Carbonation in soda comes from carbon dioxide gas dissolved into water under high pressure. When you crack open a can or bottle, that pressure drops, and the dissolved gas escapes as the familiar stream of bubbles. The process is straightforward, but the science behind why it works, why it fizzes on your tongue, and why a warm soda goes flat faster than a cold one is worth understanding.
How CO2 Gets Into the Water
The core principle is simple: gas dissolves more readily in liquid when you increase the pressure. This relationship, known as Henry’s Law, means that if you pump carbon dioxide into a sealed container of water at high pressure, the water absorbs far more CO2 than it could at normal atmospheric pressure. Bottling plants exploit this by chilling water and then forcing CO2 into it at pressures typically ranging from 25 to 40 PSI, depending on the product.
Different sodas get different levels of carbonation. Colas like Coke and Pepsi are carbonated to about 3.5 to 4 volumes of CO2 (meaning each volume of liquid holds 3.5 to 4 volumes of gas). Lighter citrus sodas like Sprite or 7UP tend to run lower, around 2.5 to 3.5 volumes. Tonic water sits at the lower end, around 3 to 3.5 volumes. These targets determine how much pressure the plant applies during bottling.
What Happens Inside the Can
Once the CO2 is dissolved and the container is sealed, the gas stays trapped in the liquid because the pressure inside the can matches the equilibrium needed to keep it dissolved. A sealed aluminum soda can holds an internal pressure of roughly 50 to 60 PSI at room temperature. In hot weather, that pressure can climb as high as 90 PSI, which is why cans left in a hot car sometimes bulge or even rupture.
The moment you open the can, the pressure above the liquid drops to normal atmospheric levels (about 14.7 PSI). The water is now holding far more CO2 than it can keep dissolved at that lower pressure, so the gas begins escaping. This is the hiss you hear, and it’s the start of your soda going flat.
Why Bubbles Form Where They Do
You’ve probably noticed that bubbles in a glass of soda don’t appear randomly. They stream upward from specific spots on the glass wall or bottom. These spots are called nucleation sites: tiny scratches, imperfections, or dust particles on the surface where dissolved CO2 molecules can gather and form a bubble large enough to detach and rise.
A perfectly smooth surface would actually produce very few bubbles. That’s why some beer glasses and champagne flutes are deliberately etched at the bottom, giving the dissolved gas reliable starting points. In a regular drinking glass, microscopic scratches from washing, fibers from a towel, or even a grain of salt can serve the same purpose. Dropping a mint or a rough-textured candy into soda creates an explosion of bubbles for exactly this reason: the candy’s surface is covered in thousands of tiny pits that act as nucleation sites all at once.
Why Cold Soda Stays Fizzier
Temperature has a dramatic effect on how well CO2 stays dissolved. Cold water holds gas much more effectively than warm water. According to data from the NIST Solubilities Database, water at 0°C (32°F) can dissolve roughly twice as much CO2 as water at 25°C (77°F) under the same pressure. This is why bottling plants chill the water before carbonating it, and why a warm soda tastes flat even if you haven’t opened it yet: the gas is already separating from the liquid inside the container, building up pressure in the headspace rather than staying dissolved.
This also explains the practical advice of keeping soda in the fridge. A cold soda releases its carbonation more slowly after opening, giving you more fizz per sip. A warm one loses its gas quickly, sometimes within minutes of pouring.
The Tangy Taste Isn’t Just Bubbles
Carbonation doesn’t just create a physical sensation. It changes the chemistry of the drink. When CO2 dissolves in water, a small portion of it reacts with the water molecules to form carbonic acid. This weak acid is what gives plain sparkling water its slightly tart, tangy flavor and drops the pH to mildly acidic levels. In soda, this acidity works alongside added flavors and sweeteners, but it’s carbonic acid that provides the sharp, clean bite distinct to carbonated drinks.
The “sting” you feel on your tongue is a separate mechanism from the bubbles popping. Research published in The Journal of Neuroscience showed that CO2 diffuses into cells on the tongue’s surface and causes a drop in pH inside those cells. This internal acidification activates pain-sensing nerve fibers (the same type that respond to mustard oil and cinnamon), triggering that familiar prickling, stinging sensation. So even if you could somehow remove all the bubbles while keeping the dissolved CO2, the drink would still tingle. The fizzy feeling is chemical, not just mechanical.
A Brief History of Fizzy Water
Humans have enjoyed naturally carbonated mineral springs for centuries, but artificial carbonation dates to 1767. Joseph Priestley, an English chemist, figured out that combining sulfuric acid with chalk produced a gas (which we now call carbon dioxide). He collected this gas in a pig’s bladder and used it to infuse water, creating the first artificially carbonated beverage. Within a few decades, entrepreneurs had scaled up the process, and by the late 1800s, flavored carbonated drinks were a commercial industry.
Modern soda production uses the same basic principle Priestley discovered: dissolve CO2 into water under pressure. The equipment is obviously more sophisticated, with precise temperature and pressure controls that let manufacturers hit exact carbonation targets, but the underlying chemistry hasn’t changed in over 250 years.