Will A Kettle Filled With Alcohol Boil Dry?

The average home kettle is set up to switch off automatically when water reaches its boiling point. But would a kettle filled with alcohol, which has a significantly lower boiling point, actually turn off? [Steve Mould] set out to find out.

The prediction was that a kettle full of 40% strength vodka would boil dry, as the vodka would evaporate before it actually got to a hot enough temperature to cause the kettle’s cutout mechanism to kick in. The experiment was done outside to minimise the dangers from the ethanol vapor. As it turns out, the vapor from the boiling vodka is about 80% ethanol and just 20% water, so eventually the mixture left in the kettle is mostly water and it boils hot enough to trigger the cutout mechanism.

However, the experiment doesn’t end there. Trying again with 99% ethanol, when the fluid started boiling, the kettle switched off even more quickly. So what’s going on?

The kettle in question uses a bimetallic strip, which trips the switch off in the base of the kettle when it gets too hot. There’s also a tube inside the kettle that carries vapor from the internal cavity and lets it pass over the bimetallic strip. When the liquid inside the kettle boils, it forces hot vapor through the tube, out of the kettle and over the bimetallic strip.

This strip triggers at a temperature significantly lower than the boiling point of water; indeed, as long as the liquid in the kettle is fairly hot and is boiling enough to force vapor out the tube, the kettle will switch off. [Steve] points out that it’s a good mechanism, as this mechanism allows the kettle to respond to boiling itself, rather than the arbitrary 100 C point which water technically only boils at when one is at sea level.

It’s an interesting look at a safety system baked into something many of us use every day without even thinking. It’s not the first time we’ve seen [Steve] dive deep into the world of tea-making apparatus, either. Video after the break.

30 thoughts on “Will A Kettle Filled With Alcohol Boil Dry?

  1. Unlike water which if you have given it any thought dissipates heat better than Alcohol. Hence part of the reason for the higher boiling temperature, plus water is heavier. That’s why the alcohol floats in water. The kettle trips because there is nothing to attempt to cool the thermistor or bimetal switch. Now if you mixed antifreeze in the water( by the way, there is a certain amount of alcohol, in a compound with propylene or polypropylene which combined with water acts the opposite way as pure alcohol ). As you know this raises the boiling point and lowers the freezing point but the fluid needs to be kept under pressure to keep from boiling. You also know that water at lower elevation boils at a higher temp than it does at higher levels above sea level. It doesn’t matter what level it is to freeze it though. Weird. Alcohol is a carbon compound and water is not. Carbon compounds are volatile and the only way to get a fire from water is to split the Hydrogen from the Oxygen, then you have the hottest burning substance in the universe. The main source of heat from the Sun. We all are familiar with what happens when a large source of Hydrogen is ignited here on Earth. Alcohol is also a solvent but then so is water up to a point. Carbon compounds do a much better job at dissolving stuff.

    1. Water and alcohol are miscible, meaning once mixed they will never naturally separate. So the alcohol will not float to the top of water. And the mixture as a whole will have a boiling point in between the bp of the pure liquids alone. Boiling is a very efficient way to transfer heat, even when it is alcohol that is boiling vs water. Once boiling commences, the inside surface temperature in the kettle will actually decrease as compared to right before boiling starts (assuming a constant heater rate). That’s the reason you can’t use a high limit surface temperature to determine if boiling has started. You have to over shoot 100 C to get to boiling, and then you’ll be dragged back close to 100 C once boiling starts.

    2. – the boiling point is determined by the vapor pressure of the liquid, not the thermal conductivity
      -water isn’t heavier, it’s denser
      -freezing isn’t fighting against the pressure of the atmosphere in the same way that boiling is so it’s not directly comparable
      -hydrogen isn’t the hottest burning substance, Dicyanoacetylene is
      – the heat from the sun isn’t the hydrogen burning, it’s the hydrogen fusing

    3. The sun isn’t hydrogen and oxygen burning. It’s hydrogen fusion. Completely different.

      Hydrogen flame isn’t even the hottest combustion flame; oxyacetylene burns significantly hotter than oxyhydrogen.

