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Re: [Phys-l] Definition of Substance

Yes, I would agree to much of this, but...

For physical science the Bohr atom is understandable, and may be necessary
step towards understanding. One is not going to teach QM at this level.
Similarly Lewis dot diagrams provide a picture which students can readily
use to generate a mental model of what is going on. Some of these concepts
help students build understandable mental models.

Part of the problem with understanding atoms and molecules is that they
can't be seen. Lawson has shown that entities that can't be seen are
extremely difficult to work with mentally. He has even shown that there is
a level of thinking above the Piagetian formal operational level, the
theoretical level. When students test at this level they have much less
difficulty in understanding things which are invisible.

Students are taught about atoms starting at the elementary level, but they
still can't use the ideas to make a coherent mental model. In HS the big
battle is get them to understand that most of the atom is empty, ignoring
the electon "cloud", and that atoms make up matter, rather than are just
floating around in a sea of matter.

Actually even scientists use very primitive mental models when they think
about many phenomena. Some of the outdated models are so good at predicting
physical facts that they are still useful. Scientists generally adjust the
model they use for the particular task at hand. Most of chemistry does not
need QM, just a few results from it to predict a wide range of things. I do
draw the line at telling students that electrons are in a cloud because they
travel so fast you can't tell where they are. This particular idea set them
up for more problems later, and is not needed for the physical science
level.

When you have over 75% of the students unable to use proportional reasoning
and over 50% unable to do 2 variable reasoning, the big battle is NOT using
a correct particle model. The reason why students can't think is not
because they have been exposed to incorrect models. It is because they have
not been exposed to the correct physical tasks in the correct fashion to
help them build reasoning ability. Also it is because they have not been
helped to build good mental models of math and science. They have been
noodled by many facts, some of which are very difficult to understand when
you are at a low thinking level. Remember that 5 YEARS of rigorous physics
did not build good reasoning ability in Chinese students.

John M. Clement
Houston, TX



On 03/21/2009 08:50 AM, LaMontagne, Bob wrote:
Why do science educators make up these idiotic narrow definitions of
comon terms. My encyclopedia uses substance to distinguish between
what makes up the physical essence of something versus merely is
form or other attributes. Substance simply means "stuff". We sound
like total jerks when we go around correcting students for using
words the way 99 percent of the rest of the world uses them.

Agreed!

Why do science educators make up these idiotic narrow definitions of
comon terms.

I can tell you exactly where this comes from. It comes from
macroscopic chemistry. A very great deal of the material that
is taught in high-school chemistry (and sometimes introductory
college chemistry) was worked out before 1900, i.e. before there
was any serious understanding of atoms. Working this out was
truly a tour de force.

In particular there was (and is) a macroscopic notion of "element"
and a macroscopic notion of "compound". There was (and is) also
a macroscopic motion of "mole" as -- you guessed it -- amount of
substance. Most physicists think of a mole as just a number, in
the same way that a dozen is just a number ... but this is *not*
how SI and IUPAC define it. In SI, a mole is one of the base
units, expressing the "unit amount of substance". I am not making
this up!
http://www.bipm.org/en/si/si_brochure/chapter2/2-1/mole.html

My reaction to all this is to say that the 1800s have been over
for a while now; wake up and smell the atoms.

We can replace the archaic view with the modern view as follows.
These are not equivalences, but upgrades:
element --> atom
compound --> molecule
mole as amount of substance --> mole as number (like dozen)

There is work in progress, laying the groundwork for redefining
the mole as a number, but official redefinition has not happened
yet. If/when this happens, "substance" will no longer be a required
part of the SI vocabulary.

Meanwhile, chemistry texts and chemistry courses continue to teach
the pre-1900 view of the world. Presumably this is because they
believe in the "historical approach" as a method for organizing
and motivating the study of science. Apparently they think that
pedagogy must recapitulate phylogeny.

This is part of a package including
-- element / compound / mole as amount of substance,
-- significant figures,
-- distinguishing chemical change from physical change
based on simple macroscopic observations,
-- Bohr atoms,
-- Lewis dot diagrams,
-- The principle that "like dissolves like",
-- oxidation numbers,
-- thermodynamics without entropy,
-- et cetera.

Sometimes people wonder why students seem to be lacking in the ability
to think. I'll tell you why: In general, as William James pointed
out, thinking involves forming connections between new things and
previously-learned things. In this case, it is pointless to think
about things that cannot possibly be reconciled with the known facts.

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