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Why does a kettle go quiet just before boiling?

Why does a kettle go quiet just before boiling?
David Gray from Dorset (Age 45-54)

Why does the noise from a boiling electric kettle get quieter just before it boils?
Mark Rawling from Hampshire (Age 5-14)

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2 Responses

  1. Hi David. Hi Mark :). 

    Well Spotted !
    This IS an interesting phenomenon.
    Some kettles DO demonstrate this audible volume change more obviously than others and the reasons lie in their spout design and the size of their jug. Vibration creates the pressure waves which stimulate our hearing as I am sure you know so clearly the answer to your question lies in what produces and what affects the vibration in a kettle as water is heated to boiling point ?
    This is a Big Question and demands an equally Big Answer !
    Pressure again lies at the heart of this effect. The greater is the difficulty experienced by the steam as it escapes from the kettle, the more these changes in audio volume may become apparent. So, if a kettle possesses an outlet filter which has become partly blocked it is likely that the reduction in overall volume just prior to boiling will be greater.

    But before we look at that, may I suggest that you investigate the behaviour of gas bubbles in the following experiment with a bottle of lemonade?!
    Stand such a bottle on a suitable table and leave it to settle for a few minutes – until the lemonade has no bubbles visible on it’s surface.
    Look closely at the clear liquid and notice what happens inside the bottle as you carry out the following activities…
    Hold the lower part of the bottle in one hand so that it doesn’t fall over and holding the top of the bottle in your other hand carry out the following experiment :
    Gently shake the upper part of the bottle and look at what happens inside.
    Bubbles of gas will appear within the liquid of course but it is what happens to them and understanding why these developments occur which is important.
    Stop shaking the bottle and keep looking at these bubbles.
    They will continue to form for a few seconds, rising to the surface then vanishing as they pop, as you would expect.
    After a minute or two the only bubbles visible will be a few that have attached themselves to the inside of the bottle. These bubbles can be disregarded.
    The important thing to note is that no ‘new’ bubbles are appearing from within the lemonade.
    Now, while holding the bottle still by grasping the lower part, very slightly open the cap until you hear the gas escaping then retighten the cap firmly again. Keep looking.
    Notice that more bubbles develop within the liquid, rise to the surface and pop. Then again it all settles as before.
    Repeat this experiment a few times and think about why the bubbles stop escaping from the lemonade on each occasion.

    This interruption in the production of bubbles is due to pressure. The bubbles which burst at the surface contribute the gas which they contain to what gas is already there and thereby increase the total gas pressure exerted upon the liquid. This process continues until a threshold pressure is exerted on the liquid then bubbles of dissolved gas cease to appear. The repetition of this experiment clearly demonstrates the intrinsic inter-dependence of these factors.

    Now, to return to what happens inside the kettle of boiling water…
    As you may imagine, if the pressure of steam over the liquid water increases then the rate at which bubbles of the vaporised water can form and escape from the liquid will be reduced. You may conclude that this process contributes to the reduction in volume prior to boiling which you have observed.
    There are other processes going on which also contribute to the effect noticed but they take even longer to describe. If you wish, post a request below and I will try to explain more.
    Good Luck,
    Rockno3.

  2. Hi again David & Mark :).
    Bubbling makes a big difference to the sound emitted by a boiling kettle. This is the most obvious transformation of the thermal energy into kinetic energy during the ‘change of state’ which is occurring. Two phases of heat transfer may be identified in the process of raising a liquid’s energy state to boiling point. Firstly, the thermal energy transferred to the liquid progressively raises the temperature in the area local to the heat source – the electric element in such a kettle. At a certain temperature, dictated by the atmospheric pressure and the nature of the liquid, parts of the contact surface reach the ‘vaporisation’ threshold and bubbling begins here. These bubbles are relatively small and, as they rise, shrink further as some of their thermal energy transfers to the liquid surrounding them via a combination of conduction and convection and a localised condensation effect takes place. As the heat source reaches its’ operating temperature the activity stabilises and the generation rate of these bubbles becomes constant. The temperature of the liquid continues to climb and to become more evenly distributed throughout its volume until the boiling point temperature (again predetermined by the chemical nature of the liquid and the atmospheric pressure) is reached. That progression then reaches a plateau, no further temperature increase occurs in either the liquid or the vapour above it (these temperatures are the same incidentally) and the first phase is complete. More thermal energy is now required to produce boiling, the quantity of energy actually dependant on the chemical composition of the liquid involved. This factor is called the ‘latent heat of vaporisation’ and generates a new equilibrium within the liquid of relatively large vapour bubbles evolving simultaneously and rising to pop at the surface and thereby contribute relatively large quantities to the steam produced.
    This form of bubbling produces a different vibration pattern which is more even. Thus the tone or ‘quality’ of the sound is different. The sound is more rounded, and more pure with less ‘harmonics’ and seems quieter as a result.
    I hope that this explanation describes more thoroughly the processes behind the apparent change in volume noticed during the boiling of water in an electric kettle.
    Good Luck,
    Rockno3.
    .

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