Carbonated Beverages: Tapping the can

[Tiassa]

This may properly be a chemistry question, but I suspect at a fundamental level it becomes one of physics. If I'm wrong, my apologies.

Once upon a time I learned a curious practice. If you accidentally shake a can of carbonated beverage (soda, beer, &c.), you are to lightly and repeatedly tap on the top of the container before opening it. This in some way is supposed to calm the pressure overload that causes the liquid to explode out of the container when the seal is broken.

I've done this many, many times, for more years than I can remember, but in any proper terms I think the best I could say is that the result is a correlation suggesing that the practice works.

But I have no idea why it works.

Yes, I happened to think of this as I walked away from a vending machine with a plastic bottle of root beer. Yes, the machine action shocked the hell out of the bottle. Yes, I tapped it as per custom. No, the stuff didn't explode all over my hand when I cracked the seal.

Is this a real effect, or just a common superstition? I mean, I'm not the only one I know who does this, and I'm hardly the source of the practice in my circles. It seems that "everybody", so to speak, knows about this.

But how does it work? That's the thing I don't get. I've never thought of CO₂ escaping from soda as a genie that can be put back in the bottle.

[James R]

I agree that it seems to work. I have no idea why.

[Anti-Flag]

I've never heard of this, so a couple questions:
Does it still fizz at all?
Secondly, how long do you keep this process up for? As unless I'm much mistaken if you simply wait a while then a can will settle again and be ok to open. I'm wondering if perhaps this process takes long enough for a can to settle.

[kenworth]

i dislodges the bubbles from the sides so they arent all released at once?

[Michael]

i dislodges the bubbles from the sides so they arent all released at once?

I think so, it dislodges the CO2 bubbles that ppt out of solution - they go to the top of the can. Then when you open it the CO2 is all at the very top and not around the bottom and sides. The little bit of CO2 that came out of solution will leave the can from the top without pushing the soda out with it (as would happen if the bubbles were coating the sides and bottom too).

The same amount of CO2 that came out of solution would still leave the can.

I think.

[BenTheMan]

Hmmm...it'd be easy enough to test, right?

My guess is that waiting the equivalent amount of time would produce the same result---i.e. superstition, as you alluded to. The CO2 is in equilibrium in the coke. That is, under the pressure it's under, the CO2 is mostly suspended in solution. When you disturb the can, some CO2 jumps out of solution and into its gaseous form. This increases the pressure in the can initially. Eventually (given time) the CO2 will settle back to equilibrium, as the higher pressure in the can serves to drive molecules back into solution.

Now, there may be some trickiness that I'm not accounting for: for example, the liberated CO2 may live inside of bubbles---the fizz on the top of the drink. In this case, tapping may help to break the bubbles up, and allow the CO2 a better chance to re-equilibrate.

[rpenner]

Shaking the can violently doesn't undissolve the gas to any large extent. (A pressure gauge could confirm this.) But it can increase the area of the liquid-gas boundary via the creation of bubbles. If the bubble is caught on an irregularity of the can, it could form a classic nucleation site, growing streams of bubbles while remaining affixed to the can. The combination of a large number of bubbles, many of those bubbles tied to the irregularities of the interior surface of the can below the normal liquid edge, the buoyancy of bubbles, and the pressure change when the top is opened causes the sometimes dramatic fountaining.

Gently mechanically agitating the can (tapping) may dislodge bubbles affixed to the interior of the can below the surface of the liquid and hasten the coalescence of the gas at the top of the can, reducing the effects of large surface area and buoyancy of bubbles from submerged nucleation sites.

[Uno Hoo]

During many years of clumsiness, was sprayed by previously disturbed carbonated beverages. In recent years heard of tapping trick. Never been sprayed when doing tapping thing.

Time interval between disturbance and tap does not seem to be a factor in my own experiences. My technique is one tap, delivered in a quick motion but at a moderate force. Have always used tap on metal tops, not plastic, and on glass and metal bottles. I make sure to tap with protrusion of fingernail, not fatty part of fingertip.

Can not contribute to theoretical musing. I don't understand how it can work, but it does seem to be effective.

I have noticed that there has never been any sign of unusual pressure release after progressing through the disturbance/tap/open routine. It is as if the tap made any unusually high pressure vanish.

[Blindman]

A hot day here in Western Australia and after reading this thread a mate and me did a simple test. Two cans of beer shaken vigorously for ten seconds. One was tapped 10 times the other not. Both when opened waisted much preciouses amber fluid.

Its an urban myth.

[Pinwheel]

What we need is a transparent can, so we can observe what happens inside when the can is tapped.

A hot day here in Western Australia and after reading this thread a mate and me did a simple test. Two cans of beer shaken vigorously for ten seconds. One was tapped 10 times the other not. Both when opened waisted much preciouses amber fluid.

Its an urban myth.

If it was XXXX then its no great loss.

[S.A.M.]

To reiterate Blindman here:

Dear Cecil:

Why do people, before opening a carbonated drink which has been shaken, tap the top of the can with their finger so that it doesn't explode upon opening? I have always laughed at this. After lengthy arguments, we even performed a semiscientific experiment by shaking a drink and opening it with and without tapping the top, but with no solid scientific conclusion. We would like to know what you, in your infinite wisdom, think of this.

— Benjie Balser, Dallas, Texas

Cecil replies:

This is not a problem that requires infinite wisdom, Benj. This is a problem that requires enough neural organization to qualify as a vertebrate, apparently a stretch for some folks these days.

