How do I keep my soda from going flat?

When I buy a 2 liter bottle of soda, i squeez all of the excess air out of the bottle when i close it up. I think this prevents the soda from going flat much longer than if you just put the cap back on. My roommate says this actually makes the soda go flat faster. Who is correct?

Have you considered getting two 2 liters of soda and performing an experiment? Drink some from each one, and squeeze the air out of one. Come back a few days later and do a double-blind taste test, or pour 1 shot of each into a glass and measure the fiz.

My WAG is that it depends more on how tight you screw on the cap.


My WAG is going to be, and always has been, that squeezing the air out makes it go flat sooner or, at least has no carbonation-preserving effect.

My reasoning behind the WAG:

When you open a bottle of soda, the pressure is released and you see a lot of bubbles escaping. My understanding has always been that the pressure in the container keeps the carbon dioxide bubbles in suspension or saturated, or whatever the proper word is for bubbles of gas in a liquid. I have always thought that a good product would be a re-pressurizing cap for bottles. Maybe along the lines of a tiny bellows with a one way valve and a stopper. The idea being that if you apply a higher atmospheric pressure to the inside of the bottle, the bubbles wouldn’t be released. then again, maybe I’m nuts.

It doesn’t make a difference. From the AFU FAQ on Science:

My 2 liter bottles never last long enough to get flat.

That is probably why they sell them at grocery stores.

They sell them at grocery stores because I thought it would be a good idea? Cool! I’m famous!


But seriously, I’ve never seen them. Is the design similar to what I’ve mentioned?

We used to have a little gizmo that did that. Worked like a charm. It was a little pump that increased the pressure inside the bottle. My guess is that the increased pressure is a less suitable for the carbonization in the soda to go, so it would rather stay put.

Here a way to get max carbination. when you want a drink, break the seal and crack it up side down with the mouth inside a glass. this will get your fingers a little wet. Let the pressure fill the glass. THe close it while still slightly pressurized. Don’t squeese.

Also clean the cap and rim before you open it

The amount of gas dissolved in a liquid is proportional to the pressure exerted. This is why you get the bends, in deep waters the pressure is higher and more nitrogen is dissolved into the blood.
To preserve the carbonation in a drink, keep the pressure high. Keep it capped and under pressure.

I used to have one of those cap pumps, and it did work, but after a month or two, it was such a pain to keep doing it… pressurizing is pretty easy if you’ve only poured a few cupfuls of soda out, but that’s really not when you need it. It’s when it gets near the bottom, and the more empty volume in the bottle, the more pumping you have to do, and, well, it wasn’t fun. :frowning:

This has been discussed ad nauseum before, here and here and here and here.

I believe it has to do not with air pressure, but the partial pressure of carbon dioxide. Unless you increase the pressure of CO[sub]2[/sub] above the liquid, the dissolved CO[sub]2[/sub] will continue to come out of solution (until equilibrium is reached).

If you want to keep your soda fizzy, simply tighten the cap as tightly as possible when you are done with it. Works like a charm.

  • Also, just a note for the home-experimenter: for drinks, you need 8-gram* CO2 bottles, not the 12-gram ones. The 12-gram ones are for air guns, and have lubricating oil inside them. The 8-gram ones only contain clean CO2. - MC

I’m not much of a coke drinker myself but my step-dad buys two 1 litter bottles of coke at a time to preserve carbonation.

I’ve been doing that for years, to the chagrin of my parents, who more than once needed pliers to open a Coke bottle I had capped.