Judging by what we see on television, you’d think that the major role of champagne in American culture is to hose down Super Bowl winners in locker rooms.
Somewhat younger children do the same thing with soda pop, making sure to shake the bottle well before moving the thumb partially aside on the top in order to aim better. Well, you know the rest. (DO NOT TRY THIS AT HOME!)
If I said that shaking a bottle of champagne, beer or pop raises the gas pressure inside, ninety-nine out of a hundred people, even chemists and physicists, would agree. But it’s not true. When you shake an unopened bottle or can of carbonated beverage the pressure inside does not change.
It certainly does seem as if the pressure is increased by shaking, and it’s easy to dream up smug theories as to why that should be. But I won’t muddy the waters by quoting those theories here, because they’ve turned out to be all wet.
Then why does the liquid squirt out with so much force when you open a shaken bottle? It’s only because shaking makes it easier for gas to escape from the liquid, and in its eagerness to escape when the bottle is opened it carries some liquid along with it.
It was two chemists named David W. Deamer and Benjamin K. Selinger at the Australian National University in Canberra who in 1988 settled the question in the simplest possible way: by measuring the gas pressure inside a bottle of pop before and after shaking it. They adapted a standard pressure gauge, not too different from a tire gauge, so that it could be screwed onto the top of a soda bottle.
Their results (which would have been the same if they had splurged and used champagne): If an unopened bottle has been standing quietly at room temperature for a day or so and is then shaken, the pressure of carbon dioxide gas in the head space (the space above the liquid) does not change.
The reason is that the gas pressure is determined by only two things: (a) the temperature and (b) how much carbon dioxide can dissolve in the liquid at that temperature (Techspeak: the solubility of the gas in the liquid). There is only so much carbon dioxide gas in the bottle; some of it is dissolved in the liquid and some of it is loose in the head space.
When an unopened bottle of soda has remained at the same temperature for some time, the amount of gas dissolved in the liquid, and more important, the amount of gas that is not dissolved in the liquid, settles down to whatever the appropriate proportions are for that particular temperature. (Techspeak: The system comes to equilibrium.) You can’t change those proportions by doing anything short of changing the temperature or adding more carbon dioxide.
(If you put the bottle in the fridge for twenty-four hours or so, more of the gas will dissolve in the liquid, because gases dissolve to a greater extent in colder liquids. There will then be less gas in the head space, and the pressure will be less. That’s why you get less of an outburst of gas when opening a cold bottle than when opening a warm one.)
The point is that shaking alone can’t change the pressure because it doesn’t change the temperature or in any other way change the amount of force or energy that is available inside the bottle. So never fear that manhandling your beer, soda or champagne on the way home from the store will make the bottles explode. On the other hand, make sure not to let the bottles heat up in the trunk of your car, because the higher temperature will indeed raise the pressure of the gas.
Now we can take a more educated look at what causes the explosive emission when we open a recently shaken bottle. It is caused by an increase in the amount of gas that is set loose, not by heating, but by the mechanical “outing” of some dissolved carbon dioxide from the liquid when the bottle is opened.
Here’s how.
First of all, a bunch of dissolved carbon dioxide molecules can’t just decide to gather together in one spot and form a bubble. They need something to gather upon, a microscopic speck of dust or even a microscopic irregularity on the surface of the container. These congregation spots are called nucleation sites, because they serve as the nuclei, or cores, of the bubbles.
Once a small gang of carbon dioxide molecules has gathered at a nucleation site and formed the beginnings of a bubble, it is easier for more carbon dioxide molecules to join up, and the bubble grows. The bigger the bubble gets, the easier it is for even more molecules to find it and the faster it grows.
Now when you shake a closed bottle of pop, you’re making millions of tiny bubbles of gas from the head space that become trapped in the liquid. There, they serve as millions of nucleation sites upon which millions of brand-new bubbles can grow. If the bottle is then left to stand for a long time, the new baby bubbles will be reabsorbed and all the contents will return to normal, in which condition it is no longer a threat.
But those new nucleation sites and their newly hatched bubbles don’t disappear very quickly; they remain for some time in a recently shaken bottle, just waiting for some unsuspecting soul to come along and open it. When he does, and the pressure in the head space suddenly drops to atmospheric pressure, the millions of baby bubbles are free to grow, and the bigger they get the faster they grow. The large volume of released gas erupts abruptly into a gigantic blurp that carries liquid out of the bottle.
Shaking a bottle or can of beer or soda pop does not increase the pressure inside.
Oh, the champagne? Same thing. The best way to handle it is to leave it undisturbed in the refrigerator long enough for it to “come to equilibrium”, at least twenty-four hours. Then be careful not to either warm or agitate it before or during opening.
After removing the wire twist, ease the cork upward with your thumbs. All of the champagne will stay in the bottle and the cork won’t become a lethal missile.
