Soda is bubbly because of carbon dioxide gas dissolved into the liquid under high pressure. Inside a sealed can or bottle, the pressure of CO2 above the liquid sits around 1.18 atmospheres, roughly three thousand times higher than the trace amount of CO2 in the open air (about 0.0004 atmospheres). That pressure difference is the entire reason fizz exists.
How CO2 Gets Into the Liquid
At bottling plants, carbon dioxide is forced into the beverage using microporous stones or inline carbonation systems that break the gas into extremely fine bubbles. These tiny bubbles dissolve into the liquid far more efficiently than large ones would, because they create more surface area for the gas to contact the water. The whole process happens inside sealed, pressurized equipment, and the drink is capped or sealed before that pressure can escape.
The amount of CO2 varies by product. Most soft drinks contain between 2.5 and 4 “volumes” of CO2, a unit that describes how much gas is packed into the liquid. Colas like Coke and Pepsi sit at the higher end, around 3.5 to 4 volumes. Club soda and tonic water range from 2.5 to 3.5. That’s why a cola feels more aggressively fizzy than tonic water, even when both are freshly opened.
Why Opening the Bottle Releases Bubbles
The physics here follow a principle called Henry’s Law: the amount of gas that can stay dissolved in a liquid is directly proportional to the pressure of that gas above the liquid. Double the pressure, double the solubility. Inside a sealed bottle, the high CO2 pressure keeps a large amount of gas dissolved. The system is in equilibrium, meaning CO2 molecules are entering and leaving the liquid at equal rates, so nothing visible happens.
The moment you crack the cap, that equilibrium breaks. The CO2 pressure above the liquid plummets from 1.18 atmospheres to essentially zero. The liquid is now holding far more dissolved gas than it can sustain at normal atmospheric pressure, so CO2 begins rushing out. That’s the hiss you hear, and the bubbles you see are the visible evidence of gas escaping.
Where Bubbles Actually Form
Bubbles don’t just appear anywhere in the liquid. They need a starting point, and this is where nucleation sites come in. These are tiny imperfections: microscopic scratches on the inside of a glass, bits of dust, lint fibers, or small pockets of trapped air on surfaces submerged in the drink. Each of these provides a pre-existing gas pocket where dissolved CO2 can gather, grow into a visible bubble, and eventually detach.
For a bubble to survive and grow, its starting cavity needs to be large enough. In a freshly opened soda at room temperature (with about 6 grams of dissolved CO2 per liter), the opening of the cavity needs a radius of at least 0.5 micrometers, roughly one-hundredth the width of a human hair. Smaller cavities can’t overcome the surface tension of the liquid, and any gas pocket that forms simply collapses back into solution. Larger cavities readily support repeated cycles of bubble growth and release, which is why you’ll often see a steady stream of bubbles rising from a single spot on the wall of a glass.
This is the same principle behind the famous Diet Coke and Mentos eruption. Mentos candies have surfaces covered in cone-shaped pits with openings well above that critical radius, providing thousands of nucleation sites all at once. The result is an explosive, simultaneous release of CO2.
Why Fizz Feels Like a Tingle
The sensation of carbonation isn’t just the physical popping of bubbles on your tongue. A significant part of what you feel is chemical. When CO2 dissolves in the moisture on your tongue, an enzyme in your saliva called carbonic anhydrase converts it into carbonic acid. This mild acid activates pain-sensing nerve endings in your mouth, which send signals to the brain through the same nerve pathway that handles sensations like spicy heat and the burn of strong mustard.
So carbonation is, in a literal sense, a very mild form of pain. The “bite” of a fizzy drink comes from localized drops in pH on the surface of your tongue, not just from bubbles bursting. This is why flat soda that still contains some dissolved CO2 can still taste slightly sharp, and why carbonated water in a pressurized chamber (where no bubbles form) still produces a tingle.
Why Warm Soda Goes Flat Faster
Temperature plays a major role in how well a liquid holds onto dissolved gas. Cold liquids retain CO2 much more effectively than warm ones. As temperature rises, dissolved CO2 molecules move faster, making it easier for them to escape from the liquid’s surface. The diffusion rate of CO2 through the liquid increases steadily with temperature, accelerating the entire degassing process.
This is why a warm soda erupts more dramatically when opened and why a cold one stays fizzy longer in the glass. It’s also why bartenders chill carbonated beverages before serving: not just for taste, but because the carbonation physically lasts longer at lower temperatures. If you want to keep a soda as fizzy as possible, refrigerate it and minimize the time the bottle is open. Every second the liquid is exposed to the low-pressure atmosphere, CO2 is leaving.
Why Soda Eventually Goes Flat
Once a bottle is opened, the carbonation clock starts ticking. CO2 escapes the liquid in two ways: through bubbles rising from nucleation sites and through direct diffusion from the liquid’s surface into the air. Both processes continue until the dissolved CO2 reaches a new equilibrium with the atmosphere, which, given that atmospheric CO2 pressure is only 0.0004 atmospheres, means almost all of the gas eventually leaves.
Resealing the bottle slows this down by trapping some CO2 in the headspace above the liquid, partially restoring pressure. But each time you open the bottle, you release that built-up gas and restart the process. A two-liter bottle that’s been opened and reclosed several times will go flat much faster than one opened once, because each opening reduces the total amount of CO2 remaining in the system. The liquid can never get fizzier than it was at the moment of the last seal, only less so.