Why are coke bottles that much fizzier if you shake them?

We've all been there: you drop your bottle of soda at some point and when you try to open it, it bursts into foam.

My question is, then: why does shaking a carbonated drink make the dissolved gas escape?

Although Why can't CO$_2$ mix back with the liquid after a soda bottle has been shaken? is a duplicate to this question, I'm not sure it's a great answer and it would be good if someone could come up with a better answer here.

The accepted wisdom is that shaking the bottle creates tiny bubbles and these act as nuclei for bubble formation when the pressure is released. Without these nuclei bubble formation is mostly by heterogenous nucleation on the bottle walls, and that is slower.

However I have never seen a paper that demonstrated this is the case, for example by light scattering measurements on the shaken but unopened bottle. The nearest I've seen is this paper, which seems reasonably authoritative though it's not from a peer reviewed journal. If anyone knows of such papers, or feels like doing the experiment, I'd be interested to see the data.

It is known that the pressure isn't increased by shaking/dropping the bottle (the article I linked measured this) so some form of enhanced nucleation does seem the most plausible mechanism. It's just that it would be nice to see it proved.

• Nucleation is obviously right, do you really need light scattering formal experiment to be persuaded? You can see the little bubbles form in the soda with your eyes. The sound waves from an impact or turbulent shaking make lots of little low-pressure regions that nucleate gas, and these millions of little bubbles nucleate when you open. – Ron Maimon Jul 18 '12 at 16:58
• I have just deliberately dropped a bottle of lemonade, and while I could see some large bubbles immediately after the drop, after a minute or two I couldn't see any bubbles in the bulk of the lemonade. Although I didn't sacrifice this particular bottle, experience tells me that a dropped bottle will foam vigorously if opened ten minutes after dropping. I'm not doubting that nucleation is the mechanism, but from a casual inspection I can see no experimental evidence for it. – John Rennie Jul 18 '12 at 17:03
• When I drop cola, I see little white dots in the soda for a minute or two. The lemonade might be too bright to see the light-scattering. The bubbles have to re-dissolve, it will take a while (I don't think as long as 10 min., maybe 3). – Ron Maimon Jul 18 '12 at 17:18
• Bearing in mind I used to be a colloid scientist, and am very used to using light scattering to see what's floating about in suspension, it seems an obvious and easy experiment to do. It should be straightforward to watch the bubbles appear then redissolve after some suitable disturbance. You could even track bubble growth (the initial stages at least) as the pressure is released. Sadly I no longer have access to that sort of kit and I doubt my former employers would see it as a good use of ther shareholder's money :-) – John Rennie Jul 18 '12 at 17:29
• In my experience, the bottle does feel like the pressure increases when I shake it. It does seem like it's worth a few hours of my time (and some off-hours lab equipment) to do this experiment properly. Hmmmmm.... – Floris Aug 30 '14 at 14:47

Think of this in terms of Le Chatelier's Principle.

An unshaken bottle should be in a state approaching thermodynamic equilibrium of concentration, temperature, volume, and partial pressure. The rates of the chemical reaction describing solution of the CO2 components is equal in both the solution and the dissolution directions.

When you shake the bottle, you mix it and speed up the reaction in the direction of solution. (Just like you can speed the dissolution of sugar in tea by stirring.) This causes the liquid to become super-saturated with respect to the CO2 component. Uncork the bottle and pressure of the gas is suddenly lowered and the super-saturated solution boils. Super-saturation is a metastable state.

As a test of this hypothesis, I also think you would find shaking a closed bottle of pop will slightly lower the temperature and slightly lower the pressure of gas and slightly change the volume of liquid.

• This seems to argue in the opposite direction of observation: the pressure in the gas goes up when you shake the bottle, not down. You can feel a plastic bottle become "tight" when you shake it. – Floris Aug 30 '14 at 14:46
• @Floris: Does it? If it does that means something else has to change, according to the ideal gas law: the volume, temperature, or density. So it doesn't. Pressure doesn't actually change, and shouldn't. In fact, school experiments routinely shows this case. – Pwnna Nov 14 '14 at 0:45

Normally I would dismiss the “persistent microscopic bubble” theory because very small bubbles would have very high internal pressure making them unstable and their large surface area to volume ratio which would accelerate the rate at which the gas would redissolve - it would be practically instantaneous. However, in the presence of proteins (or other surfactants), the bubble surface will get occupied by the proteins and the bubble will redissolve only to the point where the proteins form a rigid shell (This is why bubbles in beer, which is high in proteins, are much more rigid than bubbles in Champagne). It is quite plausible that the time scale for the protein structures to redissolve might be as long as the fifteen to twenty minutes reported.

• Seltzer water that has very little protein etc also fizzes if you shake it first. We could get a sense how much difference the extra proteins etc make by measuring the difference in fizz in the different liquids. – J Thomas Oct 16 '20 at 12:12

I don't have an answer, but I want to try to state other answers clearly.

John Rennie's answer: When you shake the bottle, it somehow creates many tiny bubbles. When the bottle opens the pressure is reduced, and each bubble expands. That expansion throws a lot of liquid out of the opening.

Also, we observe big bubbles in the liquid. The tiny expanding bubbles must be coalescing and also they attract more CO2 from the liquid to get bigger beyond their expansion from low pressure.

Mark Rovetta's answer: CO2 dissolved in water is in equilibrium between more than one state. One of the states is simply CO2 molecules bumping around in the water. A second state is CO2 molecules that are hydrogen-bonded to water, which can be represented as H2CO3. (There may be other states, these two are enough for the explanation.) See, acetic acid is like CO2 with a methyl group attached. Formic acid is like CO2 with a hydrogen attached. Carbonic acid is just CO2. The minimal carboxyl group.

Maybe shaking it gives the energy to break some of the hydrogen bonds? And over time they get re-established.

When the pressure is released, the merely-dissolved CO2 comes out of solution and forms bubbles.

No wait, that isn't what Mark Rovetta said. Here's a different way he may have meant it. You get an equalibrium between the CO2 that's dissolved in the liquid versus the CO2 that's in the gas layer over the liquid. When you shake the bottle, you provide energy that puts more of the CO2 into solution. Also you provide a whole lot of surface area for that to happen in. Later when the bottle is still, it takes longer to restore the old equilibrium because of the reduced surface area.

When you open the bottle, the extra CO2 dissolved in the liquid quickly comes out of solution. Because there's more CO2 in solution.

When I think about it, it looks important that you can prevent the fizz by opening the bottle slowly. You can open it MUCH sooner without excessive bubbling than if you wait and then open it fast. That has to fit into the puzzle somehow.

Possible experiment: Count the bubbles.

If the number of nucleation sites goes down over time there will be fewer bubbles the longer you wait to open the bottle.

If each nucleation site keeps getting smaller, there will be the same number of bubbles but they will start smaller and might take slightly longer to get big.

So that doesn't tell whether nucleation sites are important, but it does show the difference between one nucleation model versus another nucleation model or generalized solution model.

Possible experiment: Bottle some soda water with enough oil to form a layer between the water and the gas. The oil should make it take longer to reach equilibrium between CO2 dissolved in the water versus CO2 in the gas layer. When you shake it, you get a big surface area between CO2 bubbles and water, and that surface area gradually gets smaller as the oil droplets coalesce. If the issue is carbonic acid versus CO2 in the water, that should not matter at all. If it's nucleation sites in the water that disappear over time without moving to the gas layer, it also should not matter.