Challenge Questions

Just before "true" boiling, there is nucleate boiling. The bulk of the water has not reached boiling temperature, and the bubbling is from local boiling. As the boiling process goes from subcooled-nucleate to saturated-nucleate the bubbles stop for a short period then restart.

No, son. That's not steam. It's disolved air comming out of solution, since the solubility of air in water is inversely proportional to temperature. Eventually all the disolved air is out of the water, so no more bubbles can form. What do they teach you in science class now days? From now on, it's home schooling for you.

Huh? Why would dissolved air come OUT of solution as water heats, and why stick to bottom?

Ried is right. Local nucleate boiling at the wall occurs first, and as soon as they become big enough to pull away the main volume of the water still below saturation collapses them. This is actually the time when heat transfer is occuring most efficiently. When bubbles restart after saturation is reached the bubbles no longer collapse, they collect into big steam voids that rise to the surface and release into the air. That's why there's such turbulence in the water and the steam cloud forms over the pot. It's also very inefficient at that point since the water can't get any hotter at atmospheric pressure and all that steam is a waste of heat and also water volume once it recondenses on all the cool surfaces around the kitchen. That's why you put a cover on the pot, to retain the collapsed bubbles with that heat and condensate inside. And it can give a small buildup of pressure which actually allows the saturation temperature to increase, i.e your water is hotter. This is why some recipes call for a "pressure cooker" which is a sealing lid with a weighted relief valve. Be very careful with these and ensure the relief is open! While pressure cooking green beans (in jars) for canning, my wife blew up a pressure cooker. The lid went through the ceiling and green beans as well as shards of glass went everywhere in the kitchen! Lucckily the pot itself stayed intact and directed almost all of the shrapnel over her and the kids heads! She has since thrown away that cooker and does no more canning.

It's magic. if he is young enough... he will take it as such... or tell him to do some resaerch and report back to you with the anwser.

Well, There is a middle ground, as usual.

The bulk water holds dissolved air to about 23 PPM at 0c rising to 0 PPM at the boiling point.

So 1 liter will have 2.3cc of trapped air. As the water temperature rises this streams off the surface with no boiling as to boil it would have to come to a local 760MM in pressure.

Down at the hot surface where the heat is applied, you see small bubbles rising and then collapsing. These are steam with a tiny amount of trapped air. The rate of collapse should tell you it is not air dissolving into the water, but vapor cooling and collapsing into water....water that is already suprrsaturated with air

Re comment by Aurizon on air solubility - this isn't quite right.

Solubility of air doesn't fall to zero at 100°C. With partial pressures oxygen 0.21 atm and nitrogen 0.79 atm in both cases, air solubility at 0°C is ~ 38 mg/l and at 100°C it's still ~ 15 mg/l.

The reason air can be completely removed from water by boiling is that the water vapour pressure = atmospheric, so the air partial pressure falls to near zero in the steam bubbles, the air comes out of solution and is carried out with the steam.

That is true, if you hold water in a sealed container above 100C it can still hold air, however I was referring to the equilibrium position of water at 100C and normal 760 MM, as you approach 100C the sum of the vapor pressure of water and the dissolved air will be above 760MM and the water will boil below 100C as both the water and air vapor pressure combined boil off. You will get a slight depression of the boiling point(hardly measurable). Any air is soon gone in free air.

The boiling temperatures of air and water are widely disparateIf you boiled methanol and water you would find the mixture boiled below 100C and the lower boiling ethanol would contribute proportionately more to the vapor than the water. As the methanol was gone you would reach 100C.

If we ever have an opportunity to have a beer together, I will not ask if its less filling or tastes great!

However, you are correct in your point about air and steam being released from heated tap water.

23 Parts Per Million of a liter is 23μl or 0.023ml not 2.3ml which is 2,300 PPM even though the dissolved atmospheric gasses would need to condense out before the water reached 100ºC

My three year old grandchild asked her mother why there is snow on the tops of the mountains. My daughter's answer -- "Because the clouds are made of snow and they rub against the tops of the mountains".

