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I've read that the partial pressure of oxygen in water will be the same as the partial pressure of oxygen in the atmosphere, but that the saturation of oxygen in water is dependent on factors like salinity and temperature. If the partial pressure of oxygen in water is equal to its partial pressure in the atmosphere, how is it possible that the concentration of dissolved oxygen in water can vary?

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Oxygen will tend to cross the water-air interface in the direction of lower partial pressure. For example, if you extract all of the oxygen from an air volume (perhaps by burning a candle in a sealed container), then you would expect dissolved oxygen to diffuse out of the water to make up the deficit.

However, water can’t hold an infinite amount of dissolved oxygen. So if the water is saturated with dissolved oxygen, the oxygen in the air has nowhere in the water to dissolve to.

Compare to dew formation, which occurs when air gets saturated with water vapor. In a sealed container, liquid water will tend to evaporate until the air is saturated (or the liquid is gone). But if your sealed container has a warm end and a cold end, you’ll tend to get condensation at the cold end, where the saturation pressure is lower.

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  • $\begingroup$ Thank you! So, if you decrease the solubility of oxygen in water, say, by increasing the salinity, will this cause oxygen to diffuse out of the water, and increase the partial pressure of oxygen in the air? $\endgroup$
    – Elhammo
    Commented Oct 5, 2022 at 19:49
  • $\begingroup$ Apparently so. If you’re not particular to oxygen, some reading suggests you can test this by adding salt to carbonated water, which increases the rate at which the dissolved gas escapes the liquid. $\endgroup$
    – rob
    Commented Oct 6, 2022 at 3:16

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