Curious water phenomenom I can't explain.

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I was boiling water today on the stove, and when the water get really hot, it ,obviously, starts bubbling and realeasing foam and steam. Curious, I lifted the cauldron and in a few seconds, the water suddently stopped releasing steam and foam and gradualy stopped bubbling, and when I put it back on the stove it quickly started bubbling and steaming a lot.


Any body got an explanation?

It takes heat to break the polar bonds. No heat, no boiling.

Because you took the lid off.

The steam escaped into the surrounding air and the water calmed (but not cooled) down.

When you put the lid back on the steam cant escape so it gets recycled into the water and the water bubbles and foams again.

I'm not understanding if you lifted the whole pot or just the lid. But if it was the whole pot, it will stop boiling quickly because when water turns into steam it carries away a large amount of energy. So, when you take it off of the heat source, the steam quickly carries away enough energy to reduce the temperature below the boiling point.

I think the wispy and basically invisible-ness of steam makes it seem mysterious -- there's a ton of energy in there, but we can't see it.

You've just discovered for yourself the principle of enthalpy of vaporization (click).

Looks more complicated than it really is: a substance doesn't instantaneously ungergo a phase change at a certain temperature; rather, heat must continue to be added or removed.

Thanks for explaining! Physics are fascinating!

It is! And chemistry too, because it's related and this was part of my chemistry course.

Try this... put a thermometer in and wait for it to boil. Play with the temperature. As long as it isn't too close to the metal of the vessel, it will not be above 95-98 degrees C (if your thermometer is accurate). You can never get it to the boiling point unless there are impurities that raise the BP because as it undergoes a phase change, it will pull away a lot of heat energy with it. Look into heating curves of mixtures too... they level off as one component is evaporated, then continue rising. (This is how they get poisonous methanol out of ethanol so we can drink it. )

I have thought about this. If you look at water closely water doesn't go from water to air, there is some kind of barrier in between. This barrier has properties like tensile strength. You can test the tensile strength by trying to poke water gently with a small stick, if you do it just right you can create little divit in the water (much like a water bug does when it stands on top of the water).

I think what happens is under higher pressure the tensile strength of the water-air barrier increases, so that the bubbles pop more slowly, when you lower the pressure it causes the bubble's surface layer to weaken causing it to pop more readily. (That's what I think is going on).

That's why the bubbles rise - I think - I could be wrong though.

I'm not sure what you mean about the bubbles popping more slowly or readily.

The way water has surface tension and likes to stick to itself are both caused by hydrogen bonding. The tensile strength of this might weaken somewhat if the molecules are more excited by heat (and thus moving faster and have less time to hydrogen bond as they pass), but the atmospheric pressure pushing down on a substance is going to have a larger effect on the vapor pressure of the fluid.

As you lift the vessel, it will transfer its heat to the water until it equalizes. If the vessel is over 100 C, it might make the water on the bottom boil for a moment and carry enthalpy away with it in the form of heat energy.

You're right, I don't think it has much to do with the surface tension. It probably has to do with the pressure inside the bubble vs the pressure outside.

When the pot has a lid on it, what happens is that the pressure outside the boiling bubbles becomes closer to the pressure of the vapor inside, the bubbles don't have as much of a tendency to pos. So they build on top of one another. When the lid is opened the pressure drops and the bubbles pop.

I was only addressing the "foaming" observation. The fact that it stops releasing steam probably has to do with the temperature drop that occurs when the pressure drops when the lid is released. PV=nRT

Ah yes, PV=nRT! That brings back all kinds of memories of chem class.

It's also because a material (at least a relatively pure one) will start a phase change very suddenly--at one temperature it is not boiling (merely evaporating quickly) then a fraction of a degree later it will be boiling. Since boiling liquid is always going to stay at a temperature which it will boil at, and not above, once you remove it from the heat source it's temperature will very quickly fall below that which it can boil at.

i think your thermometer is off Koan, or what is your altitude?
its pretty easy to heat water well above its boiling point, just make sure its distilled.
the classic heat a beaker of water in the microwave, take it out and put a fork in it for example.

also the 3 inches of water in the pan increases the boiling point just a hair by its own pressure.

Thermometer is a Thermopen, calibrated and renowned for their accuracy. Pure water boils at ~100, assuming standard pressure (1 atm, sea level?) and that you're using a vessel that allows it (always use a boiling chip ). Altitude isn't very high here, but enough that I wouldn't be surprised if it boiled at 96-98 in most of the world (most people are above sea level). The microwave thing is an example of superheating... you can heat it above boiling, but that doesn't actually change its boiling point. Cool stuff.

The reason behind this is because once the radiated heat originating from the stove was removed, the 'stresses' on the bonds were also removed, hence the warm steam molecules, upon contact with a colder atmosphere are immediately bonded back together again as concentrated water molecules as the molecular vibrations from the intense heat were ceased. As the temperature of a liquid, such as water increases, the kinetic energy also increases, which causes increasing molecular motion, which are vibrations caused by molecules moving past each other due to the kinetic displacement. If heated for a long enough period of time, these molecular motions eventually become strong enough that the forces of attraction between the said molecules and polarity is disrupted to the extent the molecules break free of the liquid and become steam.

Its kinetics. The atoms are moving at such a fast rate it changes the phase of the water from liquid to vapor/gas. Its a physical change: still h2o when changed from liquid to vapor/gas phase. Chemical change would be h20+hcl---->h3o + cl. Put a colder temperature about a foot high or more and you can change the phase back to liquid or solid. Lovely chemistry.