Not a bad answer for a history major. A dissertation on isentropic expansion probably would not have cut it.

For the bright /smart kid:

The temperature of the water at the bottom of the pot of water is the same as the temperature at the top surface of the water, causing the air bubbles to be reduced in quantity and instead causing the water to start evaporating as steam (look at the steam rising from the surface son).Now, if you notice and look at the inside of the top, there are small bubbles being formed inside and raising a little at a time as a result of the water molecules move over the inside rough surface of the pot, causing them to create small watercooled molecules.

For the younger kid:

The big bubbles dissapear when the water starts boiling because most of the water has the same boiling temperature of 212 degree F, and the few little bubbles appear because some of the molecules cool off when they hit the inside of the pot. while rushing to raise to the top

For the very smart kid:

The bubbles tend to dissapear as the dynamic temperature and static temperature of the water equals 215 degrees F ( assuming the boundary layer created as the water molecules approach the inner walls of the pot, remain primarily laminar). Imperfections in the wall surface and material density cause the boundary layer to become turbulent as the water molecules experience temporary cool down where the localized static temperature is lower than dynamic temperature; thereby showing up as localized "cool spots" ,manifested as a gas, trapped by the surrounded liquid water molecules heated to dynamic temperature of 212 degrees F ( the smoother the pot walls, the lesser the chance to create the small amounts of air bubbles. In ideal conditions where the inner walls have a minute coefficient of friction and the pot wall thickness is 100% uniform without voids and capable of inform heat transfer from outer to inner surface, it is ideally possible to speculate that the bubbles will stop once the entire mass of water has reached equality between dynamic and static temperatures.

Usually the dissolved air is pretty much gone as you place a pot on to boil.

The bulk water is cooler, as is shown by the condensation of the bubbles as they first start and do not make it to the surface. A 1 liter pot full of water will have a max of 2.3CC of dissolved air, even if it all came out at once it would not make the quantity of collapsing bubbes seen. That is heat transfer from wall/bottom to bulk and at first you get condensation in the bulk...the bubbles vanish. As the bulk temp rises the bubbles reach the surface and are in thermal equilibrium with the water all the way = 100C. If you heat the walls and the fluid falls below the heated place on the wall, you will get superheating of the steam above the water.

Ordinarliy you will get less than 1C superhaeting in the bulk water, unless it is deep and you get elevated boiling bt pressure. each 32 feet of water add 14.7 PSI to the pressure, which elevates the temp.

Pressure cookers do this, but the depth is not normally a problem in kitchens

This is called a roiling boil and the loss from surface by evaporaation = wall input = steady state until the water drops

Well I,m off to make a (_)? and test these therory's, will report back later. It could be the removal of superheat as the steam bubbles rise through the relative subcooled water above!!!!!!!!!!!!

Its 'cause the bubbles are like "WHOA! We gotta boil now!" So they get together and hide and get ready for the big finish!

Interesting that we get such varied answers from apparently (seemingly?) learned people, at least judging by the scientific terms they use. Some seem to believe that dissolved air plays a large part, namely that the small bubbles are (almost?) entirely air and not water vapor or steam. Some make absolutely NO mention of air in their answers. Still another group seems to be reaching for a compromise answer that includes parts of both answers.

Is this a case of the three blind wise-men who are each asked by a group of children to feel a part of an elephant and tell what it is? One feels the trunk and says, "It is muscular, sleek, and twists about, it must be a snake!". "No," shouts another, feeling the side of the elephant,"It is large, hard, and flat. It must be a wall!" The third, feeling the leg says, "You are both wrong. It rises up from the ground, yet I can put my hands around it and feel its round shape and firm surface, it is a tree."

Along comes a fourth blind wise-man, hearing what the others are saying, and without feeling the elephant himself, he speaks. "You are all partly right and partly wrong," he says. "It must clearly be a snake in a tree which is growing against a wall!"

Then the elephant gives a snort and moves off leaving the "wise" men shaking and the observant children laughing.

Honestly, I did not even know that the little bubbles disappear before steam bubbles start streaming to the surface, either partially or all the way. I always adhered to the old adage, "A watched pot never boils" and usually I was off doing something else besides watching the pot, waiting for it to boil.

