Why do bubbles form when water boils
When water’s temerature exceeds it’s boiling point (100 degrees celcius at normal atmospheric pressure) bubbles of gaseous water form and boil off. This is called steam.
If you mean the little bubbles on the side of the pot, those are caused by gasses that are dissolved in the water leaving the water. The gasses leave the water because as the water gets hot, the solubility of the gas dissolved in the water reduces. So it bubbles out.
Depending on exactly what you are asking, there are several answers to this question.
First, the bubbles that form are water vapor. It is the same stuff that comes out of the tap, but in gas form. Water turns into a gas (like all substances) when the average kinetic energy of the molecules overcomes internal cohesive forces and atmospheric pressure. Basically, what happens is that the molecules are moving so rapidly that they move past the point of no return, like a rocket that has reached escape velocity.
Rarely does this happen instantaneously throughout the liquid. Internal cohesive forces tend to be stronger for molecules completely surrounded by other molecules. So, the first molecules become gaseous at the edges, usually at some imperfection in the vessel. Once a few molecules become gaseous, others that are adjacent follow. The surface tension of water tends to hold smaller bubbles on the side of the vessel. When the bubble of water vapor gets large enough to overcome this force, it breaks free and rises to the surface.
I hope that answers all your questions, and welcome to the SDMB!
Welcome to the board.
Well, water is a liquid until it is heated to 100° C. Then it turns into a gas. Those bubbles are water turning into a vapor form, humidification. The molecules will float around until they condense and fall back to the earth when the temperature cools.
Big question: What happens on the molecular level to make the bubbles instantly form and and be set aloft on their gravity defying journey?
Well, there are several theories to answer your question. Actually the changes happens on such a microscopic level, way past atoms and quarks or whatever that nobody really knows. It is all theory. Water, H20 is made up of two parts hydrogen and one part oxygen. The bond holding the hydrogen atoms and oxygen atoms breaks apart, making two H’s and one part O. The H2 cannot exist by itself so it breaks up too. We cannot see that small yet by conventional means, and would not really ever be able to because the atoms are moving way too quickly. Just like nobody has ever seen an atom split. It was predicted by theory. The only thing we actually know is that water consistently boils at 100°C and freezes at 0°C. The bubbles are the individual hydrogen and oxygen atoms being set free to float around with their brothers and sisters in the sky.
Another good question is “Does it mean that water molecules are surrounded by more heat when there bigger bubbles in a pot of boiling water?” The answer is yes.
On second thought, I might be wrong about the water molecules breaking apart. I imagined pumping pure hyrogen into a tank with pure oxygen, and couldn’t proove to myself if they would combine and become water vapor, which I imagine they might not. They might layer because hydrogen is lighter than the oxygen. I apologize for attempting to answer a question that was out of my league.
Two words: nucleate boiling. Imperfections, dirt, or crevices in the sides and bottom of the container cause disturbances the create bubbles (nucleation) which grow until their size and buoyancy cause them to break free from the nucleation site and come to the surface. If the temperature rise is too great, and conditions in the pot are right, this form of boiling can change to another form called film boiling.
In a power plant, nucleate boiling is desired as it is a process that has a relatively stable heat flux through the wall of the container (the furnace water wall tubes). Film boiling leads to a very high container temperature, and uneven cooling, and thus can destroy very expensive water wall tubes.
wishbone, as he suspected himself, is incorrect about the molecules breaking apart during the boiling process.
Well, thanks for the retraction, but I thought I’d post this anyway:
Steam is the bubbles rolling hard out of boiling water. Steam is simply hot, unattached water molecules. The molecule does not break up.
You can prove this to yourself by holding a lit match over a boiling pot. Did you blow yourself up? No? Why? Pure hydrogen and pure oxygen are what are in the space shuttle’s belly tank. Put together with a flame, they make a terrific bang. Notice how no one has ever warned you about not igniting the escaping hydrogen from boiling water?
The H2 molecule survived just fine in all those Zeppelins, until one of the ships happened both rip and supply the activation energy for the hydrogen to combine with atmospheric oxygen and form… water! The Germans discontinued using hydrogen after that, as you might expect. Would you ride in one?
BTW, they of course knew that was a remote possibility. It’s just that the USA wouldn’t sell them helium.
Even I, a wannabe scientist, know that it’s polar, that its bond angle is 109.5 degrees, that it forms comparatively strong dipole-dipole bonds with its neighbors. This last fact is what makes it so hard to boil (compared to other liquids with its molecular mass). All of its atoms can be attracted to other atoms of molecules around it, making the molecules hard to tear apart from each other.
Well, they won’t until you supply them some sort of good heat source. They will be decently enough mixed just from currents, unless you seal the room off for a while. Even then, though, the layers have to touch. You’ll still get a good explosion I expect.
When I helped teach a chem class, we had the kids evolve H2 into a test tube that was upside down, filled with water (in a basin of water). They put the rubber hose in the tube, and the hydrogen gas displaced the water. They then stuck a glowing splint in it, and got a nice pop. Some kids got even 3 pops if they didn’t burn it all up with the first one.
The Master has already spoken on this.
Steam is condensed water vapour. Water vapour, gaseous water, is transparent.
An interesting side note on the OP: I believe it is possible to superheat liquid water above the boiling point by heating it in a container which does not provide nucleation sites for bubbles to form. This is easily demonstrated by using a microwave oven to heat water in a ceramic cup with a clean smooth glaze. If the ceramic is smooth enough, the vapor can’t form bubbles on the surface and the water will superheat slightly. Dropping in a tea bag, spoon, or other object that provides nucleation sites will cause a violent boil as the superheated liquid vaporizes. Quite dramatic or dangerous depending on your care.
I did this very thing the other day heating sake in its little round bottle in the microwave. I took the bottle out of the microwave and placed it on the counter just a little too hard. BOOM! Hot, sticky sake all over my face. Luckily, I wear glasses.