Soda fizzes because of carbon dioxide gas dissolved under pressure. Inside a sealed bottle or can, CO2 is forced into the liquid at pressures typically ranging from 20 to 40 PSI, far above normal atmospheric pressure. When you crack the seal, that pressure drops instantly, and the dissolved gas rushes out of the liquid as bubbles. That’s the fizz.
How CO2 Gets Into the Liquid
Gas dissolves more readily in liquid when pressure is high. This relationship, known as Henry’s law, is the entire basis of carbonation. At a bottling plant, chilled water is exposed to CO2 at elevated pressures, forcing the gas molecules into solution. A typical soft drink is carbonated at roughly 25 to 30 PSI at near-freezing temperatures, which dissolves enough CO2 to give the drink about 3.7 to 4.2 volumes of gas per volume of liquid. A highly carbonated soda might be pressurized up to 39 PSI to reach 5 volumes of dissolved gas.
As long as the container stays sealed, the pressure inside holds the CO2 in solution. The liquid looks perfectly still. But the moment you open the cap, the pressure above the liquid drops to normal atmospheric levels, and the water can no longer hold all that gas. CO2 molecules begin escaping from the liquid, forming bubbles at any available surface.
Why Bubbles Form Where They Do
You’ve probably noticed that bubbles in a glass of soda tend to stream upward from specific spots, not from the liquid at random. Those spots are called nucleation sites: tiny imperfections, scratches, or bits of debris on the glass surface where gas molecules can cluster together and form a bubble large enough to break free. A perfectly smooth glass produces far fewer bubbles than one with microscopic roughness.
This is also why dropping a grain of salt or sugar into soda causes a sudden eruption of foam. Each tiny crystal provides thousands of microscopic surfaces where CO2 can nucleate all at once. The famous Diet Coke and Mentos geyser works on the same principle: the candy’s porous surface is covered in countless tiny pits that trigger rapid, massive bubble formation.
Temperature Changes Everything
Cold liquid holds gas much better than warm liquid. According to solubility data from the National Institute of Standards and Technology, water at 0°C (32°F) dissolves roughly twice as much CO2 as water at 25°C (77°F) under the same pressure. This is why a warm soda explodes with foam when opened while a cold one releases its gas more gently.
The practical takeaway is straightforward: a cold soda stays fizzier longer because the CO2 escapes more slowly. A warm soda loses its carbonation fast, both because the gas is less soluble and because higher temperatures give gas molecules more energy to escape. If you want your drink to hold its fizz, keep it cold, and keep the cap on until you’re ready to drink it.
What Sugar and Other Ingredients Do to Bubbles
The ingredients in soda don’t just add flavor. They also change how bubbles behave. Sugars interact with the liquid’s surface tension, which is the force that holds the skin of a bubble together. Research on foam stability has shown that increasing sugar concentration can lower surface tension in certain mixtures, making it easier for bubbles to form but also changing how long they last. This is one reason diet sodas, which replace sugar with artificial sweeteners, often seem to foam differently than regular sodas when poured.
Phosphoric acid and citric acid, common in colas and citrus sodas respectively, also influence how the dissolved CO2 interacts with the water. These acids contribute to the overall sharpness of the drink, layering on top of the sensation that the carbonation itself creates.
Why Fizz Feels Like a Sting
The tingling, slightly painful bite of carbonation isn’t just bubbles popping on your tongue. It’s a chemical reaction happening on your taste cells. When dissolved CO2 contacts the moist surfaces of your mouth, it reacts with water to produce a weak acid. That acid triggers specific pain-sensing nerve fibers in your mouth and nasal passages.
The key player is a receptor channel on sensory neurons called TRPA1, the same receptor that responds to the burn of cinnamon oil and mustard. Researchers found that when CO2 diffuses into these nerve cells, it lowers the pH inside the cell, essentially making the cell’s interior slightly more acidic. That internal acid shift activates TRPA1, which sends a mild pain signal to your brain. In experiments with mice that lacked this receptor, the neurons showed dramatically reduced responses to CO2, confirming that TRPA1 is the main molecular sensor responsible for that characteristic carbonation sting.
This means fizz is really two sensations layered together: the physical feeling of bubbles bursting against your mouth, and a genuine low-level pain signal from acid forming on your nerve endings. That combination of texture and sting is what makes flat soda taste so different from fresh, even though the flavor ingredients are identical.
Why Soda Goes Flat
A soda goes flat when enough CO2 has escaped from the liquid that there’s no longer enough gas to produce noticeable bubbles or that acidic bite. This happens fastest when the drink is warm, when it’s been poured into a wide glass (more surface area for gas to escape), or when the container has been opened and resealed repeatedly, letting pressure drop each time.
Every time you open a bottle, some of the pressurized CO2 above the liquid escapes. When you reseal it, the remaining dissolved CO2 slowly comes out of solution until a new, lower equilibrium is reached. After enough open-close cycles, the pressure inside the bottle is barely above atmospheric, and the drink can’t hold much dissolved gas at all. Keeping a half-empty bottle tightly sealed and refrigerated slows this process, but once the pressure is gone, so is the fizz.