Tricking a Kettle

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Electric kettles are designed to shut off when the water inside them boils. But what does that mean exactly? In this video, Steve Mould explores that question by trying to trick his kettles into boiling off ethanol, a liquid with a lower boiling temperature than water. Steve figures that, if a kettle is designed to shut off at 100 degrees Celsius (water’s boiling point), then it will overboil ethanol since its temperature will never get that high.

I’ll let you watch the video and see what happens…

… all done? It turns out that kettle designers are clever enough to trigger shut off based on vapor pressure rather than temperature. As pointed out in the video, using a strict temperature cut-off doesn’t account for differences in boiling point with pressure. (Here in Denver, water’s boiling point is less than 95 degrees Celsius!) Instead, both kettles sense the build-up of vapor associated with boiling and shut off then. This works great — except if you leave the lid open! (Video credit: S. Mould)

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