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Water in my electric kettle makes the most noise sixty to ninety seconds before the water comes to a full boil. I have been fooled many times by the noisy kettle, only to discover that the water was not yet hot enough for tea. The kettle is only at a full boil after the noise has subsided.

I have noticed the same phenomenon with many other kettles, including conventional kettles on kitchen ranges; it is not a peculiarity of this electric kettle.

Why does the boiling become quieter as the water reaches full boil?

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It is a complete coincidence that the question three before this one is about coffee making; I had not seen it when I came here to ask this one. –  MJD May 9 '12 at 17:08
    
Think about this. When water is hot in your kettle, and it reaches an internal pressure greater than that outside of the kettle, it must release that pressure. However, since gas is released (aka the sound produced), heat escapes and thus cooling the water somewhat. That's why when the sound has subsided has the water only reached a more energized state. –  Brent May 9 '12 at 17:49
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1 Answer

There are three phenomena that occur before vigorous boiling of water that produce sound.

1) Air dissolved in water on heating forms small air bubbles at the bottom of the container. These air bubbles get released from the bottom of the container on reaching a sufficient size. The process of release produces a sound of frequency ~ 100Hz.

2) On boiling, small vapor bubbles get produced at the bottom of the container and also produce sound of ~ 100 Hz on release. However, they cool down before they reach the surface of water and collapse. This collapsing produces a sound of frequency ~ 1 KHz.

3) Collapsing vapor bubbles agitate the water to release small micro air bubbles from water and also from the air trapped in the vapor bubble. This production of micro air bubbles produces a sound of ~ 35 - 60 kHz.

I guess you were talking about either the first or the second case.. Both of them occur before you observe vigorous boiling of water and you can hear them.

There was an interesting problem posed at APHO 2008 which is the same as your question which estimates the frequencies I just quoted. It also contains references to the experimental measurement of these frequencies. I hope you will find it interesting to solve it than me answering your question. http://apho.phy.ntnu.edu.tw/download/past_APhO_roblems/2008%20APhO/APhO2008_theory_prob1.pdf

You can also find the solutions here in case you need help on this http://apho.phy.ntnu.edu.tw/download/past_APhO_roblems/2008%20APhO/APhO2008_Theory_sol1.pdf

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@forsudee With increasing temperature the excess pressure of dissolved gas increases, the dissolved air is liberated and air bubbles appear at the bottom and walls of teakettle. Why small air bubbles are formed at the bottom of the container while heating? what happens to the air bubble density? –  Eka May 9 '12 at 22:28
    
Do you have any references that discusses such phenomena? –  metacompactness Mar 30 at 15:32
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