In fact water boiling in any receptacle makes some noise – you may have noticed a similar noise when heating water in a saucepan, for instance – but it is more noticeable in a kettle because it is in a confined space and comes to the boil more rapidly (I presume we’re talking about the actual rumbling sound of water boiling, rather than the whistling noise you get with an old fashioned type of kettle).

The noise is caused by the same process that causes bubbles to form as the water heats. What happens in a kettle is that the heat coming from the heating element (or the base of an old style stove-top kettle) first causes the water near it to heat and then, as it reaches boiling point, to turn to steam and to rise to the top or surface. Steam isn’t as dense as water and so it will rise higher (and heat always rises, as you can see if you put your hand above – obviously not too close to – an open fire or similar heat source).

When the steam bubbles from the bottom of the kettle are big enough, they are released and move upwards. There is a sound accompanying the release. As the bubbles move further away from the heat source, ie the heating element, they cool down – which causes them to lose density – and collapse. When vapour bubbles collapse, they disturb the surface of the water, which causes the release of smaller air bubbles from the water – this also produces a sound.

So in fact the boiling process produces different sounds at different stages. This is why, if you switch on your kettle and start listing, you will hear a small ‘boiling’ sound almost from the beginning. This grows louder as the bubbles start to be released and make their way towards the surface, and is loudest when they cool and collapse at the top. This occurs just before all the water has boiled, which is why the kettle will make more noise shortly before it boils than it does at the actual moment of boiling.

Once all the water has reached boiling point, all the gas bubbles are able to get to the surface and are released into the air. This is a quieter process than the sound of all the bubbles collapsing that occurs earlier. A recently boiled kettle being boiled again is also a little bit quieter than one being boiled from cold, because the dissolved gas has escaped into the atmosphere and this was what made most of the noise.

It is interesting to compare this with what happens when you heat water in a microwave. In a microwave, if you watch your container of water turning round it gives no sign of boiling, and as far as you can hear it above the sound of the microwave it doesn’t seem to make the ‘boiling noise’ either. But if you take it out you will see that the water is very hot indeed. This is the result of a process known as superheating, where the heat is actually higher than the boiling temperature of water (100 degrees Celsius) and heats it all at once rather than through a gradual process involving the formation and release of bubbles as described above. Also, in a microwave you will use a plastic container or china cup, for instance – these will be cold, while the inside and element of a kettle will be made of metal, which conducts heat so again it spreads gradually rather than all at once.

You can learn more about this subject here.