1.
> - its not
> raining anymore. But the puddle is getting smaller, i.e., H2O(l) -->
> H2O(g). The problem is describing the equilibrium situation
> at the puddle
> surface from T and P not at the surface and essentially an
> open system.
Let's consider *any* clear day with no puddles on the ground. What would
you call the gaseous phase of water that is present in the atmosphere?
I would call it gaseous phase water, aka, H2O(g). But I might slip up and
say water vapor, too.
2.
On more thing, I suspect "vapor" might be getting into the students
vernacular from relentless marketing of products directed toward
student-age persons or inhalers, where vapors of a liquid or a solid
are
inhaled.
>>>>>>>>>>>>
This inhaler reference changed my perspective on all this. I have
asthma, so I carry an inhaler when I'm active. The inhaler creates an
aerosol of the medication--that is, fine droplets of liquid, which is
quite different from a gas. (ie the usage above is not quite right, IMO.
The inhaler does NOT produce a vapor.) Are Tina's students confusing
aerosol and gas/vapor?
Here's one particularly bad dictionary definition, which will only
impede student learning:
aer7o7sol Pronunciation Key (br-stl, -sl)
n. A gaseous suspension of fine solid or liquid particles.
I don't think the definition is confusing: a gas (solvent) which contains
solid or liquid solute.
Two types of inhalers: 1) the type above, which do create an aerosol
composed of tiny droplets. These, as correctly pointed out, are not
"vapors" and I'm sorry for any confusion.
2) Inhalers of the back-alley type: glue, etc. I don't have any direct
experience with these, but the point of huffing glue is to inhale the
solvent vapors.
3.
> So, in my opinion, a "gas" is w/o any condensed phase present and a "vapor"
> is a gas in equilibrium with its liquid or solid or both.
That doesn't sound right to my ears.
I cite a few observations:
-- Thoughtful experts refer to gaseous H2O as "water vapor"
even in situations where it is nowhere near being in
equilibrium with any condensed phase. Examples include
meteorology and astronomy.
-- Thoughtful experts commonly refer to the "SVP" (Saturated
Vapor Pressure) to designate a vapor that is in
equilibrium with a nearby condensed phase. They consider
the word Saturated to be a necessary, non-redundant
qualifier.
-- Sodium vapor lamps and mercury vapor lamps operate nowhere
near the saturated vapor pressure.
I think the "experts" should call it water gas phase. In the context of
astronomy, where the molecules are far apart, gas or vapor is probably not
confusing. In meteorology, where H2O is constantly evaporating/condensing,
maybe vapor is more appropriate. (How far are you from the nearest
lake/ocean? Maybe water vapor should be reserved for the water that just
"evaporated" from liquid and is on it way to condensing over my
garden?) If the vapor pressure is not saturated, its not at equilibrium
with the condensed phase. I've used both "high" and "low" pressure Hg
lamps; the hp should contain Hg(l), lp, do not, but they really should be
called Hg gas lamps. The [vapor] pressure of Hg and Na are not really that
high (compared to H2O.) Break a fluorescent lamp and you will see a few
droplets of Hg.
For substances that are usually a gas at RT conditions, e.g., CH4, "gas" is
probably the best/only term. However, for substances that can have both
gas and condensed phases, perhaps "vapor" is better (i.e., CH4 vapor on
Jupiter.) A few days ago, somebody pointed out the differences between gas
and vapor are subtle and not important [to phys I students]. Perhaps, but
if there is a difference, this list is one place to clarify misconceptions.
Finally, anybody remember when the experts said "the molecular weight of
water is 18 grams"? The gen chem textbooks are finally getting it correct:
molar mass.