If I buy a bottle of Coke, shake it up, and open it I will end up with soda all over me and wherever I happen to be. However if after shaking the bottle I put in in the fridge and leave it for a week and then open it it will have the normal amount of carbonation. So does the CO2 gas go back into solution if you just leave it alone, and if so, why?
Actually, the CO[sub]2[/sub] gas comes out of solution and that’s what eliminates the soda spray. When you shake an unopened can or bottle of soda, the carbon gas that’s floating at the top of the container outside of the liquid gets mixed in with everything else. Opening the top relieves the pressure which means the bubbles of gas can increase in volume. The gas wants to rise out of the container but there’s all that soda on top of it, so it brings the soda too.
Letting the container sit unopened for a while gives the carbon gas a chance to work its way back out of the liquid to float in the top of the container and when you open it, it can escape joyously into the sky without having to bring lots of soda with it.
chemically when things are left alone then you will frequently get an equilibrium state. this equilibrium state, in this case, would have carbon dioxide coming out of solution at the same rate that it is going into solution, so there is no net change.
equilibrium exists because that’s what Mother Nature wants. things spontaneously to states of lowest energy.
now all of this is complex with thermodynamics, kinetics and how closed the system is. so how and why are complex but in general nature is relaxed.
Yup, things will reach the same equilibrium regardless of previous states, if you give them time to.
There is clearly a tremendous increase in pressure caused by shaking, which is the opposite of what you’d expect if it was causing gas to go into solution. In fact those large plastic Coke bottles can actually explode if sufficiently agitated. I believe what actually happens is that the agitation disturbs the equilibrium and causes CO2 to come out of solution and form bubbles in the liquid, which bubbles are then ready to energetically burst forth when pressure is relieved (it may also help dissociate some of the carbonic acid which forms a small part of the total CO2 present). If the container isn’t opened, the bubbles eventually redissolve into the liquid again. You can see this same effect where Coke that is poured vigorously bubbles much more than when poured gently.
No. If this were true even still (i.e., non-carbonated) drinks in sealed bottles would foam out if shaken before opening. When pressure is released on the shaken soda (or beer), bubbles of the dissolved gas that is normally inertly in the liquid suddenly form in the body of the liquid, forcing the foam thus formed, out. I think the effect of the shaking is probably mainly via the turbulence thus produced within the liquid, which causes pressure inhomogeneities that, in turn, induce some of the dissolved gas to come out of solution. However, it may be that bubbles of the gas originally on top that have now been mixed into the liquid by shaking may also act as nuclei for the further formation of more bubbles from the dissolved gas. I think you would find though, that even if you wait a few seconds to allow any bubbles of originally undissolved gas that have been mixed in to rise to the top again, there will still be foaming when you open the top and relieve the pressure.
No again. As johnpost and Chronos say, any gas that did get dissolved during the original shaking will dissolve back in (although there is not likely to have been very much of it actually released, whilst still under pressure). Perhaps more to the point, any turbulence will have died down while the bottle waited in the fridge. When it is opened calmly, without shaking, only a small proportion of the dissolved gas comes out of solution immediately because there is a dearth of the nuclei which bubbles need to form upon. Because of this lack of nuclei, the solution of gas will remain supersaturated at normal atmospheric pressure for some time, and only gradually go flat. It also helps that the bottle, in the OPs scenario, has been refrigerated. Gasses (unlike solids) are more soluble in colder water than in warmer.
I do not think the small amount of free gas in the container plays much role in the foaming process, except inasmuch as it provides some space for the fluid to be shaken into, producing a lot of turbulence. In a completely filled container, a brief shaking would only induce a relatively small amount of turbulence.
Yes.
Why ? Well the CO2 is always coming out and going it, the pressure stays steady because the rates are at equilibrium.
SO when there is less CO2 in the water, its ready to accept more CO2… so the rate of CO2 dissolving into the water is faster than the rate leaving… so the pressure reduces… until it again reaches equilibrium.
Why does CO2 dissolve in water ? well you can research electric fields - polarity for example - of the molecules…
I agree, and we’re saying the same thing, though you’re saying it more thoroughly. But that first statement needs a bit of clarification. It’s not that “bubbles … suddenly form” when pressure is released on the shaken soda, it’s that the shaking actually produces the bubbles by causing CO2 to come out of solution, and this is due to the creation of thousands of little centers of pressure inhomogeneities, just as you say. So instead of having dissolved CO2 (and a small amount of carbonic acid) that gradually bubbles out of solution when the container is opened, in the shaken container you have a big mass of bubbles already out of solution that have built up great pressure. The thing explodes in your face, and the resulting Coke will be found to be quite flat since most of the CO2 is gone. If the bottle is left sealed, the pressure eventually drives the CO2 back into solution; the bottle is safe to open and the Coke will be properly carbonated.
Among other things, thermodynamics ignores surface tension when describing phase equilibria.
Foaming is a chemical engineer’s worst nightmare and very difficult to model with physics based models. What happens when you shake the bottle is that you create foam. The CO2 inside the foam bubbles are shielded from the liquid by surface tension in the bubble. This results in the gas phase above the liquid to seem like it has lower partial pressure of CO2 thereby driving more CO2 out of the solution.
In short the CO2 inside the bubble is immune to the liquid-vapor equilibrium. It’s as if you removed CO2 from the bottle without really removing it.
Probably not polarity. CO2 is a symmetrical molecule, and has approximately the same polarity as hexane (which is not very miscible with water)
There’s chemical reactions involved:
CO[sub]2/sub + H[sub]2[/sub]O <—> H[sub]2[/sub]CO[sub]3[/sub] (aq)
H[sub]2[/sub]CO[sub]3/sub <----> H[sup]+/sup + HCO[sub]3[/sub][sup]-/sup
HCO[sub]3[/sub][sup]-/sup <----> H[sup]+/sup + HCO[sub]3[/sub][sup]2-/sup
In the highly acidic Coke, the equilibriums will tend to favor the less ionized forms, but still, that reaction chain draws more CO2 into the water than if there were no reactions at all (as is the case for hexane).