So one of our readers asked us this question the other day. I just dropped a can of root beer… does tapping on the lid actually fix the liquid/gas equilibrium? It seems to have worked for me in the past, but I’ve heard from some people that it’s just an urban legend.
The short answer is that tapping the top of the can does almost nothing to prevent the liquid from fizzing, this is mostly a myth. The best you can do is simply to wait for the gas in the bubbles to settle back into the liquid.
If you want to dig deeper, it turns out that not only is the tapping part a myth, but the very premise of this myth (the notion that the liquid/gas equilibrium needs “fixing”) is itself a myth! This seems to be a very common misconception, but it’s simply not true. It turns out that the equilibrium between the partial pressure of the CO2 in the headspace and the gas dissolved in the liquid is re-established pretty quickly (see this blurb from a science education journal for more details).
The real explanation for why shaken bottles of carbonated drinks fizz so much has to do with the rate of nucleation, or bubble formation. Initially the CO2 in the liquid is supersaturated, which means that it wants to escape. However, before it can do so it needs to first form bubbles.
The first step of bubble formation, the nucleation can be fairly slow because it is an activated process, meaning that it needs to pass over an energy barrier, which we call the activation energy (Ea), as shown in the above diagram. When you shake the can you provide enough energy to overcome this barrier, creating many, many bubbles in the process. Now here’s the interesting thing, once you produce some bubbles, these bubbles will actually make it more likely for more bubbles to form (if you want to get technical, this is a process we call heterogeneous nucleation). This is why all hell breaks loose when you open a bottle after shaking it, the millions of tiny bubbles rapidly produce more bubbles and much of the CO2 trapped in the liquid rushes out, taking the liquid with it.
The trick is to be patient and to simply wait for the gas in the bubbles you form when disturbing the can to settle back into solution. At that point it will be as though you had never disturbed the can in the first place (i.e. the process is fully reversible).
The key myth I wanted to debunk was the notion that tapping can change the equilibrium of the gas dissolved in the liquid and in the head-space. As for the idea that tapping the sides can dislodge part of the trapped bubbles and reduce the fizzing, I admit that this is more plausible. However, the evidence I’ve seen one way or another is all over the place, always anecdotal, and largely inconclusive, like this article.