I guess you learn something new every day. However, listening to these wise men, I am just not sure what I have learned!

I guess I must wait for the "official" answer to the challenge. However, after reading the "official" answer to the "Bilge water" challenge, which ignored the fact that transverse ribs in a boat, the Titanic example notwithstanding, always have drain holes, and that the challenge mislead us by use of the word "seemingly" to describe a leak, I am inclined to take any "official" answer with a big grain of salt!

A watched pot never boils? Now you tell me, I have been standing at the stove waiting since last night!

Actually, I too have never noticed this phenomena but as I write this, my wife is preparing supper. Fortunately, it includes pasta.....

I,m off to watch the pot!

That's hilarious! But seems to be all-too-common with these challenges sometimes.

Thanks. The part about the three wise men is an old tale, from India I believe, but I added the part about the fourth guy, because it seemed to fit CR4 to a "tee".

You do realize that an elephant is a mouse built to military specifications while a mouse is an elephant built on a government budget which leaves us with a camel being a horse designed by a committee.

That reminds me why the standard gauge railway tracks are 4 feet 8½ inches apart. It turns out that when Stevenson built the first railway line he laid the tracks along a disused piece of road and laid the tracks in the groves which were 4 feet 8½ inches apart. The reason for the groves being that far apart was that they made by carts with wheels that far apart and they made the cart that wide because it matched the ruts made by a roman chariot and the chariot wheels were that far apart because that was the width of a horses arse. So we have railway tracks that are designed to fit a horses arse.

Boy can we ever get off track on this site. It dose however make it interesting.

Yes, I had heard this before with a couple of exceptions. Firstly, why would the first railroad tracks be laid in grooves of a road? That makes no sense. There actually were horsedrawn railways that predated the locomotive. They used carts that were orginally used on the old (rutted) Roman Roads in Britain. The cart wheels had to fit in and ride in the ruts, otherwise a very bump, axle-breaking ride would be the result. The ruts were indeed originallyl made by Roman chariots, which were the width of TWO horses rear ends side by side. (4ft. 8in. would be an extremely WIDE horse!)

So, the common railway gauge is that wide because of a couple of "horses arses"!

But what has a railway got to do with steam bubbles in a pot? I guess because after the water boils and you make spaghetti, you have to "Choo-choo"!

Once upon a time, steam ruled the rails. Now it is mostly electric power that moves the freight and passengers. (either diesal-electric, electric catenary or electric third rail)

Son, If your father was a chemisty major he would be able to tell you.... Now If you asked me something about something with a motor in it, Sure Id be able to tell you...For now son....Um.Well...Ergh......Go Ask your mother.

It's a matter of bubble size, pressure in the bubble, temperature, solubility and surface tension. Small bubbles have a very high pressure in it - look a good physics book. The reason for this is the curvature. Since the gas - no matter if air or steam - pushes against the wall of the bubble, some of the gas will be redesolved in the water (or some steam 'condensates' back to water). This leds to a even smaller bubble with higher pressure thus a 'self-elimination'... But why do bubbles appear at all? Some nucleus in the water - an alien molecule may be sufficient (think of sodium, calcium, chloride, carbonate ion or simply dust) to act as starting point. Due to the temperature water evaporates at this nuclei (for thermodynamic reasons - also take a look at a good book) thus a bubble forms. The higher the temperature, the more steam with time is formed - and the bubble gets the chance to grow that large that it might withstand the collapse explained before....

BTW: if you use very pure water - deionized and degased - and put it in a well polished pot and heat it: better use a cap! You will experience boiling retardation - no nuclei are present to form the small bubbles. So when the water starts to boil, you will immediately have 'large' ones. The following is a matter of imagination what might happen... especially to unprotected skin parts ;)

What a sadly trivialised and incorrect answer from the experts at GlobalSpec. In brief, the cause is dissolved air, partial pressure of steam, surface tension and boyancy.

