First let me copy what has and had been said starting below with Dear Cecil (two words that I fear may spark an White House inquiry one day)
Dear Cecil:
Long ago I noticed that the bubbles in clear carbonated beverages seem to stream from
fixed spots on the bottom and sides of the glass containing them. Boiling water seems to
behave in a similar way. What’s so special about these spots? --David Peterson,
Washington, D.C.
Dear David:
They’ve got a fancy name, for starters, The places from which the bubbles stream are known as nucleation sites. They’re microscopic defects or bits of crud on the glass. When water is changing
phase (e.g., boiling, condensing, freezing), it needs a place where the vapor bubbles, droplets, crystals, or whatever can congregate until they’re big enough to survive. That’s what nucleation sites
provide. Snowflakes and raindrops, for instance, typically form around dust motes. When water reaches the boiling point, the scratches in the container provide havens where microscopic bubbles can collect long enough to become big bubbles.
The carbon dioxide bubbles in beer and soft drinks work the same way. Before you uncap the bottle, the pressure inside keeps all the CO2 in solution. After uncorking, the reduced pressure enables the gas to slowly boil away, which is where the nucleation sites come in handy. If you want to see some serious bubble action, try sprinkling salt in your beer. The salt provides an abundance of nucleation sites, producing not only a fascinating demonstration of physics but pots of fun besides.
Okay smarty pants…(geee who invented smarty pants)…tell us something in plain english we don’t already
know. Soooo…there called “nucleation sites” and they are places where water “needs a place where the vapor bubbles, droplets, crystals, or whatever can congregate until they’re big enough to survive” how did streaming bubbles/droplets/crystals suddenly become intelligent and living little beings where they need a night life so they can hang out with their buddies until they reach adulthood. These little beings we have overlooked for so long suddenly have a better social life than me and I want to get to the bottom of it. How come they have such well developed social instincts…so much so… that they tend to party when they all get together. In other words…I would like to see a better answer to Daved Petersons question. How the “tarnation” (Webster’s description of this word is hilarious) do these benign molecules know there is a party happening over at Defects pad or Cruds place and they all go racing over there. I find it amazing that as I watch a pot of water boil that bubbles come streaming out yet the amount of liquid stays at the same level. Does not the air in the bubbles represent some mass within the water?
BTW I realize it was from an early column and your skills had not yet been honed by hoards of whanabee Cliff Clavens.
> (geee who invented smarty pants) (Alan Poole)
Old Navy, probably…and we’ll start seeing the annoying commercials soon…
> how did streaming bubbles/droplets/crystals suddenly become intelligent and living little beings where they need a night life
(Alan again)
They didn’t. It’s convienent to anthropomorphize in discussions, to say that they ‘know’ something, but it is just a way of speaking. The bubbles do not ‘know’ that there’s a good place to start collecting, physics says that the gas will start collecting there because the rough edges are easy for small bubbles of gas to find a place to stick. When you get enough small bubbles together, they join together into one big bubble, and when big enough the buoyancy becomes too much for the adhesion to the nucleation site, and pulls it upward.
I tried to contain myself, but I escaped.
Thanks!!
Now that makes a lot more sense.
And…should have been the answer…I ahate to say…in the first place. A lot more simpler abd two the point.
How do you explain that there is no drop in the level of the liquid as the gas/vapor escapes. As you dip a jug into water. The air escapes and is replaced by a volume of water.
In pop and even as water boils there is an enormous amount of air coming out but the water level seems to stay the same.
Oooooooohh well…I must be simple minded to dwell on such small matters
It’s not simply a matter of finding a place for the smallest bubbles to stick together to form a larger bubble, but it actually has to do with the formation of bubbles at all.
In an ideal liquid, it is impossible for bubbles to ever form in a liquid. This is because the pressure exerted by a sphere of gas is inversely proportional to the radius of the sphere. You can observe this by hooking two unequal sized balloons to one another. The smaller balloon has the larger pressure and therefore inflates the larger one until only the large one has gas in it.
In the extreme case of the formation of a bubble from the infinitely small, it would require an infinite amount of pressure. (1/0 = infinity) This, needless to say, is somewhat difficult. Realistically, this is not exactly true because of the molecular nature of gases. The role of the “nucleation site” is to disturb the ideal nature of the liquid and give the newly forming bubble some imperfection from which it can form the bubble without having to start from the infinitely small.
Yes, the air in the bubbles constitutes some of the mass of the liquid, but most gases are about 1000 times less dense than the liquid phase. So the bubbles make up such a small amount of the liquid, the level doesn’t drop.
TheDude
Addendum to Dudes answer. His explanation is quite accurate, but he didn’t carry quite as far as I feel neccessary. He succinctly explained the need for nucleation sites, but as for the “gas knowing” to go to these locations is misleading. The gas dispersed in the liquid is not stationary, it is constantly moving about the fluid at very high speeds and bouncing off the glass, and other molecules. Because of this it is simply random chance that these air molecules will eventually meet at the nucleation sites. Now without getting into intermolecular forces and entropy, the gas is inclined to stay in place with the other molecules at the nucleation sites, therefore over time congragating to a level sufficient to float to top of the glass. So they aren’t sentient, just lucky, and aided by the abundance of them in the fluid.
