How to Keep Fizz in Your Soda

Explore the properties of carbon dioxide in your soda pop.

Rarely do you ever drink an entire 2-liter bottle of soda at one sitting. Since leftovers are inevitable, the soda tends to go flat. Besides screwing on the cap, is there anything that can be done to keep the fizz in a bottle of soda?


  • Wintergreen Lifesavers
  • 2-liter bottle of soda
  • Clean, empty soda bottle (1-liter or smaller works best)
  • Fizz Keeper (or a similar pressurizing pump)
  • Small, fresh marshmallows


You don’t have to shake a soda in order to produce a flowing, foaming river of scientific inquiry.

  1. Open a brand new 2-liter bottle of soda and drop in three or four Wintergreen Lifesavers. Oh, by the way, stand back! The dissolved carbon dioxide gas makes a mad dash for the breath mints (lots of nuclei sites–see below) and forms big bubbles that burst out of the liquid. Please remember that this is not a way to keep the soda from going flat. This little gem of an activity actually speeds up the bubbling process. But isn’t science fun?


Here’s a great activity to explore the properties of air pressure (and it will keep the kids busy for hours!):

  1. Fill the bottom of the empty bottle with marshmallows (a dozen or so). Screw the pump on the bottle.
  2. Carefully observe the marshmallows as you start pumping the piston up and down. Hey, they’re shrinking! The marshmallows are shriveling up. Why? The Fizz Keeper is like a miniature bicycle pump that forces molecules of air into the bottle. The increased pressure, in turn, pushes on the marshmallows. Since marshmallows are just puffy pockets of air, the increased pressure compacts the molecules and the marshmallows shrivel up.
  3. Release the pressure by unscrewing the cap, but don’t take your eyes off the marshmallows. Let’s just say the rapid decompression is well worth all of the effort of pumping!

What else might work in place of marshmallows? Are all gases compressible? Can you compress a liquid using the pump? Notice the change in temperature of the bottle when you’re pumping the piston. Why does the bottle get warmer? Does the bottle temperature drop when the pressure is released?

How Does It Work?

There are really three factors that you should keep in mind when searching for the ultimate solution to the fizz factor dilemma:

Reactions with Carbon Dioxide Lucky for soda drinkers, carbon dioxide gas easily dissolves in water forming carbonic acid. That’s what gives soda a tangy taste. The goal is to keep the molecules from escaping. Everyone knows that shaking a soda makes it explode (what fun!). Why? In order to escape, the molecules have to find some rallying points, some unique meeting places at which they can congregate and form groups (bubbles) that are big enough to push their way up and out of the bottle. Scientists refer to these congregation sites as nuclei. Shaking a soda allows it to mix with air molecules trapped in the bottle. These air bubbles are the best possible nuclei for the further growth of bubbles. Carbon dioxide molecules latch onto the air molecules, which grow bigger and bigger. And before you know it, the foaming mess is out of control. In other words, if you want to keep your soda from going flat, don’t shake it. Duhhh!

Pressure At first glance, this part of the equation makes the most sense. Here’s what we know… the higher the gas pressure above the liquid in the bottle, the more gas will be pressed into the liquid. Makes sense. However, here’s the kicker. Once you open the bottle, the vast majority of the carbon dioxide molecules that were forced into the soda at the bottling plant come flying out. It’s that unmistakable sound of PSSSSST!

What about those gadgets called Fizz Keepers (you know, those pumps sold at the grocery store that supposedly re-pressurize the soda)? Do they work? When you pump the piston up and down, you’re pumping plain old air (mainly oxygen and nitrogen) into the bottle. However, scientists tell us that the escaping gas can be pushed back into the liquid only by forcing more molecules of that particular gas (in this case carbon dioxide) into the space above the liquid. In reality, there isn’t one more molecule of carbon dioxide in the liquid after pumping the Fizz Keeper gadget than if you had simply screwed the cap on tight. You guessed it, the gadget is just a glorified soda bottle cap. Yes, it gives you that satisfying PSSSSST! sound, but that’s all it does. So, in terms of pressure, there’s not much that can be done.

Temperature Let’s cut to the chase. The secret to keeping your soda alive with plenty of reusable fizz is to keep the soda cold, plain and simple. The higher the temperature, the less the carbon dioxide molecules will dissolve. (By the way, try letting the bottle of soda sit out in the sun before dropping in the breath mints and watch out!) It’s especially important to keep the bottle tightly sealed while it is out of the refrigerator since the higher temperature makes the gas want to leave the liquid. Pour yourself a glass of refreshing soda, cap the bottle, and put it right back in the refrigerator. Keep it cold… keep the fizz.

Source Information: If you find this sort of stuff interesting, you’ll want to rush out and pick up a copy of What Einstein Didn’t Know by Robert Wolke. The book is packed full of great information that is sure to get your creative ideas flowing.