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Re: Density of gas mixture



On Tue, 20 Jul 1999, Leigh Palmer wrote:

It is like broadjumping in a dense fog in unfamiliar territory.

Speaking of fog, I've found that the terms "vapor" and "steam" are sources
of misunderstanding. I suspect "moist air" is another. In everyday
language, clouds and fog are "vapor," as is the white cloud that shoots
from a tea kettle's spout. If "vapor" means "cloud of suspended
droplets", and if moist air contains "vapor", then moist air is denser
than dry air. This is easily demonstrated by observing the white fog
produced by an ultrasonic humidifier while its internal fan is disabled.
The fog pours across the tabletop almost like a liquid. But isn't moist
air full of H2O gas only, and lacks suspended droplets? If so, then
humidifier fog is not moist air, and moist air is less dense than dry air
because of the low-mass H2O molecules.

If water vapor is dry gas with no suspended droplets, then water vapor not
a vapor under the common definition of "vapor". If it contains no
droplets, then water vapor is not wet, it is dry. For example when fog
evaporates, it changes from "vapor" into water vapor. Yet moist air is
full of evaporated water...

So foggy air is both "moist" and wet, and dry water vapor is not a vapor,
yet droplet-vapor makes air denser, while steam from a kettle is a visible
vapor, but steam *is* water vapor and therefor is not white and not a
vapor, and clouds are full of moist air which actually is dry vapor
between wet droplets, and moist air is not wet air. Gah!! Obviously the
problem is not simply one of prior misconceptions; there's also a serious
collision between scientific and common terminology.

Is water vapor made of tiny wet droplets? Is "moist air" wet or dry? Is
"steam" colored white or is it transparent & invisible? Is fog a vapor?
One path out of this morass is to temporarily stop using any terms which
have multiple definitions. Avoid "water vapor" and instead say
"evaporated water" or "H2O gas." Don't say "steam", say "cloud of
droplets." Avoid "moist air" and replace it with "air full of H2O gas."



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