And as for the loss of volume in the fluid, yes the level does go down. I am fully aware of the loss of fluid in boiling water, and it is noticable everytime I leave the kitchen while boiling water for some Mac&Cheese. Now comparing boiling water to carbonated water in not acceptable. The nucleation sites both occur, but the motivation of the gas leaving the fluid are quite different. Water boils by changing that state of the substance and the formed vapor needs a place to go. This vapor is thousands times less dense and the volume loss in gradual, but apparent. Now the carbonation in pop is much different. This gas is lost to the energy of the particles of gas exceeding the surface tension of the fluid. So in essence the volume of the fluid never changes, unlike boiling water. The fluid simply becomes more pure. The volume of the gas in the pop is low, again thousands of times less voluminous, so you need very precise measuring tools to notice it.
The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…
Omniscient; BAG
There is a drop, it just happens slower than you expect. Leave a pan of water boiling on the stove for a half hour (watch it carefully so it doesn’t run dry) and you’ll see a drop. Water expands ALOT as it turns to steam, so a little water makes a bunch of steam.
‘Seems’ is the key word here when talking about water and steam. With pop and CO2, the liquid isn’t changing to vapor, rather there is vapor dissolved in the liquid, which comes out of solution when the pressure is lowered. I’m not actually sure how this effects the volume of liquid, but it’s not much, that’s for sure.
Ugly
Thanks to all!
But now you have me wondering.
What happens if we remove the crud and the defects where (as you call them “nucleation sites”) these particles have a chance to congregate. Lets get really picky and create a “PERFECT” vessel to boil our water in. Lets make sure that no matter what it costs we manufacture a vessel that is absolutely perfect with no perceptible defects or “jagged edges” where these bubbles of gas get a chance to form. How would the bubbles have a chance to form?
Geeee…small things amuse my small mind.
But I am facinated by it!!!
Sorry you didn’t get it, Alan, but I think we’ve got a system here now. I’ll explain it to the Teeming Millions, and they’ll explain it to you.
We don’t even need a perfect vessel. We can use a cup of water heated in a microwave. Normally convection in heated water aids the process of boiling by circulating water past the nucleation sites. (On a stove, the water heats from the bottom up, and the differential in temp gives rise to convection.) In a microwave, however, the water heats up more or less uniformly, so it doesn’t always boil, and “superheating” can occur - the temp actually rises above 100 degrees C. Putting a spoonful of instant coffee into a cup of superheated water can cause it to come to an instant boil (lotsa nucleation sites), possibly splashing over the side of the cup and scalding you. I’m telling you this, Alan, because I worry about you and want to be sure you stay out of trouble
This reminds me of something I learned in college; not in class, but in the “Schlitz Beer School” (true!!!) that was conducted by the brewery of the same name for all the social chairmen of the fraternities. Schlitz wanted their product to be served correctly, so they had a slide show and Q&A sessions regarding beer trivia and serving do’s and don’ts.
Anyway, to get back to the thread topic, one of the things they demonstrated was that an unclean beer glass would exhibit a lot more streaming bubbles than one that was very clean. “Unclean glass” is a relative term; minute traces of soap from dishwashing would create a lot of streaming bubbles, whereas if you wet the inside of a glass, sprinkle salt on it and rub the inside of the glass with your finger and rinse it well, there will be a lot less streaming bubbles. They also warned against ever pouring milk into beer glasses; evidently, milk is very difficult to completely clean off of glass, and will produce hoards of bubbles.
Ahh, the joys of an advanced education…
TT
P.S. Anybody else remember the Schlitz Beer School?
While the early bird may get the worm,
it’s the second mouse that gets the cheese. --Dr. Ray Sheldrake
TT,
My understanding that the cleaning of glasses was especially important regarding head retention, which is a totally different effect. It is well known that detergents and milk and oils are pretty good at breaking up foams. So the reason you don’t want them on your glass is so that they do not mess with the head. In fact, one of the common ways to get the foam to go down (in case it is too obnoxious) is to rub the side of your nose and stick you finger in the beer. The oils from your face do an admirable job at breaking up foam.
TheDude
Wait, wait…
I’m trying to think of a time when beer foam would be more obnoxious than nose oil…
The overwhelming majority of people have more than the average (mean) number of legs. – E. Grebenik
Thank you Cecil and all.
I thought I would explain something why this facinates me. Coffee experts (buyers & tasters) use a system for tasting brewed coffee. They say that you need to submerse the grounds to get the full bouquet, flavor essence of coffee. Since I like the taste of coffee so much…that’s what I do…I boil a pot of water each morning and then dump the grounds in…I then filter it through a gold mesh filter ( I have used the same filter for 10 years still bright and shiny as the day I bought it). Some mornings in my pre-coffee stupor I would watch the water come to a boil wondering …what’s up with those streaming bubbles from the boiling points. Now I know…and my coffee Cecil…I put the grounds in before it comes to a boil so I won’t scalled myself…hehehe.
I it black BTW…Starbucks…YUCK