OK, I know it’s trivial but it seems to me, from my reading of this board that there’s nothing that can’t be answered. So…
Is there any evidence to suggest that opened bottles of soft drink are less likely to go flat if store upright or laying flat?
Off the top of my head, I’d think you would lose carbonation faster if a bottle is stored on its side - more surface area for gas to escape from (a reason why Champagne flutes are designed so thin).
This is something that you could perform at home, of course. Take two bottles of soda, open both, and pour one into a casserole dish (or similar wide container). Leave overnight, check in the morning.
The surface area difference is probably the major effect here. However, the solubility of carbon dioxide increases with increasing pressure, and the liquid in a pop bottle on it’s side isn’t as deep as the liquid in a vertically stored bottle. That decreases the pressure at the bottom, so the CO[sub]2[/sub] in a shallow container will fizz out more quickly.
I’d imagine that, unless you plan on opening the bottle back up in a matter of minutes, it doesn’t really matter. Once enough CO[sub]2[/sub] comes out of solution, its partial pressure in the bottle will be such that no more will escape the soda. This partial pressure will be the same whether the bottle is standing up or lying on its side.
So, after a reasonable length of time, two identical opened bottles, one lying on its side and one standing up, will be equally as flat.
My experience is that on days when I’ve had lunch at home, and had pop with it, the bottle is clearly softer that night, and that 2 liter bottles of coke still haven’t reached their maximum hardness after a day, so CO2 is presumably still coming out of solution after even a day.
Thanks all. I often find myself with a 2 litre bottle of Coke opened one day and quite flat the next. For space-saving reasons, I tend to lie the bottle flat in the 'fridge and wondered if I could keep the drink fizzier for longer by standing the bottle upright.
ZenBeam, I can’t say I’ve noticed any change in the hardness of the bottle itself. Now I’ve got something else to take note of. 
jmizzou: Fascinating flute explanation.
I’v eheard that if you squeeze all the air out of the bottle before recapping it, it will last longer
I would think the exact opposite would happen actually.
If you squeeze the bottle then cap it, the liquid in the bottle at the top will be exerting pressure on the walls, trying to make it go back to its original shape so it can sink down and find it’s lowest level. This will create a volume of reduced pressure at the top, which will “suck out” the dissolved CO2. You’ll end up with a bottl that looks unsqueezed, but the “air” above the liquid will be all CO2.
I would guees you’d do better to pump air into the bottle somehow. With high pressure above the liquid, less CO2 will be able to fizz out of solution.
/Chemistry student
//But it was a long time ago damnit.
I’ve heard this too, but it’s wrong; you’re creating a lower-pressure closed environment and the lower pressure will allow more gas to come out of solution. Ideally, repressurising the bottle with some kind of pump would ensure the best preservation of fizz, but just pumping in air, rather than Carbon dioxide might cause other problems such as making the drink taste stale.
True, it would be best to pump in CO2 or an inert gas (eg. Nitrogen?)
It would be best to pump in CO[sub]2[/sub]. That would ensure that no CO[sub]2[/sub] came out of solution, and maybe some would go back in. That’s how a soda siphon works. A PET bottle couldn’t withstand the same kind of pressure as a soda siphon, though.
Nitrogen or air would be next best. They’d stop the soda going flat, but I don’t know how they’d affect the taste.