Boiling water removes oxygen by significantly reducing the amount of dissolved gases present in the liquid. This process, driven by heat, forces the oxygen molecules out of solution and into the air. The resulting water is scientifically distinct from its original state, lacking the gaseous components that are naturally absorbed from the atmosphere.
What is Dissolved Oxygen
Water in its natural state contains oxygen physically mixed into the liquid, known as dissolved oxygen (DO). This is gaseous, diatomic oxygen (O2), which is the same molecule found in the air we breathe, not the oxygen atom chemically bonded to hydrogen to form the water molecule (H2O).
This free oxygen enters the water primarily through two natural processes: direct absorption from the atmosphere and as a byproduct of aquatic plant photosynthesis. Dissolved oxygen is a necessary component for most aquatic organisms, including fish and bacteria, which use it for respiration. The amount of gas water can hold is its saturation level, influenced by temperature and atmospheric pressure.
The Mechanism of Oxygen Removal
The removal of dissolved oxygen from water is a direct consequence of the inverse relationship between temperature and gas solubility. As water is heated, the kinetic energy of the water molecules increases, causing them to move faster and further apart. This increased motion breaks the weak attractive forces, or hydrogen bonds, that temporarily hold the gas molecules in solution.
This phenomenon begins long before the water reaches its boiling point. The dissolved gases, including oxygen and nitrogen, start to come out of solution, a process called exsolution. This leads to the formation of micro-bubbles on the sides of the container or heating element. These initial bubbles, known as nucleation sites, are composed of the expelled air, not water vapor.
Once the water reaches a vigorous boil, the combined effect of high temperature and constant agitation ensures maximum gas expulsion. The partial pressure of the dissolved gases in the liquid drops rapidly as the gas escapes into the atmosphere. To achieve the most complete deoxygenation, the water must be boiled vigorously and held at a rolling boil for several minutes, rather than simply brought to a brief simmer.
Practical Effects of Boiling Water
The most noticeable effect of boiling water is a change in taste, often described as “flat” or dull. This alteration is due to the significant loss of all dissolved gases, primarily oxygen and carbon dioxide, which contribute to water’s crisp, fresh flavor. Once the water has cooled, the lack of these gaseous components leaves the water tasting noticeably different from unboiled tap water.
The practical importance of this deoxygenation is most evident in the context of aquatic life. Water that has been boiled and then cooled is hazardous to fish and other aquarium inhabitants because it contains insufficient dissolved oxygen for them to breathe. The fish would quickly suffocate in the deoxygenated water, regardless of the temperature being safe.
Fortunately, the process is reversible, and the water will naturally re-absorb oxygen from the air until it reaches a new equilibrium. This process of re-aeration can be accelerated by agitating the cooled water, such as by pouring it back and forth between two containers or by vigorous stirring. This simple action reintroduces the lost gases, restoring the water’s original, more palatable flavor and making it safe for aquatic use again.