Sample 8
Sept. 28, 7:45AM EDT
No surprises. One cold flat sample and one warm flat sample. I can see bubbles in the Pepsi, slightly more bubbles in the cold sample, but no significant feel in my mouth from either one.
OK. Two two-liter bottles of Diet Sunkist Orange Soda. Both opened and two full glasses removed from each. Both put back into the fridge next to each other in the middle of a shelf so that neither was closer to a wall. One bottle just capped securely. The other bottle squeezed to remove as much air (and no soda) as possible then capped as tightly as possible.
Four days later: The squeezed bottle appeared to be just about as squeezed as before. It had notably more carbonation than the other. This was noticeable both in pouring and in taste.
Now excuse me. I have lots more soda to drink.
I think the growing air space in a bottle as it’s used up is a big factor here.
That’s exactly what I thought all along and why I tried to remove the air space by squeezing the bottle and capping it. I ran the experiment, but it’s a practice I’ve followed for a few years.
Were the bottles kept upright? Did you squeeze the bottle enough that the surface area of the liquid was decreased (i.e., it was pushed up towards the cap where the cross-section narrowed)? Was the unsqueezed bottle notably pressurized over ambient?
Yes both bottles were upright. The squeezed bottle leaned a little, And yes the surface of the squeezed bottle was up in the neck and smaller. There was a bit of a hiss as the unsqueezed bottle was opened. A louder hiss from the squeezed bottle.
I feel like surface area probably plays a significant part here. I’d be interested in an experiment where you lay the squeezed bottle flat.
I don’t understand how you could ever get a hiss from the squeezed bottle, though. It has no pressure difference from the outside. If anything, it should be slightly negative as the plastic tries to pop back into shape.
The hiss occurs as you start to unseal the bottle cap. I might have been holding the squeezed part so it couldn’t expand, or it might have taken a moment for the plastic to relax, I don’t recall
If people have reliably observed soda going flatter in the fridge, I wonder if that’s anything related to the plastic seal being deformed by lower ambient temperature. Maybe it contracts and/or becomes more brittle, resulting in an imperfect seal.
I get that, but the hiss comes from gases which are escaping due to the pressure difference. No pressure difference, no hiss. And it’s a very significant difference with a new bottle, on the order of 3 atmospheres.
Of course it does. The total pressure in the bottle will be the pressure of the air sealed in the bottle (atmospheric) PLUS the pressure from the CO2 that comes out of solution once the bottle is resealed. That will be some amount greater than the atmospheric pressure outside the bottle. The smaller amount of airspace in the bottle, the greater the CO2 pressure in the bottle will be, since the AMOUNT of CO2 coming out of solution will be the same.
Except that the bottle has been squeezed, and so will just expand slightly when the CO2 comes out of solution. It can only start holding pressure when it’s back to its original shape. Or there’s a mechanical clamp on it, etc.
A squeezed bottle does not return to its original shape when you open it unless you’ve only squeezed it a bit. Actually before I put a squeezed bottle into recycling, I often blow into it hard to reshape it. I have no idea why I do that actually, jut seems like the bottle’s shape should be right I guess.
It will if there’s even the tiniest bit of internal pressure (as you found by blowing into it). If you squeeze it kinda flat, the sides have an area of maybe 50 square inches. Just 1% of an atmosphere is 0.14 psi, so there’s 7 pounds of internal force trying to pop it back into shape. That’s more than enough for the typical thin plastic. Anything that’s a more reasonable fraction of an atmosphere will not just pop it into shape, but make the bottle noticeably rigid.
As a basic rule of thumb, if there is any detectable floppiness to a flexible container, then it doesn’t have significant pressure. If it’s pressurized, it will feel rigid. Hence why spacesuits are so difficult to move in despite only containing ~1/4 of an atmosphere.
why do you say this? It’s not true. 2 liter bottles can and do absolutely start getting pressurized the moment you put the cap back on (assuming there is CO2 still in solution and the soda isn’t completely flat), regardless of the shape it is in. There is a little headspace above the liquid in the squeezed bottle above the soda in the neck of the bottle. That headspace will absolutely get pressurized as CO2 comes out of solution.
Are you saying this because you think the bottle will go back to its “uncrushed” state as soon as the internal pressure rises above atmospheric? It’s true that the bottle will return to its fully inflated state if the pressure is significantly greater than atmospheric (just try forcefully blowing into an empty crushed 2 liter bottle to demonstrate this), but the rigidity of the PET plastic will allow the vapor pressure inside the bottle to rise somewhat above atmospheric while still retaining its crushed shape. It’s not like a balloon. In my experience, the vapor pressure of the CO2 that comes out of solution in soda is not generally enough to return the bottle to its uncrushed state, so the vapor pressure in the bottle will necessarily be above atmospheric.
The rounded top and bottom of a 2L bottle are thicker material and have compound curvature; if you cave those parts in, it will take significant pressure to pop them back out.
If you just squeeze the cylindrical sides in, those have a lot of area, exhibit one-dimensional curvature, and they tend to be thinner material; it doesn’t take nearly as much pressure to pop those back out to a cylindrical shape
Not completely, but it will expand progressively until it does. There’s no way the pressure is significant. There might be some slight residual deformity if you creased it, but it won’t be much. The top and bottom parts (as Machine_Elf pointed out) will probably pop out last, but they still won’t survive even half an atmosphere over ambient (much less than fresh soda).
The reason it doesn’t fully return to its uncrushed state is probably just because there isn’t enough gas left. There’s only about 3 L of CO2 per liter soda in the first place. If you drank half the bottle, then some of the 3 L will be lost into the atmosphere while it’s open, and the rest has to expand into the (1 L) remaining headspace, and stay partially dissolved into the remaining soda (due to the 1 atm partial pressure). There’s not much left even under optimal conditions.
You are right that squeezing the sides will not take significant pressure to pop back out. For some reason, I was more envisioning crushing the bottle down from the top, creating significant corrugations in the sides and increasing the rigidity. This would noticeably increase the vapor pressure required to “pop” it back out to original shape.
That might require slightly more pressure since the accordioning action will require creasing and work hardening the plastic, but it won’t be that different, at least not until you get to the last several percent of volume difference.
All I can say is I’ve done it. If I squeeze the bottle and cap it, it will hold that shape at least for a while (days) when a carbonated beverage is inside it. I believe when I blow into it, I’m putting a fair bit of air into it, likely quite a bit more than comes out in the form of CO2 while it’s just sitting in the fridge.