Soda Can Shake-Up

Shaking a can of soda before you open it isn't too smart, unless you know the science going on inside

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In some circles, it’s considered bad manners to shake a can of soda just before giving it to a thirsty friend. The unexpected w-h-o-o-s-h of soda and foam is a sure way to spoil someone’s day. If a “friend” of yours thinks it’s cute to hand you a can that’s just been shaken, smile, do what you need to do, and then open the can to a gasp of amazement… from the other person. Science know-how can help you in the funniest ways.


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Here's What You'll Need

  • Unopened cans of cola (diet and regular)
  • Water and paper towels for clean up
  • A handy sink or pan

Let's Try It

  1. (You might want to practice with cans of club soda. It’s clear and not sticky.) Vigorously shake a sealed can of regular cola.

    Invite a willing friend to immediately open the can over a sink or pan. It’s possible that there may be a few taps given to the top of the can. Many people do this out of habit.

    Odds are, however, when the shaken can is opened, there will be a surge of cola and foam out of the opening.

    Clean up the mess and share what’s left of the soda in the can.

  2. Grab another unopened can of regular cola and shake it as you did before. Before you open it this time, however, flick the the sides of the can hard with a finger. Go all the way around the can, too. Give the can a quarter turn and snap again. Snap and turn the the can about six times before opening it.

    Open the can slowly to share the non-foaming and non-spewing drink with your friend.

    Do the same shaking and can tapping with a diet cola but open it over the sink. Your technique is not bad; it’s the diet formula in the can.

How Does It Work

Since the fizz in soda is actually dissolved carbon dioxide gas (CO2), the goal is to keep as much of it in the liquid as possible. Soda fizzes when dissolved CO2 is released in the form of bubbles. At the bottling plant, carbon dioxide molecules are forced into the liquid in quantities much greater than would dissolve in normal atmospheric pressure. There’s a lot of pressure being applied to do this. In addition, the liquid is very cold. As it warms up in the sealed can, the CO2 wants to come out of solution but the can is strong enough to hold it in there. As soon as you open the can, some of the excess gas escapes but the liquid and the remaining gas stay inside.

Shaking an unopened can of soda causes bubbles of CO2 to cover the inside walls of the can. They form on imperfections in the metal called nucleation points. When you open a shaken can, the pressure in the liquid drops quickly and the volume of a billion or so CO2 bubbles increases instantly (check out Boyle’s Law). Whoosh! The quickly expanding bubbles force any liquid above them out of the can as a foaming mess.

Some people think tapping the top of the can before opening it prevents this mess. This actually does very little to the bubbles. The trick is to dislodge them from the side walls and bottom of the can so they can scoot to the top of the liquid (because gas is less dense than a liquid). Once the bubbles are at the top of the can, there will be little or no liquid blocking their escape when you open it. As a result, the soda doesn’t spray. Remember, snap the side instead of tapping the top.

Take It Further

Once you’ve mastered the technique on regular cola in cans, hone your talents on different kinds and containers of soda. You’ll find that it’s probably easier to set the containers (even the cans) on a hard surface to smack them with a finger. It gives you better control and allows more energy to go into the smack.

Watch out for diet sodas, however! There’s no guarantee that even snapping the side of a can of diet soda will prevent getting sprayed when you open a shaken can. Some scientists speculate that diet sodas contain more CO2 while others believe there’s some unique interaction between the artificial sweetener, the preservatives used, and the carbonated water. At this point, no one fully understands the reason. So, if you choose to open a shaken can of diet soda… well, you’re on your own.

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