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Posted by on Friday, October 30, 2009 at 8:34pm.

When we breathe we inhale oxygen, O2, and exhale carbon dioxide, CO2, plus water vapor, H2O. Which likely has more mass, the air that we inhale or the same volume of air we exhale? Does breathing cause you to lose or gain weight?

I thought that the air we breathe out has more mass since the water vapor has more mass than the CO2 and O2 gasses. This should cause me to lose weight as I lose water. Is this right? Or am I missing something? Thanks.

  • Chemistry - , Friday, October 30, 2009 at 8:50pm

    I don't know that you are missing anything but I think it answer isn't complete. The air we inhale also has nitrogen (about 80%) and the density of air at STP is about 1.29 g/mL. The air we exhale is CO2 and H2O so that is much more dense I would think, even at an elevated temperature (the body T is about 98 or so F and air is about 75 or so F. I agree that the mass of air we exhale is more likely to weigh more than the air we inhale. I agree that we lose weight when we exhale. But we regain, at least part of it, when we take the next breath in. So yes, you lose water but you also lose CO2 plus the nitrogen taken in with the oxygen.

  • Chemistry - , Friday, October 30, 2009 at 10:32pm

    So what happens to the nitrogen we inhale?

  • Chemistry - , Friday, October 30, 2009 at 10:36pm

    It isn't used. The nitrogen there dilutes the oxygen in air; otherwise, we would be breathing pure oxygen (along with small amounts of other gases).

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