carbonation in soda

Ok, so my wife has been doing this for years and I’ve seen it firsthand that this method works. When a (say 2 liter) bottle of soda gets roughly half empty, she squeezes all the air out of it to keep it carbonated. Of course at first I scoffed and thought this would remove the fizz faster but she said it would make the soda retain it’s carbonation. She was right. Multiple times and I just don’t get it. WHY does creating a vvaaccuumm keep the soda carbonated? Here at work, we think it has to do with the actual air pulling the carbonation out so if there’s little air (oxygen) then the carbonation won’t be released. Any thoughts or proof one way or another?

I have thoughts only, no proof. I did that for a while (with tonic water), thinking that less air in the bottle would be the concentration of CO2 would be higher and therefore it would stay carbonated longer. Then I got to thinking…when I open the bottle, you can hear the pressure release meaning some of the carbonation has left the liquid and pressuized the air in the bottle. I assume once an equilibrium has been reached, the carbonation stops coming out of the water. If that’s the case, squeezing the bottle would give all that carbonation alot longer to dissapate out of the water before the equilibrium is hit. Of course I don’t know what I 'm talking about, but it seems to me that squeezing the bottle would make it lose it fizziness sooner.

I’ll chime in to say that there is a scientific answer to this, and I used to know it.

I remember my chemistry teacher in high school counseled us all to squeeze the air out of the bottle if we wanted the beverage to stay carbonated. It had something to do with Raoult’s Law, Henry’s Law, and the Ideal Gas Law. He then gave us a thorough scientific explanation which I have utterly and completely forgotten. I do remember though, being perplexed by the counterintuitive nature of the solubility of gasses in liquid.

Since I have forgotten it, I can not vouch for its accuracy. In fact, the more I read about it and try to recreate his (my chem teacher’s) logic, the more I think he may have been wrong. Ah well. Here is a helpful link until someone else comes along with an actual explanation:

Former chemistry teacher here. Here’s my hypothesis to explain what’s going on.

First of all, to a good approximation the other gases in the air space above the liquid in the closed bottle are irrelevant. Assuming the gases are ideal, gases don’t interfere with each other.

What’s happening is that a characteristic partial pressure of gaseous CO[sub]2[/sub] builds up in the closed air space in the bottle. This pressure is dependent on the temperature of the liquid, and by Henry’s Law, on the concentration of dissolved CO[sub]2[/sub] in solution. If the air space is small, only a relatively small amount of dissolved CO[sub]2[/sub] comes out of solution to build up the equilibrium partial pressure of gaseous CO[sub]2[/sub]. If the air space is large, more CO[sub]2[/sub] comes out of solution to build up that characteristic partial pressure. If the bottle is left open, the air space is really large, and most of the CO[sub]2[/sub] comes out of solution until equilibrium is reached with the very small amount of CO[sub]2[/sub] present in the air around us, resulting in a flat drink.

Joey, you started out on the correct track, but came to the incorrect conclusion, I believe. Rick, it’s not that you are “creating a vacuum;” as I stated above, to a good approximation, the other gases present are irrelevant. The key is that when you squeeze the bottle, you are decreasing the volume of the air space over the liquid.

I have not had the same experience as you.

I squeeze the air out of my soda bottles so that my soda is less carbonated. Mine are much less carbonated because of it.

Wow. I sure do think it wouldn’t work as the OP says. But I have no experimental evidence on the question. If things are conducive I’ll experiment. Meanwhile, I’d probably take a $10 bet or two on this one…

Wouldn’t this only be true if you had a rigid vessel? Won’t a plastic pop bottle, being flexible, just expand with increase internal gas pressure? Only if the pop bottle doesn’t reexpand would decreasing the space over it be a benefit.

I think what confuses here is the difference between the gas pressure, which would push the bottle out, and partial pressure which affects the rate of CO[sub]2[/sub] diffusion from gas to liquid. Partial pressure is related to concentration as well as the overall gas pressure so a relatively high concentration will give a relatively high partial pressure without necessarily requiring a high overall gas pressure. Therefore the partial pressure of CO[sub]2[/sub] may reach equilibrium levels without exerting much additional pressure on the sides of the bottle.

I used to do this, on the theory that less air space would mean there would be nowhere for the carbonation to escape into. Then I read a Mythbusters-style website that tested it and found that doing that with an ordinary plastic soda bottle will greatly increase the rate of decarbonation, unless you somehow put pressure on the outside of the plastic to keep it scrunched up. They mentioned that there are specialized lid-pumps that fit most 2-liter soda bottles and can be used to pump in air to increase the pressure inside the bottle, which does keep the drink carbonated longer.

No idea if either assertion is correct, but they had lots of data!

I’ve had good experiences with the little air pumps; IME they work great. Just the other day, however, in a moment of brain lapse, I scrunched my half-empty bottle of fizzy water to make less airspace. When I came back later, the bottle had popped back out and the water was not very fizzy any more.