Carbon monoxide (CO) is only very slightly soluble in water. At 25°C and normal atmospheric pressure, about 1.48 mg dissolves per milliliter of water, which makes it a poor aqueous solute compared to gases like carbon dioxide. CO can technically form an aqueous solution, but the amount that actually dissolves is so small that it’s often described as practically insoluble for everyday purposes.
How Much CO Actually Dissolves
At 25°C, the mole fraction solubility of CO in water is roughly 0.000057, according to IUPAC reference data. To put that in perspective, carbon dioxide is about 26 times more soluble than oxygen in water at the same temperature, and CO sits even lower on the solubility scale than oxygen. You’d need to dissolve enormous volumes of CO gas to get a meaningful concentration in water.
The Henry’s law constant for CO at 25°C is approximately 0.00095 mol per kilogram of water per bar of pressure. That number tells you how much gas dissolves at a given pressure, and for CO it’s extremely low. In practical terms, if you bubbled pure CO through a glass of water at room temperature, the vast majority of the gas would simply pass through and escape.
Why CO Barely Dissolves in Water
Water is a highly polar molecule, and it dissolves polar or ionic substances best. CO has a triple bond between its carbon and oxygen atoms, but its overall polarity is surprisingly low. One of those three bonds is a “dative” bond where both electrons come from the oxygen atom. This unusual bonding shifts electron density in a way that actually counteracts oxygen’s natural pull on electrons, leaving the molecule with a very small net dipole moment. The result is a nearly nonpolar molecule that doesn’t interact strongly with water.
This is the same reason CO dissolves more readily in organic solvents than in water. Nonpolar molecules mix well with other nonpolar substances but are essentially excluded by water’s tight hydrogen-bonding network.
Temperature Changes the Picture Slightly
Like most gases, CO becomes less soluble as water temperature rises. Measured solubility data spans from 0°C to 80°C, and the trend is consistent: colder water holds more dissolved CO, warmer water holds less. Between 0°C and 30°C, the change is modest because the baseline solubility is already so low. This is the same principle behind why a cold soda stays fizzy longer than a warm one, though with CO2 the effect is far more noticeable because so much more of it dissolves in the first place.
CO Can React With Water Under Extreme Conditions
While CO mostly just sits as dissolved gas molecules in water at normal conditions, it can chemically react with water at high temperatures. This is the water-gas shift reaction: CO combines with water to produce hydrogen gas and carbon dioxide. Industrial facilities run this reaction between roughly 200°C and 480°C (400°F to 900°F) using metal catalysts. It’s a key step in hydrogen production and synthetic fuel manufacturing. At room temperature, though, this reaction doesn’t happen to any meaningful degree, so dissolved CO simply remains as CO.
Why This Matters in Biology
The low aqueous solubility of CO has real consequences in the body. Very little CO stays dissolved in blood plasma as a free gas. Instead, hemoglobin picks it up with 200 to 300 times the affinity it has for oxygen, forming carboxyhemoglobin. This is what makes carbon monoxide poisoning so dangerous: even tiny amounts of CO in inhaled air get locked onto hemoglobin and block oxygen from binding. Once a single CO molecule attaches, it also changes the shape of hemoglobin in a way that makes the remaining binding sites grip oxygen more tightly, making it harder for tissues to extract the oxygen they need.
So while CO is technically present in aqueous solution in the blood, the dissolved fraction is negligible compared to the amount bound to hemoglobin. The gas’s poor water solubility and extreme hemoglobin affinity work together to create its outsized toxic effect.