    4. Your reaction is so full of misunderstanding of chemistry and physics that I hardly know where to begin.
      The heat dissipation and weight of water have nothing to do with its boiling point being higher than that of ethanol.
      That water freezes at the same temperature at any pressure is not weird, if you understand that freezing has nothing to do with molecules having to escape into the atmosphere.
      “Carbon compounds are volatile” Proteins are carbon compounds, are they volatile? Tar is a carbon compound.
      “Carbon compounds do a much better job at dissolving stuff” Depends on what that “stuff” is. Try dissolving table salt in pure ethanol.
      And as already pointed out: the sun does not burn any hydrogen.

      1. I did read the article

        I’ve replaced the switch in more than a few kettles

        I’ve also had this same argument a few times, easiest way to prove it is to get a piece of tubing and blow into the cut off switch

        it clicks off

  2. This is wonderful. I have a kettle of that sort, and one of the finest things in all the world is learning how something works.

    I’ve looked in there countless times, notices that tube, and a small fraction of my brain cells wondered just why it was there and what it was all about — and now I know!

    Thank you.

  3. At first I thought the second kettle tended to shut off when the liquid inside boiled because vapor doesn’t conduct heat as well as liquid. When a significant portion of the hot plate is covered by vapor, heat conduction decreases (think Leidenfrost effect), and the plate gets even hotter, eventually tripping the temperature switch.

    But then, a tube in the second kettle was revealed, and its effect on the kettle behavior resulted in the expected performance when covered.

    I’m forced to conclude that all kettles have a tube that direct vapor to the temperature switch which trips at a lower than expected temperature due to heating by vapor, and is the normal operating mode. The temperature switch mounted to the underside of the hot plate is actually a safety switch, preventing the hot plate from heating without limit when the pot is either removed, or boils dry.

    The ideal experiment fails because the engineering prevents it from occurring…. Clever engineers!

  4. An e-kettle is uncommon in the US. Anyway I knew a x-Brit who used one and I never have seen a tube or anything but a boil-dry thermostat embedded in the bottom of the one piece kettles I have seen. Our water tends to be hard and that Calrod element died of encrustation overheating in the one I experienced. Europe is different.

    1. I think they are becoming more common in the US, and have been pretty common in my area (SF Bay Area) for 10-15 years. But the US kettles are 115V instead of 230V and are noticeably slower (same amp ranging, so half the wattage). The most powerful kettles in the US are only 1500W, while 3000W is pretty common in the UK.

      My kettle has a sight glass, which also connects to a hidden tube. It becomes more obvious if you unscrew some things. I do use the kettle with the lid open to clean it, and it will boil dry before shutting off. (anyone who has taken a few chemistry classes will have some respect for the reactive abilities of boiling hot vinegar)

    1. Most kettles have a secondary thermal switch, possibly integrated with heating element.
      Each time I bought a new kettle I properly tested it by running it without water. Kettle without such protecion has no place in my household!

        1. So far as I’ve been able to tell, that’s usually the thermal switch actually failing; they’re quite sensibly built to fail open, but they’re meant to be able to cycle at least a few times. I’ve replaced a few and had the same device hold up through an overheat cut-out and then later fail open.

  5. I’ve heard this is one of the reasons kettles have a minimum fill line, on top of keeping the heating element submerged – too little water and the vapour tube won’t fill, so it might boil dry.
    Also overfilling can sometimes flood the tube, so an overfull kettle might not shut off until the liquid level allows the tube to pass gas
    Depending on the exact layout and design of course

    1. It wouldn’t work at all, even at sea level. The temperature sensor would have to be better than extremely accurate. If it turned off at 99.5°C, it would never boil, and if it turned off at 100.5°C, it would boil dry before turning off.

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