Science Questions

How do you keep the fizz in a bottle of fizzy drink?

Sun, 27th Jun 2010

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Steve Slack asked:

What's the best way to stop a half empty bottle of fizzy drink going flat? Should I squeeze the air out?


Chris -  Okay.  What do we think here in the studio?  Helen?

Helen -   My guess is that you should not squeeze the air out because at least that by having the air, the carbon dioxide thatís already been released when you first close it that somehow must help stop more carbon dioxide coming out too.  Otherwise, you've got a sort of vacuum to fill and that the carbon dioxide will quickly come out of your drink if thereís nothing for it to push against.

Andrew -   Yes, I absolutely agree with Helen actually...

Chris -   You sound surprised!

Helen - I am only a biologist!

Andrew -   Yes, exactly.  We can't have biologists answering physics questions!  I absolutely agree.  Basically, thereís loads of carbon dioxide dissolved in these drinks and the fizzing happens when the carbon dioxide is coming out of solution and turning back into a gas, and the rate at which that happens is determined by the pressure against which itís pushing.  So, you want as high pressure as possible in the vicinity of the liquid to stop the gas coming out of solution.


Chris -   Because you can buy those gadgets that will pump up the bottle again and put some pressure above the liquid.  Some people have said this won't work because itís putting in air, not carbon dioxide.  But the point is that air is 80% nitrogen.  Nitrogen is really poorly soluble in water.  So for the carbon dioxide to come out and go into the air above the bottle, itís got to increase the pressure because hardly any nitrogen is going to dissolve.  So if you pump up the pressure even higher above the liquid, itíll make it even harder for the CO2 to come out, so I reckon thatís the reasonable strategy.

Andrew -   Yes, thatís right.  You just want the pressure to be as much as possible to stop it coming out of the solution.


Chris -   The biggest determinant of keeping the drink fizzy actually is putting it in the fridge, because as the temperature of the liquid rises, itís ability to dissolve a gas like CO2 or oxygen for that matter drops.  And therefore, the colder the liquid is the easier itís going to hang on to its CO2, so whacking it back in the fridge is absolutely what youíve got to do.


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Slack, Steve asked the Naked Scientists: I have a question: To retain the fizz in a half empty 2 litre coke bottle, is it a good idea to squeeze the bottle until there is very little air left, or does it make no difference? Thanks Steve Slack What do you think? Slack, Steve , Wed, 23rd Jun 2010

I suppose squeezing it might help, but to what extent I don't know. Squeezing it would of course mean that you have less gaseous volume, so your pressure would end up being higher which would push the equilibrium to have more CO2 dissolved in the coke. I think it's an equilibrium problem.

There is probably information about environments which favor dissolved CO2. If I can recall the best you can do is keep it cold, which favors dissolved CO2 (as H2CO3) over gaseous CO2. Bill.D.Katt., Fri, 25th Jun 2010

To keep the fizz the best is to open the bottle as little as possible, as well as not squeezing it. If you do that you reduce the pressure when you close the bottle again, and then more gas can come out of solution until it reaches equilibrium again with the bottle pressurised out to be rigid again. SeanB, Fri, 25th Jun 2010

That sounds accurate. But when you pour, the CO2 will come out and the gas that goes back into the bottle before the lid is placed back on will be normal air, with much lower CO2 pressure. So a high amount of CO2 will have to be released again to reach equilibrium, whether or not the bottle was squeezed before the lid placed back on. I think the best way is just to keep the lid on and keep it cold. Bill.D.Katt., Fri, 25th Jun 2010

If you expel the air above the liquid, by squeezing the bottle before re-capping, this will inevitably lead to a greater loss of fizz than if you leave the bottle fully inflated. This is because, with the bottle collapsed above the liquid, it's easier (energetically speaking) for CO2 to come out of solution and to collect in the potential airspace above the liquid.

Conversely, if the bottle remains unsquashed, any gas exiting the liquid will increase the pressure above the liquid, reducing the potential for further CO2 efflux.

The ideal solution would be to pressurise the bottle, with CO2, before re-capping and keeping the liquid cold, which increases the amount of gas that can dissolve.

Chris chris, Fri, 25th Jun 2010

Steve-science-Spangler says these "fizz-keeper" pumps don't help ...

some disagree. RD, Sat, 26th Jun 2010

A neighbor of mine always swore by dropping a stainless steel spoon into the soda and capping never tried it so I don't know what it does if anything at Karen W., Sat, 26th Jun 2010

Karen - we've discussed this very question about spoons keeping champagne fizzy on the show - it doesn't work; forum-user JnA actually reported on an experiment they'd done to prove it! (Details are in the thread).


I, for one, disagree; the increased pressure above the liquid, which will be largely air, which is mainly nitrogen and hence very insoluble. Increasing the pressure above the liquid makes it much more difficult for molecules - of any species - to leave the liquid and that includes CO2; furthermore, increasing the pressure on the liquid makes it even harder for a bubble of CO2 to form in the first place, keeping the gas insolution.

I would have thought that a parallel is the variation in the boiling point of water with altitude. Water boils at the top of Everest at a much lower temperature (69 degrees C) than it does at sea-level because the atmospheric pressure is lower, making it easier for water molecules to exit.

Chris chris, Sat, 26th Jun 2010

Thanks for the link Chris..Thats good to know. I had missed that...Nice experiments reported. Karen W., Sat, 26th Jun 2010

I was thinking more in terms of CO2 partial pressure. I thought the partial pressure of CO2 would be the only gaseous pressure that would matter in an equilibrium like this. So I agree that pumping it full with CO2 would be best, and might even increase the fizz. Bill.D.Katt., Sun, 27th Jun 2010

But the critical point is that nitrogen does not dissolve very well in water and so for CO2 to leave, to my mind, this would increase the net pressure above the liquid, increasing the odds of the CO2 re-dissolving...

Maybe one of the physics / gas gurus here can confirm this would be the case...?

Chris chris, Sun, 27th Jun 2010

If you can stop the bottle re-expanding (by tying, taping, or clamping it) after you've squeezed the air out, this should keep it fizzier than the other options of leaving a larger volume of air in a non-squashed bottle, or allowing the bottle to re-expand (creating a partial vacuum and bringing more gas out of solution).

Alternatively, you could top up the bottle with marbles to achieve the same result, or decant the liquid into smaller bottles (but you might lose some fizz in the process).

If the initial air pressure is the same, having a smaller volume of air in the bottle is better, as you need less CO2 to come out of solution to reach equilibrium. Boyle's Law? ebichu63, Sat, 3rd Jul 2010

Obviously no one saw the TV professor i think his name was Chris, he used to ride around on a bike. He took a fizzy drink to the top of Ben Nevis, i cant remember why. But he proved that pumping air into a bottle was useless because the gasses are different therefore the CO2 still escapes. filling with CO2 would of course work. I squeeze the bottle as i pour leaving no air and as long as you keep upright and refrigerated the bottle stays squashed and retains its fizz. Another way is to chill as cold as poss this always prevents the escape of CO2, pour gently from big bottles into small 250 ml ones and they will retain fizz without refrigeration and can be chilled as required. I have no major qualifications but i do have common sense in plenty and can vouch for all i say.. Forget all the other prats with their so called observations and try it for yourself. It will work i guarantee it.The small bottle one is the best and longest lasting chilled or not. But always chill to fill. Also big bottles will not stand upright in a lot of fridges, lying down will not work. and shaking about will not work. Once the lid is on a small bottle you can treat it as you like. Terry Colloff, Wed, 5th Feb 2014

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