Carbonated water is plain water with carbon dioxide gas dissolved into it under pressure. When you open the bottle or can, that pressure drops, and the gas escapes as bubbles. The chemistry behind this process is straightforward, but the way your body perceives and responds to those bubbles involves some surprisingly complex biology.
How CO2 Gets Into Water
Carbonation relies on a basic principle of gas physics: the higher the pressure, the more gas you can dissolve into a liquid. At normal atmospheric pressure, water holds only a tiny amount of CO2. But when manufacturers pump CO2 into water at elevated pressures, the gas molecules are forced into the liquid and stay there as long as the container remains sealed.
Temperature matters too. Cold water holds significantly more CO2 than warm water, which is why carbonated drinks go flat faster when they’re warm and why commercial carbonation happens at near-freezing temperatures. The combination of high pressure and low temperature is what gives a fresh bottle of sparkling water its intense fizz.
Once dissolved, CO2 doesn’t just float around as a gas trapped in liquid. A portion of it reacts with water molecules to form carbonic acid, a weak acid that lowers the pH of the water. Commercial carbonated waters typically land in a pH range of about 4.2 to 5.9, depending on how heavily they’re carbonated. For reference, plain water sits around pH 7.
Why It Tingles on Your Tongue
The fizzy, slightly stinging sensation of carbonated water isn’t just from bubbles popping on your tongue. Most of that characteristic bite is chemical, not mechanical. When CO2 reaches your mouth, an enzyme in your saliva called carbonic anhydrase converts it into carbonic acid. That acid then breaks apart into bicarbonate ions and free protons (hydrogen ions), which lower the pH inside the cells lining your mouth.
This drop in pH activates a specific pain-sensing ion channel on trigeminal nerve fibers in your oral tissue. These are the same nerve fibers that respond to mustard oil and cinnamon. Research published in The Journal of Neuroscience found that CO2 diffuses directly into cells and produces intracellular acidification, which triggers these channels at pH levels starting around 6.5 and reaching maximum activation at about pH 5.5. That’s why carbonated water produces a mild sting rather than just a neutral mouthfeel. Your nervous system is literally interpreting the acidity as a low-level pain signal, blended with the tactile sensation of rising bubbles.
This dual sensory pathway, chemical irritation plus physical bubbling, is what makes carbonation feel so different from simply drinking cold or flavored water.
What Happens in Your Stomach
When carbonated water reaches your stomach, the dissolved CO2 expands as gas, temporarily increasing your total gastric volume. A study that measured stomach distension using imaging found that drinking 300 ml of a carbonated beverage significantly increased gastric volume compared to the same amount of still water, with most of the expansion concentrated in the upper portion of the stomach.
This expansion is short-lived. The gas is either absorbed through the stomach wall or released through belching, often within minutes. By the time you reach fullness during a meal, there’s no measurable difference in stomach volume between people who drank carbonated versus still water beforehand. Carbonated water also doesn’t appear to change the rate at which your stomach empties food into the small intestine.
The hormonal picture is more nuanced. Ghrelin, the hormone that signals hunger, showed different patterns depending on what people ate alongside their drinks. With a solid meal, both carbonated water and plain water suppressed ghrelin similarly. With a degassed version of the same beverage, ghrelin suppression was actually stronger before a solid meal, a counterintuitive finding that researchers noted didn’t ultimately change how much food people ate. In healthy people, carbonated water doesn’t appear to meaningfully alter appetite or food intake.
Hydration Compared to Still Water
Sparkling water hydrates you just as well as flat water. Randomized trials measuring urine output over several hours after drinking, a method used to calculate a “beverage hydration index,” found no difference between carbonated and non-carbonated water. The CO2 doesn’t interfere with absorption, and your kidneys process the fluid identically. If you prefer the taste and drink more water because of it, that’s a net positive for hydration.
Effects on Teeth
The carbonic acid in sparkling water does make it mildly acidic, and tooth enamel begins to demineralize when exposed to liquids below a pH of about 5.5. Some commercial carbonated waters fall below that threshold, particularly heavily carbonated varieties. A laboratory study found that higher carbonation levels produced greater reductions in enamel microhardness, while lower carbonation levels, especially in the presence of calcium ions, caused significantly less damage.
Context matters here. These studies typically involve prolonged, continuous exposure to carbonated water in a lab setting, without the protective effects of saliva, which neutralizes acids and remineralizes enamel in real time. Plain carbonated water is far less erosive than sodas, citrus juices, or sports drinks. The practical risk to your teeth from unsweetened sparkling water is low, though sipping it constantly throughout the day gives your enamel less recovery time than drinking it with meals.
Effects on Bones
The concern that carbonated water weakens bones has not held up in research. A study in the American Journal of Clinical Nutrition measured urinary calcium excretion after women drank various carbonated beverages and found that carbonation itself, whether from carbonic acid or phosphoric acid, produced no increase in calcium loss compared to plain water. The only beverages that raised urinary calcium were those containing caffeine, and even that effect was temporary, compensated by reduced calcium loss later in the day. The researchers concluded that any link between carbonated drinks and weaker bones is likely explained by people drinking soda instead of milk, not by the carbonation itself.
Carbonation and Swallowing
One practical application of carbonated water’s sensory effects is in helping people with swallowing difficulties. The chemical and tactile stimulation from CO2 activates multiple sensory pathways simultaneously: pain-sensing nerves in the mouth, acidity-sensitive taste receptors via the facial nerve, and the broader somatosensory system. This heightened sensory input has been shown to increase swallowing muscle activity and swallowing pressure, which can reduce the risk of food or liquid entering the airway in people with impaired swallowing reflexes. Clinicians sometimes use carbonated liquids as a therapeutic tool for this reason.