Oxygen will dissolve in water.
Atmospheric oxygen (O2) will dissolve in water at ambient temperatures and normal atmospheric pressure. No chemical reaction takes place and the dissolved O2 will remain available for respiration by aquatic animals. The primary source of dissolved O2 in waterways is from the atmosphere and plant photosynthesis.
Dissolving Oxygen in Water
The amount of O2 that can be dissolved in water depends on a variety of factors. The thermodynamic properties of temperature and pressure define how much O2 can theoretically be dissolved in pure water. On a more practical level, water agitation and plant life can play an important role as well. For example, dissolved O2 levels in a stream will rise during the daytime because the surrounding forest is photosynthetic ally active, but decrease at night when the energy of the sun is absent.
The amount of oxygen in stagnant water will vary considerably by depth. Since the dissolution process of atmospheric gases occurs at the surface, this level tends to have the highest concentrations of O2. Although O2 will diffuse to lower levels, deeper depths have higher pressures and will therefore permit less dissolved O2 to enter. Since temperature also varies daily and seasonally, this too controls how much O2 can be dissolved from the atmosphere. Therefore, daytime and summer O2 levels in stagnant bodies of water tend to be lower than nighttime and winter levels.
Water can also become supersaturated with O2. For example, algae and weeds can supersaturate the water with O2 due to photosynthetic activity. Equilibrium in this case can only be achieved by releasing O2 into the atmosphere. Dissolved O2 levels can sometimes exceed 200% of what would be expected from the surface interacting with the air. 100% saturated water contains about 8.8 mg O2/L at sea level, so 200% would be 17.6 mg O2/L.
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“The chemistry of dissolved oxygen measurement.” Central Water Commission. N.p., n.d. Web. 3 Jan. 2013. http://cwc.gov.in/main/HP/download/11%20Chemistry%20of%20DO%20measurement.pdf.
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