Nitrogen gas (\(\text{N}_2\)) is the most abundant component of Earth’s atmosphere, making up approximately 78% of the air we breathe. Nitrogen is soluble in water, though only to a slight degree. Dissolved nitrogen is a constant feature in all natural bodies of water, from oceans to freshwater lakes, due to continuous exchange at the water’s surface. The amount of nitrogen that dissolves has profound implications for both the planet’s chemistry and the life forms that inhabit aquatic environments.
The Chemistry Behind Nitrogen Solubility
The limited solubility of nitrogen gas is a consequence of the opposing chemical properties of the gas and water molecules. Nitrogen gas exists as a diatomic molecule (\(\text{N}_2\)) held together by a strong triple covalent bond. This structure results in an equal distribution of electron density, classifying the molecule as non-polar.
Water (\(\text{H}_2\text{O}\)), conversely, is a highly polar molecule, having partial negative and positive charges. The chemical rule of “like dissolves like” dictates that non-polar substances do not readily mix with polar substances. The powerful hydrogen bonds holding water molecules together must be broken to create space for a dissolved gas molecule.
Since non-polar nitrogen cannot form strong hydrogen bonds or dipole-dipole attractions with water, the only available forces are the weak London Dispersion Forces. These forces arise from momentary shifts in electron distribution that create temporary dipoles in the \(\text{N}_2\) molecule. The energy required to disrupt the strong water-water hydrogen bonds is significantly greater than the energy released by forming these weak nitrogen-water forces. This energetic imbalance is the primary reason why nitrogen is considered sparingly soluble.
How Temperature and Pressure Affect Dissolution
While chemical structure determines why nitrogen is slightly soluble, external physical conditions dictate how much nitrogen will dissolve. Temperature is a major controlling factor, as the solubility of all gases in water decreases as temperature rises. The process of a gas dissolving into a liquid is exothermic, meaning it releases heat.
When water temperature increases, the kinetic energy of the water and dissolved gas molecules increases, causing them to move faster. This extra energy allows the nitrogen molecules to overcome the weak London Dispersion Forces and escape the liquid phase back into the atmosphere.
Pressure is the second variable that directly controls the quantity of dissolved nitrogen, following Henry’s Law. This law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas immediately above the liquid. If the partial pressure of nitrogen above the water is doubled, the concentration of dissolved nitrogen will also double to maintain equilibrium. This relationship also explains several physiological phenomena in deep-sea environments.
Environmental Significance of Dissolved Nitrogen
The slight solubility of nitrogen has significant consequences for aquatic ecosystems and human physiology. In natural waters, dissolved \(\text{N}_2\) serves as the starting point for the aquatic nitrogen cycle, a process essential for supporting life. Most aquatic organisms, including plants and algae, cannot directly utilize dissolved \(\text{N}_2\) due to its triple bond structure.
The dissolved nitrogen gas must first be converted, or “fixed,” into biologically available compounds like ammonia (\(\text{NH}_3\)) and nitrate (\(\text{NO}_3^-\)). This transformation is performed by specialized microorganisms called diazotrophs, including certain bacteria and cyanobacteria. Once fixed, this nitrogen forms the base of the aquatic food web, fueling the growth of primary producers like phytoplankton and algae.
The pressure-solubility relationship defined by Henry’s Law also creates risks for organisms experiencing rapid pressure changes. In deep-sea divers, increased ambient pressure forces excess nitrogen to dissolve into tissues and the bloodstream. If a diver ascends too quickly, the pressure drops rapidly, causing dissolved nitrogen to form bubbles within the body. This leads to decompression sickness, commonly known as “the bends.” A similar phenomenon, called gas bubble disease, affects fish when water becomes supersaturated with gases, resulting in bubbles forming in their tissues and vessels.