Science Questions

Why does the kettle get louder as the water heats up but then quieter as it reaches boiling point?

Mon, 3rd Oct 2016

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Question

Allan Grant asked:

Why does a tea kettle make an increasingly louder sound as it approaches the boiling point and then start to quiet down as it reaches 212 degrees?

Answer

We put Allan's question to physics expert Caroline Steel... Kettle

Caroline - So, in a kettle, you have a heating element at the bottom and that heats up quickly to actually above one hundred degrees. So the first thing to heat up in a kettle is the water around the heating element. This hot water around the heating element forms tiny little bubbles which then rise up through the kettle, but as these bubbles rise, they rise into an area of colder water. So the bubbles cool down and condense and kind of implode, and the bubbles sort of slaps against itself making a really loud noise.

But as the kettle heats up throughout so all the water inside the kettle is one temperature. As these bubbles form and rise through the water they donít reach an area of colder water, so they actually make it all the way to the surface and then they pop on the surface. But this popping sound on the surface is a lot more gentle and quiet than the sound of the bubbles kind of imploding on themselves within the kettle.

So, if you watch your kettle boil, as itís making a loud sound and coming up to boil youíll see lots of tiny little bubbles in the main body of the water, but you wonít see them reach the surface. And then once the kettle has reached boiling point and these bubbles make it to the surface, youíll see them expand and pop but that popping noise is actually quieter than the imploding bubbles in the main body of the water.

 

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The sound is due to bubbles collapsing. At first these are dissolved air bubbles which expand and then collapse, this produces a low frequency. As the water gets hotter vapour bubbles form on the heating elements and they collapse at a higher frequency - around 1kHz.
I'm not so sure of the mechanism nr boiling, but as the water get hotter the bubbles get smaller and the frequency rises starting to move above hearing frequencies and so appears to get quieter.
I've met people who claim to be able to tell the temperature from the sound and using a thermocouple they appear to be reasonably accurate. Colin2B, Thu, 29th Sep 2016

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