In detail, when the bubbles first form, they are principally dissolved gases and CO2 (you can check this by seeing that these small bubbes don't form if you repeat the experiment with preboiled cold water). They remain small until near the boiling point, because only a limited amount of air is dissolved. As the temperature rises towards boiling, the vapour pressure of the water rises. The bubbles will expand so that the partial pressure of the water vapour and the air are equal to the bubble pressure* (initially, the bubble pressure is relatively high because of surface tension and curvature, but falls as the bubble expands). They will detatch from the bottom when the bubble grows large enough for the boyancy to overcome the surface tension that was holding the bubble to the base of the pan.
*The bubble expands until the partial pressure of the air falls so it just makes up the difference between the required total presure and the vapour pressure of the steam.

OK..lots of very close answers...(and the one posted by the site was not correct either). Here is the correct one.

A vapor bubble in a boiling situation has two factors contributing to its increase/decrease in size. The first is the surface tension pushing in on the bubble, and the second is the force being created by the vaporization of water from its surface. The higher the temperature of the surrounding water, the greater this second force. The smaller the bubble, the larger the force of surface tension. For a given temperature, the bubble will have a critical size. If it is bigger than this size, it will grow. If it is smaller, it will shrink. If you put pure water in a bowl of spinning mercury, and boil it, the water will not boil until it is over 150 C (not sure of the exact temperature) because that is when the vapor formed by spontaneous molecular irregularities will be large enough to grow. Because the water is so hot in this case, the bubble will expand very quickly and the boiling will be explosive. In real-world boiling, trapped gases and impurities form a larger nucleus than those created by molecular irregularities, and the boiling temperature is much lower.

Back to our question:

The bubbles initially formed are created when the hottest water at the surface vaporizes around trapped gas in the surface crevices of the pot. They don't grow very big because the water immediately above the surface is not at boiling temperature. They dont' rise because the surface tension between the pot and bubble is greater than the boyancy of the bubble. Just before boiling, the boyancy becomes large enough for this to no longer be true, and the bubbles rise. However, the water immediately above is not hot enough to sustain the bubble so it shrinks away, and appears to disappear.

"Trapped gas in the surface crevices of the pot" was not my expected mechanism, so I checked it out. I tried a preboiled pot with a still damp surface, and added fresh cold water very gently. The bubbles still appeared. Then I took a fresh dry pot and added cold, recently boiled water. No bubbles appeared until the pot boiled. So it's the air in the water that is important. (I'm not saying that you couldn't create a pot that had such a poor surface that trapped gas made a contribution, just that it is not necessary and not usually significant).

Then there is the statement that the bubbles disappear before they reach the surface. I didn't expect this, so I checked this too. It just doesn't happen - the vast majority of the early bubbles visibly rise all the way to the surface without shrinking significantly (watch it and see - I even tried with the tallest, narrowest pot I could find). In fact, in most cases the water was hot enough that the bubbles sat on the top without shrinking significantly until they burst. It seems that you would have to have a convection-free pot for the temperature gradients to be sufficient for the bubble to shrink back to its original "gas-defined" size.

So contribution #23 seems to have been complete and correct (albeit not that easy to follow). But I doubt that we will ever see a correction from the editors (please prove me wrong here, gentlemen)

I forgot to say - the 'official' answer is a valid answer - but to a different question. The question would be "Why, immediately shortly before the pot boils, do bubbles that form at the bottom of the pot shrink as they rise and fail to reach the surface". [Although these individual bubbles vanish, the number and size of these bubbles increase as the pot gets hotter]

well, if we have the hot plate at the bottom there will be a gradient from bottom to top full of convecting fluid with the top cooler. Once you get local boiling at nucleating sites on the bottom these fill small volumes with water vapor and none or a mere trace of air. These small volumes we see as bubbles break away as soon as their lift exceeds their adhesion to the bottom and start to rise. They then get no more heat and seek equilibrium within the column by shrinking and heating the column and you get enhanced convection or stirred convection. At first they shrink to extinction and others follow them. With enough heat from the hot plate this proceeds until the entire column is at equibrium with water at 100C and all bubbles reach the surface. This continues until the pot is empty.