Yes, carbon dioxide is soluble in water. At room temperature and normal atmospheric pressure, about 1.5 grams of CO2 dissolve in one liter of water. That’s moderate solubility, enough to make carbonated drinks possible but far less than highly soluble gases like ammonia. What makes CO2 interesting is that it doesn’t just dissolve physically. It also reacts with water to form a weak acid, which is why carbonated water tastes slightly tart.
What Happens When CO2 Meets Water
When carbon dioxide dissolves in water, most of it stays as intact CO2 molecules sitting among the water molecules. But a small fraction undergoes a chemical reaction: CO2 combines with water to form carbonic acid. Carbonic acid is unstable and quickly splits into a hydrogen ion and a bicarbonate ion. This chain of reactions is why adding CO2 to pure water makes it mildly acidic, dropping the pH from 7 down to roughly 5.6.
Only about 0.3% of dissolved CO2 actually converts to carbonic acid at any given moment. The rest remains as dissolved gas, ready to escape back into the air when conditions change. This is exactly what you see when you open a bottle of soda: the dissolved CO2 that was held in place by pressure rapidly comes out of solution as bubbles.
How Temperature Changes Solubility
Cold water holds significantly more CO2 than warm water. This is the opposite of how most solid substances behave in water (sugar dissolves faster in hot tea), but it’s typical for gases. As water warms up, its molecules move faster and are less able to keep gas molecules trapped in solution.
Data from the CRC Handbook of Chemistry and Physics illustrates this clearly. At standard atmospheric pressure, the mole fraction of CO2 in water at 0°C is 1.337 (per thousand). At 20°C, it drops to 0.704, nearly half. By 40°C, it falls further to 0.431. In practical terms, ice-cold water can hold roughly three times as much CO2 as water at 40°C. This is why warm soda goes flat faster and why carbonated beverages are always served cold.
How Pressure Changes Solubility
Pressure has an even more dramatic effect than temperature. The relationship follows a principle called Henry’s Law: the amount of CO2 that dissolves in water is directly proportional to the pressure of CO2 above the liquid. Double the pressure, double the dissolved gas. At 25°C, the Henry’s Law constant for CO2 in water is 1.67 × 10⁸ Pa, which means you need substantial pressure to force large amounts of CO2 into solution.
This is the entire basis of the carbonated beverage industry. Inside a sealed can of Coke, the CO2 pressure is well above atmospheric levels. When you crack the seal, the pressure drops to normal atmospheric levels, and the water can no longer hold all that dissolved gas. The excess CO2 rushes out as fizz.
How Carbonated Drinks Use This
Beverage manufacturers measure carbonation in “volumes” of CO2, meaning how many volumes of gas (at standard conditions) are dissolved in one volume of liquid. Different drinks target different levels. Lemon-lime sodas typically run 2.5 to 3.5 volumes. Colas like Coke and Pepsi are more heavily carbonated at 3.5 to 4 volumes. Tonic water sits around 3 to 3.5 volumes.
To achieve high carbonation, manufacturers chill the water (usually to around 5°C) and pressurize it. Hitting 5 volumes of CO2, for example, requires about 38 PSI of gauge pressure at that temperature. The combination of cold temperatures and high pressure forces far more CO2 into solution than water would naturally hold at room conditions. Sealing the container maintains that pressure until you open it.
CO2 Solubility in the Ocean
The same chemistry that makes soda fizzy operates on a planetary scale. The world’s oceans absorb an estimated 5 to 10 gigatons of carbon from the atmosphere every year, making them one of Earth’s most important carbon sinks. Cold polar waters absorb the most CO2, consistent with the temperature effect described above. As those waters warm due to climate change, their capacity to absorb CO2 decreases, which could accelerate the buildup of greenhouse gases in the atmosphere.
Once dissolved in seawater, CO2 follows the same reaction pathway: it forms carbonic acid, which splits into hydrogen ions and bicarbonate. The extra hydrogen ions lower the ocean’s pH, a process known as ocean acidification. Since the start of the industrial era, ocean surface pH has dropped by about 0.1 units, representing a roughly 30% increase in acidity. This shift affects shell-building organisms like corals and mollusks, which rely on specific water chemistry to form their calcium carbonate structures.
CO2 Solubility in Your Blood
Your body relies on the same dissolving chemistry to move CO2 from your cells to your lungs. When your cells burn fuel for energy, they produce CO2 as waste. That CO2 diffuses into your bloodstream, where it’s transported in three different forms. About 10% stays dissolved directly in plasma, much like CO2 dissolved in a glass of water. Another 10% binds to hemoglobin, the protein in red blood cells that also carries oxygen.
The remaining 80% takes a more complex route. Inside red blood cells, an enzyme dramatically speeds up the reaction between CO2 and water, converting it to carbonic acid, which immediately splits into bicarbonate and a hydrogen ion. Bicarbonate is the primary transport form of CO2 in your blood. When that bicarbonate-rich blood reaches your lungs, the reaction reverses: bicarbonate converts back to CO2 gas, which you exhale. Your body processes about 200 milliliters of CO2 per minute at rest through this system.