Shaking does not make more fizz, so why does a shaken soda erupt?
A shaken can erupts on opening, yet shaking neither adds fizz nor permanently raises the pressure. What shaking changes is not the amount of gas but its distribution: the footholds for escape (nucleation sites) end up scattered through the liquid. So on opening it all bursts at once.
Shake a can, then open it, and the cola shoots up. Everyone knows this. But think about it and it is strange. Shaking does not create more fizz inside the can, and your hand did not pump gas in. The amount of carbonation inside is the same. Same amount, yet calm before shaking and explosive after. What on earth did shaking change?
People often answer "shaking raises the pressure" or "shaking makes more gas come out." Neither is quite right. Shaking does not change the total CO2 in the can, nor does it permanently raise the pressure. There is one decisive clue. A shaken can, left alone for ten to fifteen minutes, becomes fine again. Something returns to how it was. So what shaking changed is not the amount of gas but the state the gas is in.
Let us return to the first question. What did shaking change? Not the amount of gas, but the distribution of the footholds by which gas escapes. When even a few footholds are scattered everywhere, the moment it opens, gas bursts from every spot at once. Dropping a Mentos into cola makes it fountain by the very same principle. The countless tiny pits on the Mentos surface become a vast set of nucleation sites, so the gas bursts all at once without any shaking. Shaking, in the end, is just a way of making lots of these footholds. Temperature lends a hand too. The colder it is, the better CO2 stays dissolved, so the drink holds more gas. So "it is cold, therefore safe" is not always true. Then again, when cold, the gas is held more firmly and does not escape as easily, so temperature works both ways.
Top: pick a settled or shaken can and press Open the lid to see why the same amount of fizz erupts so differently. Bottom: raise the nucleation sites (footholds) and watch the eruption grow stronger.
First, the background. In a fizzy drink, CO2 is held dissolved in the liquid under pressure. The higher the pressure, the better the gas dissolves. But for this dissolved gas to leave as a bubble, it needs a starting point. We call this starting point a nucleation site. In a clean liquid, a bubble rarely forms on its own; there must be a small foothold where, overcoming surface tension, a bubble can take hold. Inside a sealed can, the high pressure suppresses bubble formation. So a drink left undisturbed stays settled, holding its gas with almost no bubbles. What happens when you shake it? Countless tiny bubbles form in the liquid. Each of these little bubbles is a nucleation site. And because of the swirl from shaking and the drink's stickiness, these bubbles cannot rise and vanish but stay trapped, scattered throughout the liquid. Here is the key. Shaking did not make more gas; it laid footholds for the gas to escape all through the liquid in advance. Now open the lid. With a settled drink, the gas leaves slowly, fizzing at the surface. But the shaken drink already has footholds everywhere, so the gas bursts out all at once, with no time to redissolve. That is why it shoots up. And if you let the shaken can sit, those tiny bubbles dissolve back into the liquid over time. That is why it returns to its pre-shaken state. Proof that the problem was not the amount of gas but the distribution of footholds.
So the trick is simple. A can you shook by mistake just needs to sit. After ten to fifteen minutes the footholds dissolve away and it is safe again. In a hurry, open it very slowly before fully cracking it, letting the gas trickle out a little at a time so the footholds do not all burst together. More broadly, the story is that even for the same amount, how it is distributed and what footholds it has decide the outcome. This idea of nucleation is not unique to soda. A water droplet forming in a cloud, water boiling in a pot, even a diamond being made, all follow the same principle. Without footholds, water can stay still without boiling even past its boiling point. A shaken can erupts not because the gas increased, but because the paths of escape opened all at once. The path, more than the amount, decided the outcome.