So its always bugged the hell out of me why a kettle makes most noise before it boils, then becomes almost silent when it actually boils. However when you get a high speed camera on it, it becomes very obvious very quickly! The noise is actually made by the collapse of steam bubbles in colder water. At about 70 C you have the most number of these bubbles collapsing. As the temperature of the water gets higher, the steam in the bubbles doesnt condense as well, meaning the bubbles have a longer lifetime. Finally when the water boils, the bubbles dont collapse at all, and it becomes almost silent!
Cool stuff water does under vacuum:
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