea produced along with it – satisfied me. :)
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