It is a common question whether water, seemingly a simple compound of hydrogen and oxygen, contains “air.” While water is indeed H₂O, natural water bodies are not simply pure water. They contain various atmospheric gases that are dissolved within them, rather than existing as floating bubbles. This process of gases dissolving in water is a fundamental aspect of aquatic environments and plays a significant role in many natural and industrial processes.
Gases Dissolved in Water
The “air” in water primarily consists of atmospheric gases like oxygen, nitrogen, and carbon dioxide. Nitrogen is the most abundant atmospheric gas, and it is also present in dissolved form. Oxygen, though less abundant, dissolves readily. Carbon dioxide also dissolves, forming carbonic acid, which can influence water’s pH.
The ability of different gases to dissolve in water varies, a property known as solubility. Gases like ammonia are highly soluble because they form hydrogen bonds with water molecules. Carbon dioxide also dissolves well due to its reactivity with water. In contrast, gases like nitrogen are less soluble due to their chemical inertness. Colder water generally holds more dissolved gas than warmer water.
How Gases Enter and Leave Water
Gases primarily enter water through direct contact with the atmosphere, a process known as aeration. Wind and waves create surface turbulence, increasing air-water contact and allowing gases to dissolve. Aquatic plants also contribute; during photosynthesis, they absorb carbon dioxide and release oxygen. Gas pressure above the water also influences dissolution, with higher pressure leading to greater solubility.
Gases can leave water through several mechanisms. An increase in water temperature reduces gas solubility, causing dissolved gases to escape, which is why heating water often leads to the formation of bubbles. Changes in pressure can also cause gases to be released; for instance, opening a carbonated beverage reduces the pressure, allowing carbon dioxide to escape as fizz. Additionally, aquatic organisms consume oxygen and release carbon dioxide during respiration, influencing the balance of dissolved gases.
Why Dissolved Gases Matter
Dissolved gases are important for the survival of aquatic life. Dissolved oxygen is important, as fish and other aquatic organisms rely on it for respiration, much like terrestrial animals breathe oxygen from the air. When oxygen levels become too low, a condition known as hypoxia, it can lead to stress or death for aquatic species.
Dissolved gases also have human applications. Aeration is a common process in drinking water treatment, removing undesirable dissolved gases like carbon dioxide and improving taste and odor. Carbonated beverages, such as soda, contain dissolved carbon dioxide, creating their characteristic fizz and tangy taste. For divers, understanding dissolved gases is important to prevent conditions like “the bends,” where nitrogen gas forms bubbles in the bloodstream if a diver ascends too quickly. The increasing amount of dissolved carbon dioxide in oceans also contributes to ocean acidification, which threatens marine ecosystems.