everyday-lifeheatwater
Something a little different to our usual fare. I was boiling a pan of water for cookery the other day, and got to wondering what caused the location of the bubble streams from the bottom of the pan. The stream of bubbles seems to come from the same spot of the pan, is this a quirk of the fluid flow? Imperfections in the pan? These streams of bubbles appear to continue for several minutes until the water fully boils and it becomes too turbulent to follow anything.
Any thoughts?
(source: engineeringtoolbox.com)
The graph(blue lines are what you want) only goes till $50^o C$, but we can extrapolate and say that the oxygen level will become a fifth or so by the time the water reaches its boiling point.
The solubility comes from Henry's law, but I don't know the temperature dependance of the proportionality constant $k_H$--I'll check it out tomorrow and edit it in..
Water could have small pockets of air dissolved inside it, and given enough time they will rise to the surface on their own.
Now when you boil water, what happens is that the vapour pressure at the water surface is equal to the atmospheric pressure, and liquid water gets turned into water vapour. This process does help the trapped pockets of air reach the surface more quickly, since the water molecules are moving around a lot during boiling. So after one boil, there is less water in your kettle, but there is also a lot less air dissolved in the water.
As the question you linked to pointed out, some of the sound comes from the dissolved air bubbles hitting the surface, and with less air that is less sound. Also quoted is the vapour bubbles that are produced at the bottom of the kettle (where the heating is). A vapour bubble is basically a bubble of water vapour, as the name suggests. The pressure of the water vapour is higher than the pressure of the water around it, so it rises upward to the surface. This produces a sound (quoting the paper linked in the question) of about $35 - 60$ $KHz$.
But the second time you boil the water, the water isn't at room temperature. It's quite a bit higher. Therefore I suspect that less vapour bubbles are produced near the bottom (since the pressure of the surrounding water is higher than at room temperature, because it's hotter) and thus overall less sound.
Best Answer
According to Wikipedia
At Cornell they did a study of Critical Droplets and Nucleation, and they observed that surface tension is what allows this vapor pressure change and causes the bubbles to form. This can be caused by imperfections in the pan due to heating of the pan itself, scratches in the pan, and other added areas of nucleation such as salt or sand. Not due to impurities of the water, although different impurities do heat at different rates than water.
(Very Dangerous) If you heat distilled water in a smooth container (with no nucleation points or imperfections) in the microwave it exceeds the boiling point because no bubbles are allowed to form. At the point you shake the cup or add something that breaks the surface tension you invoke a vapor pressure change allowing boiling to take place, rather violently because you've created new points of nucleation.
When you heat metals (welding and cutting included) you create something called a heat affected zone (HAZ) where the microstructure of the material and the properties of the material itself are altered by the heating process. In your pan you're causing different materials in the metal itself to shift (based on the heating element placement) which is what's helping to create [some of the] nucleation sites.
The shape of the pan, imperfections in the pan, and the surface tension caused by the bubbles and the lack of bubbles on the surface, and the pressures in the vertical flow column allow the bubbles to form uniformly, creating streams of rising bubbles.
Other obstructions in the pan like salt, rice, and noodles stuck to the bottom of the pan can help to create nucleation sites as well.