What Causes the Pinging Sounds When Boiling Water in a Kettle?

[Hornbein]

I boil water in a tea kettle. When it's starting to boil there are pinging sounds. These grow louder until there is some sort of phase transition and they quiet down. That's about when the tea kettle starts to whistle. What's going on?

 

[Borek]

Yes, that's how it goes. I don't have a real answer, only a guess: it is related to the water degassing (solubility of gases goes down with the temperature) and size/presence of the bubbles in water (which change the way sound propagates in water).

 

[Rive]

As far as I know it's about small, local bubbles of steam popping up and then collapsing when cooling down due their expansion and rise. Kind of like (mechanical) cavitation, just different reason.
When the energy available is finally enough to keep the bubbles going it quiets down (and start boiling).

 

[Hornbein]

To make coffee I pour a cup of water into the tea kettle and boil that. In this case the boiling doesn't quiet down, instead it gets louder. I (infer?deduce?) that the pinging sound comes from bubbles detaching from the metal surface of the kettle. If the water is deep enough then most of the later bubbles form away from the surface. If the water is shallow then most of the bubbles continue to detach from the metal.

 

[sophiecentaur]

To make coffee I pour a cup of water into the tea kettle and boil that. In this case the boiling doesn't quiet down, instead it gets louder.

It seems to me that there are at least two factors here. De-gassing takes place at relatively low temperatures (singing kettle due to many small bubbles) and actual boiling will happen at around 100C. The rate of boiling, after 100C, will depend on the power supplied and the noise will depend on the specific surface and even details of thermal contact between element and water. 'Bumping' is the descriptive term for what you can get under some conditions. Also, the size of a typical kettle can give resonances to the sound inside it and sound louder for some kettles than others

Your small mass of water probably goes quickly through the stages. A kettle full of water (or even an urn) can produce the different effects at noticeably different times. I notice our kettle starts of (a few seconds after switch on) with distinct 'cracking' sounds for a short time, particularly with freshly drawn water.

You should take care, here. You could get addicted to the sounds plus the taste of coffee/.