A question on Twitter

SaxLegs McGinty
Why do kettles go quiet before they boil?

Hmm, good one.

So, err, why? A first guess would be that the noise as heating up is dissolved gases being driven off – does that make sense? Dissolving solids in liquids can be easier as temp rises, but gases work the other way around? – and then the actual boiling is the creation of the steam. But there’s that little gap between driven off gases ending and steam production starting?

This is, obviously, a question where the answer has been worked out already. So, is this one of those where first guesses are missing some vital fact that thereby makes a mockery of them?

11 thoughts on “A question on Twitter”

  1. I suggest you reboil a kettle of recently boiled water – one that has not had a chance for much air to redissolve in the water – and listen carefully.

  2. Cavitation I think. As water gets hotter at the bottom, gas bubbles form above the heating element, rise and collide with each other and this plus cooler water causes them to implode releasing sound energy. As the water gets hotter towards boiling this effect gets less and stops as gas bubbles rise quickly through very hot water to the surface and form larger bubbles which burst on at the surface and gas escapes to the air.

    Noise doesn’t stop, it changes from a high frequency singing sound to a lower frequency bubbling sound caused by larger bursting bubbles at the surface.

    Cavitation in machinery or on ships’ propellers can cause metal pitting. Cavitation can also give the location of a submarine away as it can be picked up on passive sonar.

  3. I specialise in the design of industrial evaporation plant, so this is my subject! The Thunderf00t Youtube video is basically correct, a boiling liquid can’t dissolve any gas (Henry’s Law), so by boiling all gas is removed from the liquid. The noise comes from the collapse of bubbles of water vapour as they rise up into colder water & the vapour condenses (cavitation). This happens most at lower temperatures because there are few bubbles & lots of colder water (Nucleate boiling regime). As the temperature rises the mixture becomes turbulent & well mixed so cavitation reduces (mass boiling regime). Cavitation on industrial plant occurs if there is excessive pressure drop on the suction side of a pump, the liquid then boils as it is now above its boiling point, these bubbles collapse as the pressure rises through the pump & it sounds like you are pumping gravel. The bubbles pluck at the pump surface & erode it. As mentioned in another comment you can get a similar effect with ships’ propellers.

  4. Thank you so much for sharing this, @Charles.

    I love learning about stuff which is completely outside my experience, especially when it has been eloquently condensed into something understandable, without me having to trawl through a search engine or reference books without the vocabulary to do so.

    I, like many others here, also look forward to the Wisdom of Steve, but it’s these small nuggets which absolutely make my day!

  5. Isn’t the crackling noise the dissolved CO2 being driven out?
    And isn’t the amount of dissolved CO2 dependent on the water temperature and the vapour pressure of the surrounding atmosphere.(more dissolved for lower temp and higher VP)
    I am sure everyone knows this intuitively in that they know warm beer is harder to pour and loses its fizz quicker.
    And the re-dissolving of CO2 back in as the temperature cools again takes time to reach the temp vs qty dissolved equilibrium, i.e there is a lag, which I suspect is what dearieme is getting at.
    Reboil a kettle of recently boiled water and it won’t crackle near as much as it hasn’t enough time to re-dissolve all the CO2 it is capable of.
    Same effect in the other direction too. Heating water will always have a little more CO2 still dissolved in it than at equilibrium for any given temperature.
    I am sure Al Gore knows all about that lag

  6. @Nautical Nick

    I used to enjoy Thunderfoot before he came down with a severe case of TDS. Coupled with his sneering at global warming ^H^H^H^H^H^H^H^H^H^H^H^H climate change ^H^H^H^H^H climate Armageddon sceptics, turned me right off

  7. @Nautical Nick: yes, I’ve watched all the creationist mick takes. Some of his science vids are brilliant, especially the coulombic explosions, and getting a vacuum bubble to rattle.
    Seems to have a bad case of Musk Obsession Syndrome now.

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