In the last few weeks, with temperatures cooling down and the first snow of the winter coming, I drank my fair share of tea. As a European, I use an electric kettle of course to boil the water (for good reasons: faster heating process) but at some point, while waiting for the water to finish boiling, I asked myself: When does the kettle know that the water is boiling?
First instincts
The first thing that came to my mind was: it’s probably just a thermostat in there that is set to 100°C. It works for large water heaters, so why should it not work for an electric kettle?
But a kettle has to work in different locations - both sea level and also on Mt. Everest (in the extreme case). This would not work, the thermostat would never trigger in some cases, if it’s set to 100°.
So, let’s talk about thermostats real quick: most of the time (for analog versions), they are a bimetallic combination where both metals have a different rate of expansion. A bit abstract, but think about this example: you set the thermostat of your radiator to 21°, if the temperature is lower, the difference in expansion allows for warmer water to flow in. When the temperature is higher than the target, the metal disallows the water flow.
For this to work in an electric kettle, the manufacturers would probably set the target at 95-100°C. But this is a problem at higher altitudes.
Vapor pressure
When does a liquid start boiling? I’m not a chemist, but in my previous life I had to think about this sometimes (Sevoflurane for example has a surprisingly low boiling point) and also about atmospheric pressure.
Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.
Definition found on Wikipedia
In every liquid, regardless of the current temperature, some molecules will always change its phase from liquid to gas. At lower temperatures, this will not matter too much because the pressure gradient from the probably already saturated atmosphere to the pressure of the liquid is just too big.
But as the liquid gets hotter, the vapor pressure also increases. When the vapor pressure exceeds the atmospheric pressure, the liquid starts to boil. This has several implications: If you increase the pressure the liquid will boil at a higher temperature - congrats, you now know how a pressure cooker works!
Vice versa, if the atmospheric pressure decreases, water will start to boil much sooner. Water boils at 100°C at sea level, at 93°C at 2000m elevation and at 74°C at 8000m.
If an electric kettle would use a thermostat, on Mt. Everest it would just never stop heating. The water would never reach the temperature needed for the kettle to turn off.
Alternative solutions?
Have you ever tried cooking with an electric kettle? I haven’t but I have seen lots of videos where this is shown. In those videos you’ll notice that the kettle never turns off, even if the water is boiling for minutes. Interesting…
The funny thing is: the solution lies in the bottom of the kettle (most of the time at least).
So, the water is boiling, the pressure inside the kettle is rising. Parts of the lid lift a bit, some hot vapor escapes which now heats up a bimetallic switch (mostly found in the bottom) that at some point turns off the kettle.
Actually, thinking about this now, it IS a thermostat. Just not in the way we are used to! A lot of things to learn from a small kitchen appliance!