Why is water loudest just before it boils?

05 September 2013

Question

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 actually 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 it's starting to boil. These bubbles are created and then if it's 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 it's 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 it's 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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