Naked Science Forum
General Science => General Science => Topic started by: thedoc on 09/09/2013 17:34:52
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One thing that has always bothered me is why when you listen to water as you are heating it up it is loudest just BEFORE it boils. Once it is boiling, it gets a bit quieter again. Why is that?
Asked by Achim
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Are you sure it is the water or the expanding metal container it is in? A while back I asked why metal pans or pots sometimes shake or vibrate, and someone suggested that they had expanded unevenly which caused them to rock back and forth on the burner, more and more as one part of the pan touched the element and then lifted slightly off it. In your case could sound waves be generated when the pot heats up, cools slightly as steam is released and heats up some more? But once it is at a rolling boiling the sound even outs or is less noticeable?
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I suspect the noise is generated by bubbles of H2O gas collapsing back into liquid water. The hotter the water becomes, the more of these bubbles there are, so more of them are there to collapse. Once the water is boiling, they reach the top without collapsing and release all their content as steam into the air instead, and that makes far less noise.
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I suspect the noise is generated by bubbles of H2O gas collapsing back into liquid water. The hotter the water becomes, the more of these bubbles there are, so more of them are there to collapse. Once the water is boiling, they reach the top without collapsing and release all their content as steam into the air instead, and that makes far less noise.
David is correct here. The sound is bubbles of steam that form and then recollapse. The larger the bubble the louder the sound and the longer the bubble lasts. If the bubble lasts long enough to make it to the surface (boil) then it pops at the surface instead of collapsing (relatively quietly). As the water gets hotter and hotter, it forms larger and louder bubbles, but then there is an abrupt change as boiling begins because the bubbles all escape before collapsing.
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Good point about the bubble size - I'd need a kettle with a better window to study that properly. It would be interesting to know if the bubbles are formed bigger to start with or if they're only bigger because of bubble merging, but either way when they collapse it sounds reasonable that they might lead to a greater thud. By the way, the bit about bubbles collapsing wasn't something I thought of for myself - it was just the bit about why the kettle goes quiet when it boils that I worked out from the starting point that bubble collapse causes the noise.
After a kettle goes quiet and all the water has reached boiling point, it then gets noisier again as it boils more violently and the pressure of the steam switches the kettle off, and many people insist on waiting until the kettle has switched itself off before they believe that the water has reached boiling point instead of switching it off manually at the point where it goes quiet. By waiting, all they do is waste energy and lose more of the boiling water to the air.
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Could it not also be the bubbles banging against the sides of the pot, and once the water boils, they are escaping into the air above? Or the turbulence of the water in the pot before it boils? I can more easily imagine metal conducting or amplifying sound than I can imagine an surfacing bubble making sound. Is there any difference in the noise you hear if you boil water in a glass pot?
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Different devices for boiling water behave differently. Stand-alone kettles tend to make more noise than when you boil water in a pot on a cooker. I've just tried a new kettle and it boils water more quickly, eliminating the quiet phase when it boils altogether by jumping straight past it into aggressively boiling the water off.
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But cavitation is defined as a phase change due to a change in pressure, not temperature. Cavitation bubbles certainly do collapse and emit noise (and light and heat up to over the temperature of the surface of the sun), but if all this is happening because of temperature changes - its just boiling. I would bet that the noise is due to bubble collapse when they reach cooler areas. One way to check would be to use a hydrophone and look for the characteristic cavitation bubble collapse frequency. But would bubble collapse due to temperature emit the same?
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This was so poorly written. It is painful to read.