h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I have always wanted to know this:-

A kettle just before it boils makes a hissing noise. The water around the heating element is simmering away, small bubbles are being formed and they make the usual happy hiss of a kettle coming to the boil. HOWEVER, just before the water starts to boil, it goes momentarily quiet. Then it boils and makes a different bubbling noise.

Why does it go momentarily quiet?

I have never been able to explain this simple fact. I am a technologist. Physics and electronics are my life. This (no doubt really simple) every-day event has defied a simple answer from everyone I have ever asked.

Anyone know?

TTFN

Freddie.

Hello - I am not sure if this correct or true, but it may be that before it reaches the boil the small bubbles are dissolved air being forced out, then it reaches 100 C, all the air has been forced out, the element is now just heating the water, and does so until it starts to boil (the second kind of bubbling) - As mentioned before this may not be the answer, but it is possible (I think Until later......
BCNU - Crescent

What an interesting question. I know the exact phenomena of which you speak, though I've never really wondered about it - until now, that is.

Having thought about it somewhat, here is my hypothesis: the sound we hear before it comes to a "rolling" boil is due to phenomena that occurs on a molecular scale, in the microstructure of the stainless steel in the pot. On a very small scale, there are cracks and crevices in the surface of the steel, even food-grade polished stainless steel. In these cracks and crevices, lots of molecules of water reach temperatures above their boiling point before their neighbor molecules in the vast expanse of water that is in the pot, and, hence, form very tiny bubbles of water vapor - too tiny to be seen by the eye. As soon as these "micro-bubbles" are formed, however, they almost immediately lose the energy they needed to be in the vapor phase to their surrounding water molecules, and condense back to the liquid phase. This continuously happens until eventually enough heat is passed onto all of the water as a whole, and a thermal equilibrium is reached between the liquid phase and the vapor phase of the water (i.e. the steam coming out of the pot), and the rolling (steady, large bubbling, gently splashing) boil occurs. The minute shocks caused by the billions of "micro-bubbles" of water molecules going from vapor to liquid and back again very quickly in the microstructure of the steel are what I hypothesize to cause the overall vibration we can hear as a sort of hissing, rasping noise. It's also what I suspect is behind those little "bubble trails" that we see appearing out of what looks like random spots of the steel pot during this time.

I confess to not having the tools or the time to test my quaint little "micro-bubble" theory, so I leave it up to someone else to prove me right - or wrong. For what it's worth, I think some of this sort of thing might be discussed in a good Chemical or Mechanical Engineering textbook on heat, mass and momentum transfer phenomena.

Thank you for the reply. Your answer suggests a very complex non-linear occurence. Chaos perhaps? Probably domains of heated water jostling with neighbouring domains of cool water (relatively). This is quite simply the best answer suggested to me in a long while. As to why the sudden reduction in noise at a crital temperature (100 C)should occur must be down to the physical fluid activity. I doubt if it is because the air in the water has boiled out, because it happens again when the kettle has cooled down and is re-boiled. I think? In fact I will check on this last point now.

Thanx to Cresent and JD.

A reply soon, I hope...

TTFN

Freddie.