Nonpolar Gases Are Poorly Soluble in Water

The molecules of the biologically important gases CO₂, O₂, and N₂ are nonpolar. In O₂ and N₂, electrons are shared equally by both atoms. In CO₂, each CUO bond is polar, but the two dipoles are oppositely directed and cancel each other (Table 1.1). The movement of molecules from the disordered gas phase into aqueous solution constrains their motion and the motion of water molecules and therefore represents a decrease in entropy. The nonpolar nature of these gases and the decrease in entropy when they enter solution combine to make them very poorly soluble in water (Table 1.1). Some organisms have water-soluble carrier proteins (hemoglobin and myoglobin, for example) that facilitate the transport of O₂. Carbon dioxide forms carbonic acid (H₂CO₃) in aqueous solution and is transported as the HCO₃^- (bicarbonate) ion, either free bicarbonate is very soluble in water (~100 g/L at 25 ^οC) or bound to hemoglobin. Two other gases, NH₃ and H₂S, also have biological roles in some organisms; these gases are polar and dissolve readily in water.

TABLE 1.1 Solubilities of Some Gases in Water