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Re: vapor

-----Original Message-----
From: John S. Denker [mailto:jsd@MONMOUTH.COM]
Sent: Tuesday, May 21, 2002 11:14 AM

[snip]

Personally, I blame phase diagrams for part of the problem.
I think they
confuse students. Students don't realize what the P refers
to (it is
pressure of the substance at equilibrium, no?)

P means pressure. I don't see what's confusing about that.

The whole phase diagram presupposes equilibrium. I don't
see what's confusing about that.

I should first state that I haven't done any research on this.
Consequently, I may be wrong.

Furthermore, I don't teach phase diagrams so I don't know if students
understand them after being taught. My experience is with senior and
graduate science education majors, some of whom are/were chemistry majors,
who try to teach about water phases. They tend to forget that the curve
represents conditions at equilibrium and they tend to think that P means the
pressure of the air. Since atmospheric pressure is 1 atm, they tend to
think that only the stuff at P=1atm is normally experienced.

Okay, I shouldn't blame the phase diagrams. And, maybe this isn't
prevalent. I just seem to remember that something is confusing these people
and upon closer inspection it seems to be the phase diagrams that they have
misinterpreted.

and end up thinking that the
gas phase of water cannot exist below 100C

Huh? Everyday experience with atmospheric humidity should
give the students plenty of intuition about water vapor
below 100C.

I agree. I didn't initially think they would have this misconception
either.

or that gas and liquid water
cannot coexist except in rare circumstances.

Huh? The most prominent thing on the phase diagram is
the coexistence curve. I don't see how that causes students
to think there is no coexistence.

My point is that they have used a phase diagram to argue that, under normal
conditions, it is impossible to have a cup of water in a room and still have
water vapor present in the room at the same time.

What are the symptoms of confusion, and what's the evidence
that phase diagrams are to blame?

I don't have any except anecdotal reports (see above). In the fall, I'll
ask some of my students the following question:

The air pressure in the room is 1 atm. The temperature is 20C. Attached is
a phase diagram for water. Based upon the phase diagram, which phases of
water can exist in the room? Choose all that apply.
A. Solid
B. Liquid
C. Gas

and I'll ask some other students the following question:

The air pressure in the room is 1 atm. The temperature is 20C. Which
phases of water can exist in the room? Choose all that apply.
A. Solid
B. Liquid
C. Gas

Would this be sufficient to evaluate the validity of my concern?

____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301