Boiling and Removing Dissolved Gases
Date: October 2007
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?
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.
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.
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Update: June 2012