First I called the folks at Coke central in Atlanta. I did this in the interest of thoroughness, in case Coke physicists had discovered quantum mechanical aspects of beverage carbonation that had previously eluded the notice of science. However, they didn't return my calls. There are two possible explanations for this: (1) everybody was out in the plant stamping out souvenir Olympic bottles, or (2) Cecil's message was a little too detailed. This is an inherent risk in my business. If you tell some low-level gatekeeper type you have a question about poultry, you may actually get through. Tell them you want to know which end of the egg comes out of the chicken first, and they'll have security trace the call.

No matter. First let's consider the matter from a theoretical perspective. Carbonation is produced by forcing carbon dioxide into solution with H2O under pressure. Shake up the can and you create thousands of micro-size bubbles. Each bubble offers a tiny surface where CO2 can rapidly come out of solution, creating the potential for explosive fizzing should you open the can prematurely. Wait a while though, and the bubbles will float to the top of the can and disappear, and eventually all will be as before.

But suppose you're the impatient type. You tap the can. What, pray tell, is this supposed to accomplish? Are we going to noodge the tiny bubbles to the surface faster, after the manner of herding cows? Are we going to maybe dislodge a few bubbles that have stuck to the sides of the can? Maybe we are, but the difference is slight. Open that baby and you're still going to get a faceful of froth.

We confirmed this to our satisfaction out in the Straight Dope Back Yard of Science with a half dozen cans of pop. OK, so I didn't replicate my results 50,000 times. I figure if extraordinary claims demand extraordinary evidence, stupid claims demand . . . well, something a little less rigorous.

I should tell you that when I had Little Ed broach this issue recently on the Usenet he heard from a science teacher, among others, who insisted tapping the can really did reduce fizzing and bragged about a classroom demonstration he did to make just this point. No wonder today's youth are going to the dogs. But to be sure, I called up physicist Jearl Walker, who's written about the physics of beverage carbonation in Scientific American. Jearl, you'll remember, is the guy who used to plunge his hand into a vat of molten lead as a classroom demonstration of the Leidenfrost effect. This makes him either a madman or a genius--in either case somebody you want to listen to with respect.

Jearl had heard similar claims about the efficacy of tapping and had a similar reaction: these guys are nuts. He said he could only attribute the persistence of the practice to the same suppressed macho ethic that makes people tap the ends of their cigarettes before lighting up.

If you want a real solution, try this. It's an implacable fact that a warm can of pop that's all shook up will fizz more than a shaken cold can. If you absolutely must pop the top on that jug of Jolt, stick it in the fridge first. You'll chill the contents and chill the carbonation too, an inevitable consequence of increased gas solubility and Charles's law.

TAP DANCING, PART ONE

Dear Cecil:

You missed the most obvious explanation for tapping on a soda can: tapping takes time, and with the passage of time the CO2 goes back into solution. --Jonathan Cook, via the Internet

Dear Jonathan:

You raise a legitimate point, a sufficiently rare occurrence in these parts that I went right out to the Back Yard of Science to do another experiment. I got two cans of Coke Classic at room temp and shook them each vigorously 60 times. Setting both on the pavement, I opened one immediately and got a good-size gusher of froth. Then I waited 60 seconds and opened the other one. There was no gushing to speak of. Mind you, I had done no tapping.

So you may well be right that what tapping chiefly does is kill time, an issue to which I perhaps gave inadequate attention in my original column. But that doesn't change my main point, which is that tapping per se doesn't do squat.

http://www.straightdope.com/columns/...ing-on-opening

note: I did not add quotation tags to the cut and paste because its apparently irrelevant. I own the words anyway.

[D H]

A hot day here in Western Australia and after reading this thread a mate and me did a simple test. Two cans of beer shaken vigorously for ten seconds. One was tapped 10 times the other not. Both when opened waisted much preciouses amber fluid.

Its an urban myth.

Snopes.com agrees, as do spokespeople from Coke and Pepsi: It's a myth. See http://www.snopes.com/science/sodacan.asp and http://www.chow.com/stories/10800.

[MacGyver1968]

Using science to save me from spilling beer...I ..love...science (sniff).

[Dywyddyr]

How would YOU spill beer?
I was under the impression you just sucked it down in one go, can and all.

[Trippy]

Jearl walker:
Leidenfrost Essay

I like his idea of making graduates undergo firewalking to prove their faith :3.

[kurros]

Maybe it works for coke but not for beer .

[Blindman]

We could tap deep sea divers as they surface to save them from the bends.

[Pete]

Hmmm...it'd be easy enough to test, right?

Maybe not, as the thread attests. A fairly rigorous protocol would be required, controlling for temperature, initial agitation, tapping, and time. This would be a great exercise for experimental design that could be done in a clasroom.

One thing I note is that if all the subjects in a trial go fwoosh, there's probably something wrong. Overagitation or high temperature spring to mind. The test should involve enough agitation that some, but not all, subjects go fwoosh. That way, you can do a proper test for significance between tapped and untapped subjects.

[machiaventa]

Me thinks this is a very old trick and it does work only for brief disturbances. shaking a can of beer or soda for ten seconds will only result in the loss of contents. Why don't you suggest this problem to mythbusters and see what they come up with.


machiaventa

[machiaventa]

I forgot to mention temperature effects this procedure radically.