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Name: Kyoko
Status: other
Grade: 8
Location: CA
Date: July 2008


Question:
I like to know the best way of explaining oxygen density per cubic feet become less in higher altitude. I tried to explain like this... "On the ground level, let us say you have a 10 x 10 balloon. If the same balloon go up to higher altitude, balloon expands because of less pressure from outside. But the content of oxygen remains the same (floating in the expanded balloon). Therefore, when you look at same cubic feet, oxygen count is less per cubic feet in higher altitude..." Does anybody have better explanation? I am trying to teach this to my 8th grade cousin.



Replies:
Kyoko,

I find that since students have a good idea about gravity, I can have them imagine a column of air stretching from the ground all the way to space and have them answer the question of where there would be more air (as an effect of gravity). It gets a little complicated when combined with the "ideality" of gases, but that can be resolved by invoking real gases.

Greg (Roberto Gregorius)


I think where you are getting confused is that the volume of air, and its oxygen, both expand proportionally with the decrease in external pressure. The relative amounts of nitrogen and oxygen remaining the same, assuming no diffusion of either component across the walls of the balloon.

Although many students (and many of their teachers) fear using equations, this is an example where a formula helps explain most clearly what is happening. The equation relating the pressure, volume, amount of gas, and temperature is: P x V = N x R x T where P is the pressure which in this case assume that the TOTAL gas pressure is the same as the outside pressure, V is the TOTAL volume of the gas, N is a measure of the TOTAL amount of gas in the balloon, R is a constant, and T is the temperature of the gas, which in this case can be assumed to be the same as the surrounding atmosphere. The TOTAL amount of gas present, N, is the sum of the amount of each gas present. The density of the gas N / V that is, the amount of gas per unit volume then becomes: N / V = P / R x T decreases as P decreases, but the relative density of each component -- N2, O2, etc. -- decreases in the same proportion.

Vince Calder



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