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Name:Rita
Status: student
Grade: 9-12
Location: N/A
Country: USA
Date: October 2007


Question:
How does boiling water takes out the oxygen dissolved in it? And why does distilled water still contain dissolved oxygen after the process of distillation?



Replies:
Rita,

Boiling only partially removes the oxygen (or any gas) dissolved in water. This is because the process is not technically a distillation. Distillation suggests that the two substances being separated have different boiling points in the solution and that there is no way that the separated substances can redissolve into each other. Since the distillation process is done in air (in the presence of oxygen) then even if the oxygen were removed during the distillation process, it could still come back in since the water is constantly in contact with air.

The only reason that oxygen is removed in the distillation process is that the amount of oxygen gas dissolved in water is a function of the temperature of the water. The higher the temperature, the lower the amount of dissolved oxygen. So, during the distillation process, the gas is partially removed, but when the water cools down, since it is still in the presence of oxygen, the oxygen goes back into the water.

It is possible to remove practically all the oxygen in the water with an application of heat and reduced pressure. The heat reduces the amount of oxygen that may be dissolved, and by applying a vacuum, any removed oxygen is not allowed to go back into the water. Then, the air is replaced by some inert gas, like argon or nitrogen, and, for as long as the container is never opened to the air, there is very little oxygen remaining.

Greg (Roberto Gregorius)


Warm water is less able to hold oxygen than cooler water. As water boils, it makes all of the molecules move faster, increasing the number of collisions between them. The oxygen molecules leave the water more often. Gases become less soluble as temperature increases. I am sure that during the process of distillation, the same phenomenon occurs. As soon as the water cools however, gases can become more soluble and oxygen becomes dissolved in it again.

Van Hoeck


The solubility of all gases decreases with increasing temperature. This is an experimental observation that can be formulated using thermodynamics, but nonetheless, it is an experimental fact. The solubility of a gas also decreases as the partial pressure of the gas decreases, at a given temperature. This is a verbal statement of Henry's Law, which in the end is also an experimental observation. Those are just "laws of mother nature" about the solubility of gases.

Water, distilled in the presence of air -- the normal way you would do a distillation -- the distillate would still remain in contact with air and so at equilibrium would contain the amount of dissolved gases (not just oxygen) allowed by Henry's Law.

However, if a scientist wants REALLY pure distilled water, the distillation is carried out under vacuum so that there are no atmospheric gases present -- and the water would be distilled several (usually 3) times to ensure all the atmospheric gases are removed. The distillate is also kept under vacuum so that it does not come into contact with air. As an aside, getting highly purified water is experimentally challenging since even glassware can contaminate the water.

Vince Calder



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