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vapor



>
>my physics students. Everytime I say gas, they bring up vapor.
>
>
>I don't know where it comes from and I keep trying to get them to stop
using vapor but it seems stuck in their minds..
Esp. that gas and vapor are the same.

It might help to look at a phase diagram of a pure substance, e.g., water
or CO2.

The three phases of matter: solid, liquid, gas.

If a closed system of a pure substance, at equilibrium, has temperature and
pressure such that the T and P are within one of the phases, the other
phases cannot exist.

If the T and P are such that the system is on one of the phase boundary
lines, the two phases coexist. "Vapor" is the gas, in coexistence and in
equilibrium with the liquid/solid phases. The equilibrium vapor pressure
is the [gas phase] pressure of the substance in equilibrium with the
condensed phase(s) at the given temperature. The system cannot have gas
[vapor] present above this equilibrium vapor pressure at the given T; it
will be liquid or solid. If the system has pressure below this equilibrium
vapor pressure; it will be all gas and no solid/liquid can be present.

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.

For systems not at equilibrium, I suppose you could say a vapor is present,
but once the pressure = the equilibrium vapor pressure, the system is at
equilibrium. The example that comes to mind is, on a hot Summer day, the
puddle of water on the road, after the rain storm: Clearly, the vapor
pressure of water is less that the equilibrium v.p. at that T - 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.

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.

Joe Sabol
NMU
Marquette, MI