Science Questions

Why is water loudest just before it boils?

Wed, 4th Sep 2013

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Question

Achim asked:

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?

 

Answer

Dave - So, this is all to do with, if you think about what's happening before it Kettleactually boils properly, the heating element is over 100 degrees centigrade. Therefore, the water around it is going to be producing steam even quite early on when itís starting to boil. These bubbles are created and then if itís not quite boiling yet, they move up into a cooler area of water and they cool down. If a steam bubble cools down very quickly. It suddenly wants to disappear and it collapses and it forms in the cavity. If you imagine this bubble kind of collapsing symmetrically around each other, it kind of slaps into itself. This is known as cavitation and itís very, very noisy and it's actually, quite a destructive process. You get this in boat propellers and if the boat propeller goes too fast, you actually little bubbles of steam which collapse again and actually completely smash up the surface of the propeller and you can see horribly eaten up propellers. So therefore, going back to the kettle as it heats up, slowly these bubbles are getting bigger and bigger, and they're collapsing harder and harder, until eventually, they get hot enough that they get all the way up to the surface and they just pop gently.

Chris - So, the reason it gets louder just before the water boils is because that's when the bubbles are at their biggest to start with and therefore, they've got the biggest collapsing to do.

Dave - Yes, so itís the biggest bubbles which are collapsing and then they get to the point where they reach the surface, at which point, they just pop gently and then it gets much quieter.

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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? cheryl j, Tue, 6th Aug 2013

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 Cooper, Wed, 7th Aug 2013



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. chiralSPO, Sun, 11th Aug 2013

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. David Cooper, Sun, 11th Aug 2013

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?
cheryl j, Tue, 13th Aug 2013

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. David Cooper, Wed, 14th Aug 2013

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? Cavitationist, Fri, 31st Jan 2014

This was so poorly written. It is painful to read. CC, Wed, 15th Oct 2014

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