How bubble is formed?

February 21, 2009 10:58 AM

By what process do air bubbles form in water?

I left a glass of water sit on the mini-fridge overnight, and in the morning there were little bubbles clinging to the walls of the glass. More than just a few. Googling led to this page, where someone asks the exact same question. Apparently there are gases mixed in with the water, and the temperature affects their solubility (whatever that means.) Further googling led to this page, which, in section M5, seems to discuss the matter. But it's written at a level I don't understand. Why doesn't the gas just rise to the surface? What is this whole solubility thing? Temperature affects it? What happens between the time when there are no bubbles, and the time when there are bubbles?

posted by metastability to science & nature (13 comments total)

To put is simply, solubility is the amount of solvent that a solution can hold. It might be easier for you if you think about stirring sugar into water. You add a little, it stirs right in and all the sugar dissolves. If you keep adding more and more, the water eventually won't be able to hold any more, and the sugar will start precipitating out, to the bottom of the glass.

Water also dissolves other things, like gases. Some of the nitrogen and oxygen from our air is dissolved into the water as well. Here's the important part: The solubility of water isn't constant, and is affected by temperature and pressure.

Hot water will be able to dissolve more sugar than cold water. This fact is often demonstrated by making rock candy in grade school science lab. Heat water up to boiling, and stir as much sugar in as you can. Then, stick a popsicle stick in the solution and let the water slowly cool. Over a day or so, crystals will start to form.

What happened? Well, a few sugar molecules stick to rough spots on the stick. These can be called sites of nucleation. A few more molecules of sugar will start to stick to them, then a few more, and soon visible crystals will start to form. All this happens because the solubility of the water has decreased as it cools. So the water can't really hold all the sugar anymore, and the sugar desperately wants to get out of solution.

The same principle is at work with your glass and air bubbles. There are dissolved gases in the water, and imperfections in the glass serve as nucleation sites. That's why bubble stick to the edge of the glass. Over time, if the bubbles keep growing, the force compelling them to rise will overcome the forces sticking them to the side of the glass, and the bubbles rise to the top and disperse. As you've observed, you can tap the glass to help the bubbles break free from the nucleation site, and a bunch of them will rise at once.
posted by chrisamiller at 11:22 AM on February 21 [2 favorites has favorites]

From googling "solubility bubbles"
posted by rhizome at 11:26 AM on February 21

Ok, that's a really good start. More details please.
posted by metastability at 11:28 AM on February 21

Err - to fix my first sentence:

Solubility is the amount of solute that a solvent can hold in solution.
posted by chrisamiller at 11:29 AM on February 21

Well, if you want more details, you might want to pick up a chemistry book and read up on polarity and solubility. The very-low level details are somewhat complex and involve entropy changes and intermolecular forces. A very basic intro:

Water is a polar substance, which means that one side of the molecule is negatively charged (near the O), and the other is positively charged. To put it simply, this means that molecules of water are good at sticking to other polar molecules. The minuses are attracted to the pluses, and vice versa. This means that water is an excellent solvent for many substances.

Things like temperature and pressure change the way in which these molecules interact at a very fundamental level. As you heat up the liquid, there is more kinetic energy in the system, which means the molecules move faster, which means it's easier to break the bonds holding gases in solution, and more of them will rise out. (Remember that soda goes flat as it gets warmer)

Vapor pressure also affects solubility. Again, soda is a good example. The can is pressurized with CO₂, so when you pop the top, you change the pressure, and the liquid can hold less carbon dioxide. This is why you get a brief burst of rapid fizzing when you open a can of soda.
posted by chrisamiller at 11:46 AM on February 21

...the molecules move faster, which means it's easier to break the bonds holding gases in solution, and more of them will rise out.

Wouldn't that imply a decrease in solubility as temperature increases?
posted by metastability at 11:58 AM on February 21

Yep. Told you this stuff gets complicated. :)

The relationship between solubility and temperature is not the same for every solvent-solute pair. If we assume water as a solvent, the relationship between solubility and temperature is reversed between sugar and many gases. This page provides some more details and talks a little about the thermodymanics involved. A few good google searches will get you a lot more info.
posted by chrisamiller at 12:09 PM on February 21

It wasn't until shockingly recently that I figured out that water is really a gas that for some strange reason likes to hang out as a liquid at room temperature. We think of water as heavier than air of course, but its constituent molecules -- H₂O -- are actually lighter than N₂, O₂, and CO₂, our atmosphere.
posted by troy at 12:16 PM on February 21

Alright, to summarize: increase temperature => gas/water bonds break apart => gas bonds to side of glass instead of water molecules => gas sticks to itself like growth crystals = bubbles.

Thanks a lot guys.
posted by metastability at 12:34 PM on February 21

What happened? Well, a few sugar molecules stick to rough spots on the stick. These can be called sites of nucleation. A few more molecules of sugar will start to stick to them, then a few more, and soon visible crystals will start to form. All this happens because the solubility of the water has decreased as it cools. So the water can't really hold all the sugar anymore, and the sugar desperately wants to get out of solution.

There is an interesting experiment that can be done where you make sure everything is extremely clean. No dust, no rough spots on the glass, nothing extra mixed into the water.

After it cools, no solids precipitate out. Instead, what you get is called a "supersaturated solution". If you walk up to it and gently shake it, the glass fills with crystals. It's amazing to watch. Dropping in a single crystal will do it too.

(You can do other cool tricks with super-saturated solutions.)

Gas dissolved in water can do the same thing. That's why putting a pinch of salt into a class of club soda is a recipe for a mess.
posted by Chocolate Pickle at 12:39 PM on February 21

Wait. According to the graph in that second link, the solubility starts to rise as the temperature gets above 100, or so. Why?
posted by metastability at 2:20 PM on February 21

100 C is the boiling point of water. At that point everything changes again.

(Note that to raise liquid water above 100C without permitting it to convert to gas, you have to keep it under pressure. The pressure affects the solubility of other gasses.)
posted by Chocolate Pickle at 3:31 PM on February 21

Ah yes, of course. New reactions start to happen above 100 degrees, so it's believeable that more gas molecules could bond with the water.

I think what was confusing me more the most was the mention of "pockets" in that second article I linked. Now I see. The "pockets" are not the air bubbles. They are configurations of water molecules into which air molecules can be placed, or trapped. At low temperatures, these complex configurations of water molecules are able to form, and trap more air; hence, the solubility increases at low temperatures.

Again, to summarize: there are several different ways gas molecules can get absorbed in water. Polar bonds, as chrisamiller first said. Chemical reactions, which presumably happen above 100 degrees. And lastly, the pocket configurations at low temperatures. Great. Yay, internet!
posted by metastability at 7:51 AM on February 22