Pop is fizzy because it’s packed with carbon dioxide gas under high pressure. At the bottling plant, CO2 is forced into the liquid at pressures around 25 to 30 psi, well above normal atmospheric pressure. Under those conditions, the gas dissolves into the liquid and stays there, invisible and waiting. The moment you crack the seal, the pressure drops, and all that dissolved gas starts escaping as bubbles.
How CO2 Stays Dissolved Under Pressure
The physics behind carbonation comes down to a principle called Henry’s Law: the higher the pressure of a gas above a liquid, the more of that gas will dissolve into it. Inside a sealed bottle or can, the space above the liquid is filled with CO2 at high pressure. That pressure forces a large amount of carbon dioxide to dissolve into the drink and stay dissolved. The gas and liquid reach an equilibrium where CO2 molecules are constantly entering and leaving the liquid at the same rate, keeping the carbonation stable.
When you open the container, you break that equilibrium. The pressure above the liquid drops to normal atmospheric levels, and the liquid suddenly holds far more dissolved CO2 than it can support at this lower pressure. The gas has to go somewhere, so it starts forming bubbles and escaping into the air. That rush of escaping gas is the hiss you hear when you pop a tab or twist a cap.
Where the Bubbles Actually Form
CO2 doesn’t just spontaneously appear as bubbles throughout the liquid. Bubbles need a starting point, and they find one at tiny imperfections on surfaces, a process called nucleation. Microscopic scratches inside a glass, tiny fibers from a cloth, or small defects on the inside of a can all serve as nucleation sites. Gas molecules collect in these micro-crevices and grow into visible bubbles that eventually detach and rise to the surface.
This is why pouring pop into a glass produces a burst of fizz, and why a scratched or rough glass generates more bubbles than a perfectly smooth one. It’s also why dropping something into a glass of soda, like a mint or a grain of sugar, triggers an eruption of foam. Those objects introduce thousands of new nucleation sites all at once.
The Chemistry That Creates the Tingle
Fizz isn’t just about bubbles. When CO2 dissolves in water, it reacts chemically to form carbonic acid. This weak acid gives carbonated drinks a slightly tart, sharp bite that plain water doesn’t have. It’s the reason flat soda tastes noticeably sweeter and less interesting than freshly opened soda: without carbonation, there’s no carbonic acid adding that edge.
Your tongue actually detects carbonation through the same cells that sense sour flavors. Researchers at the National Institutes of Health found that an enzyme sitting on the surface of sour-sensing taste cells acts as the primary CO2 sensor. This enzyme converts dissolved CO2 into carbonic acid right at the cell surface, producing a localized burst of acid that activates the sour taste pathway. When researchers removed these sour-sensing cells in mice, the animals lost all ability to taste carbonation. So the “fizzy” sensation you feel is partly a tactile experience from bursting bubbles and partly a genuine taste, routed through your sour receptors.
Why Temperature Matters
Cold pop holds its fizz much better than warm pop, and the reason is straightforward: gas dissolves more readily in cold liquid. At near-freezing temperatures (0°C), water can hold roughly twice as much dissolved CO2 as it can at room temperature (25°C). This is why a warm can of soda explodes with foam when opened, releasing gas aggressively, while a cold one opens with a controlled hiss.
It’s also why bartenders and soda manufacturers chill beverages before carbonating them. Starting cold means more CO2 can be pushed into the liquid, producing a fizzier result. If you want your pop to stay carbonated longer after opening, keeping it cold is the single most effective thing you can do.
Not All Fizzy Drinks Are Equal
Carbonation levels vary dramatically across beverages, measured in “volumes of CO2,” which describes how many volumes of gas are dissolved in one volume of liquid. The minimum level people can even detect is about 0.6 volumes. From there, the range is wide:
- British ales: 1.5 to 2.2 volumes, barely fizzy
- Typical lager beer: 2.4 to 2.6 volumes
- Club soda and tonic water: 2.5 to 3.5 volumes
- Most soft drinks: 3 to 3.5 volumes
- Coke and Pepsi: 3.5 to 4 volumes
- Champagne: 4.6 to 6 volumes
- German wheat beer: around 5 volumes
Champagne, surprisingly, is often fizzier than most sodas. German wheat beer rivals champagne. The maximum recommended carbonation for any beverage is 8 volumes of CO2, beyond which containers risk failure under the pressure needed to keep all that gas dissolved.
Why Pop Goes Flat
Every time you open a bottle, you let pressurized CO2 escape. Resealing the bottle helps, but it can never fully restore the original pressure. Each opening-and-closing cycle lets more gas out, and the drink gradually loses its fizz.
The container itself matters too. Glass is essentially impermeable to gases, forming a near-perfect barrier. Aluminum cans perform similarly well. Plastic bottles, though convenient, are a different story. PET plastic (the material used for most soda bottles) allows small amounts of gas to slowly pass through its walls over time. That’s why a two-liter plastic bottle of soda loses carbonation faster than the same drink in glass, even if neither has been opened. For the longest-lasting fizz, cans and glass bottles win.
Surface area also plays a role. A two-liter bottle that’s half empty has a large pocket of low-pressure air sitting above the liquid, pulling CO2 out of solution. Smaller containers with less headspace keep drinks fizzier longer, which is one practical reason single-serve cans tend to taste more carbonated than the last glass poured from a big bottle that’s been in the fridge for two days.