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Re: [Phys-l] Definition of Substance

Late 18th century so prior to 1800. The definition in the Wikipedia is that
it has the same "fixed" properties throughout, which leaves out most
mixtures or solutions. It is associated with the concept of definite
proportions. It does seem rather silly to have this definition in addition
to element and compound. If you need to communicate with a chemist and
understand all of the words, it may be necessary, but for physical science
is just more "stuff". I suspect that chemists use the term rather loosely.
After all one can be given an unknown "substance" to analyze, and it may
actually be a mixture of a whole bunch of "stuff", but still look
homogeneous. Substance is now a very common word which has no point of
being several scientific definitions.

Of course physics has the same problem with weight and mass, but they are
conceptually very different. I have been somewhat convinced that we should
do away with weight as a term in the beginning course, and only talk about
the gravitational force, with maybe a small footnote that weight is a
synonym, or whatever the author decides it should be. Then after they
understand gravitational force, perhaps weight could be discussed. It has
gone into common usage in so many ways that fighting over weight may be a
useless battle. Leave it to the more advanced engineering courses that
actually use it.

John M. Clement
Houston, TX


I agree that a solution (homogeneous mixture) comes very close to matching
the description. Air and alloys, in turn, come very close to being
solutions, etc. These are certainly strained definitions, but that's
mostly
a consequence of trying to categorize prior to introducing atoms and
molecules. One has to wonder how far back these definitions go, and
whether
or not they (or some of them) actually predate actual knowledge of atoms
and
molecules (Dalton round about 1800).


So the same properties throughout could also refer to a solution, an
alloy,
or just plain old air, but not a suspension, colloid, or mixture. Then
pure
substance would be only a compound or element. I still think it is an
awfully fine definition to be inflicted on physical science students,
especially if it is not agreed upon. I would tend to agree that the
definition the same throughout makes more sense.

Incidentally the book says milk is a colloid, but every dairy farmer and
older person might disagree. Soymilk is definitely a suspension. They
forgot the important adjective homogenized. The book also calls paint a
colloid, but every painter knows it settles like a suspension. Perhaps
they
were thinking of something other than common house paint. Do they
expect
students to memorize that paint is a colloid even when they know it
settles?
No wonder students say science doesn't make sense.

John M. Clement
Houston, TX


Ah... Didn't get that distinction from your post. I can't recall if
the
terms "pure substance" and "substance" have been used interchangeably
throughout the past, but just a quick referral to my (now) limited
library
of old chemistry textbooks finds the same definition back into the
'80s.
One states that a substance is a material that has the same properties
throughout, which is a slight variation on the theme, but is consistent
with:

element or compound = substance
everything else = mixture

Overall, I would have to say, then, that the definition you have there
is
in
accordance with accepted chemistry terminology. My personal
recollections
go back to the early '60s, and it seems to me that it has always been
defined this way.

----- Original Message -----
From: "John Clement" <clement@hal-pc.org>
To: "'Forum for Physics Educators'" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, March 20, 2009 3:03 PM
Subject: Re: [Phys-l] Definition of Substance


My question was not about "pure substance" but just the work
"substance".
The book defined "substance" as either a pure element or compound. I
would
agree with "pure substance" as being either an element or compound,
but
not
just the word substance without the adjective pure.

John M. Clement
Houston, TX


Subject: Re: [Phys-l] Definition of Substance

I can't speak to the origin of this terminology, but your text is in
agreement with the chemistry texts in New York over the last 30
years
or
so. I wouldn't, therefore, ascribe this terminology to a specific
author. "Pure substance" refers to any entity which has the same
particles throughout, and cannot, therefore, be separated into
dissimilar portions by physical means. This limits pure substances
to
elements and compounds (and ignores differences between isotopes,
btw).
I think the original intent was to focus students on the particle
nature
of matter by categorizing according to the particulate composition,
and
those categories would obviously require different names. The
choices
made for those names were necessarily subjective.

A pure substance is not "opposite" to anything, merely different
based
on composition. The other class of matter is mixture, which
contains
dissimilar particles that have been physically combined, and which
can
be physically separated into dissimilar portions by physical means.
A
solution IS a mixture which has the added characteristic of being
homogeneous - the mixing is completely uniform. This typically
requires
the particles to be atomic/molecular in size. Other mixtures one
might
come up with would tend to be heterogeneous and contain localized
"clumps" of one substance or another (even if those clumps consisted
of
particles which are atomic/molecular in size). Suspensions, despite
being quite uniformly mixed, are still considered to be different
than
solutions, presumably because of the larger particle size.

I think there is ample cause/purpose for the distinction between
these
classes of matter.

On Fri, 2009-03-20 at 11:06 -0500, John Clement wrote:
In the TX IPC book the word substance is defined essentially to
mean
a
"pure" compound or element. I do not remember having encountered
this
definition, and the dictionary is no help here.

I personally think this is one particular author's definition, or
possibly a
definition pushed to be able to ask questions like "Is milk a
substance?".
Since there are alternate words which do have traditional
definitions,
this
is a vey unnecessary complication.

Would anyone agree with the book definition? This is in the
"chemistry"
part of the text, so would a chemist agree with this? I would use
"pure
substance" as the opposite to a mixture, solution...

Language is messy, but we don't have to burden students with
unnecessary
definitions. If the vast majority agrees with the book
definition,
then
it
is reasonable, but merely unnecessary.

John M. Clement
Houston, TX


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l