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