Re: The importance of being pedant

[William Beaty <billb@ESKIMO.COM>]

On Wed, 21 Jul 1999, Leigh Palmer wrote:

> Someone decided a long time ago that the three common states of
> matter were solid, liquid and vapor. That's canonical; there's no
> good reason to change our terminology.

If "vapor" means gas, then why do we say "water vapor" when we actually
mean "water gas"?  Our terminology reinforces the misconception that
evaporated water is not a gas.  When liquid oxygen evaporates, it turns
into a gas, as does N2 and CO2.  But when water evaporates, it becomes
"water vapor."

When I look back upon my own misconceptions in this area, I see that I was
taugh to equate "vapor" with suspended droplets, and my misconception
continued for decades.  I was convinced that water turned into very small,
wet droplets as it evaporated, but that such things as LN2 turned into
gas.  I've seen evidence that this is a common misconception even in
people who should know better.

I'm not suggesting that we avoid using scientific terminology, I'm
recommending that we fight specific misconceptions by giving certain terms
special treatment.  One such treatment is to continually employ the
definitions in our explanations, rather than only defining the terms once
at the beginning.

For example, in the following I keep switching back and forth between
"vapor" and "gas", rather than assuming that the reader has internalized
the technical meaning of "vapor".

  When water evaporates, it turns into Water Vapor.  This dry, transparent
  H2O-gas is similar to other gases such as nitrogen and oxygen, but its
  molecules have less mass than N2 or O2.  Water vapor is a bit like
  helium: if air is full of evaporated water, it will be less dense than
  the surrounding air and will rise.  Water-gas can condense from vapor to
  liquid.  If the vapor in the air should turn into droplets, its total
  volume decreases, and we might expect the droplet-filled air to be
  more dense than the surrounding atmosphere.  However, the process of
  condensation must heat the air and cause it to expand.  Clouds rise like
  balloons, while fog pours across the landscape as if it was a dense
  fluid.  Both contain heavy droplets of liquid water.  To explain the
  difference in their behavior, look at their temperatures.



> One reason for remaining true to the old terminology is that the
> old literature remains accessible. Another is that misunderstandings
> will not arise if both parties to a dialog are using the same
> vocabulary*.
>
> Leigh
>
> *I once lost a half night's observing time at Kitt Peak because my
> computer was interpreting my commands in the wrong vocabulary.

I've done that too, with Unix shells.  If somebody redefines the
terminology without telling us, our language malfunctions.  It's a very
eerie sort of malfunction too, it feels like a kind of insanity.  Isn't
that what people experience when hearing physics lectures?   :)


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