Just like solids, gases can dissolve in water. Dissolved oxygen is especially important. Without the dissolved oxygen in water, rivers, lakes and oceans would be devoid of life because aquatic organisms like fish need the oxygen to survive. Consequently, factors affecting oxygen solubility are an important consideration for the health of aquatic environments.
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When you have a mixture of gases like air, the partial pressure is the amount of pressure any one of those gases would have if it alone took up the whole volume occupied by the mixture. As the pressure of the mixture increases, so too does the partial pressure, and as the concentration of a gas in the mixture increases, its partial pressure increases also. The solubility of oxygen in water is directly proportional to the partial pressure, so increasing the oxygen concentration or increasing the ambient pressure causes more oxygen to dissolve in the water.
Higher temperature usually (though not always) makes solids more soluble in water. With gases like oxygen, however, it has the opposite effect. It's easy to understand why if you think about the gas at the molecular level. The higher the temperature, the faster the molecules of oxygen are moving, and the easier it becomes for the oxygen molecules to escape the water's surface. Consequently, increasing temperature causes oxygen to become less soluble, and oxygen is more soluble in cold water.
Henry's Law is a simple equation that relates the solubility of a gas like oxygen (in moles per litre) to its partial pressure at a given temperature. The equation is normally written s = kH P, where s is solubility, kH is a constant that depends on the gas, solvent and temperature, and P is the partial pressure. For oxygen in water at 20 degrees Celsius, for example, the kH constant is 1.3 x 10^-3 moles per litres atm. So if you know the partial pressure of oxygen at that temperature you can readily calculate its solubility.
High humidity reduces the partial pressure of the oxygen in the air very slightly, so high humidity reduces oxygen solubility. Sometimes water may be oxygen-poor for reasons having little to do with the solubility. During algae blooms, for example, bacteria decomposing the dead algae use up most of the available oxygen in the water, creating an oxygen-poor environment. In a situation like this, the problem is not that the oxygen is less soluble per se --- rather, that it's being consumed rapidly by the decomposers in the ecosystem.
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