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

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 that's 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 there's 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, there's 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 it's 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 it's 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, it's 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, it'll make it even harder for the CO2 to come out, so I reckon that's the reasonable strategy.

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

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