From: gary@horne.cc
Date: Tue Dec 03 2002 - 15:41:38 PST
Message-ID: <180630-220021223234138749@horne.cc> From: gary@horne.cc Subject: Re: pinhole compounds ... pure substances? (Concrete, please?) Date: Tue, 3 Dec 2002 18:41:38 -0500
This has been an interesting discussion to read about a subject that
I had taken for granted and now realize that I know very little about.
So... for the benefit of all non-chemists, could you help us
understand this all better by giving some concrete examples?
Chocolate Milk? Pure substance or mixture? Water? Scotch? Really
good scotch? How about concrete, for that matter!
Thanks!
Gary
---- Original Message ----
From: roy_mayeda@isd743.k12.mn.us
To: pinhole@exploratorium.edu
Subject: Re: pinhole compounds ... pure substances?
Date: 03 Dec 2002 10:59:44 -0600
>Sue,
>
>Yes, they intend the term "particle" to mean basic unit of that
>material. A pure substance is one which has particles which are
>identical, and can't be separated by physical methods (without
>rearranging any chemical bonds).
>
>George,
>
>Yes, there is a little confusion generated by your statement. I
>think the basic reason that we run into this problem is that we are
>talking about different categories as if they are the same.
>(Students often relate element-atom-pure substance and
>compound-molecule-mixture.) These groupings are not the same, though
>there is considerable overlap in some of the groups. The basic
>problem with what you are saying is the statement regarding most
>elements having "molecules" of only one atom. Molecules have two or
>more atoms (of which element is irrelevant) bonded together. I know
>it also depends on the grade level at which you are teaching. I know
>that in lower levels, I teach that all compounds are formed of
>"molecules," but at higher levels we distinguish between molecular
>and ionic compounds. Just depends on how important this information
>is to the kind of understanding that you are trying to achieve.
>
>I try to handle the groupings of matter regarding particles in this
>way (not that it's correct, but it has been useful in my classes):
>
>Ways to group matter:
>1) Pure substance vs mixture
> Pure substances can't be separated by physical methods (eg.
>distillation, filtration, chromatography, etc.) while mixture can be
>separated without changing the chemical properties of each of the
>components. -- this last bit gets tricky to explain, especially at
>lower levels.
>
>2) Atoms vs. molecules
> Atoms are particles which are not bonded, and so can't be broken
>down by any chemical means. Molecules, whether made of one element
>or multiple elements, can be separated into individual atoms.
>
>3) Elements vs. compounds
> Elements contain only one "type" of atom. They can be found as
>individual atoms, or as molecules (N2, H2, O2, etc.). Compounds must
>contain at least two different "types" of atoms, connected by a
>chemical bond. (In general science we leave it at that. In Chem, we
>go on to look at ionic and molecular compounds, and the range of
>those characteristics based on electronegativities.)
>
>It's hard to know how to present a lot of science material, because
>you want to generate understanding as well as stimulate independent
>thinking. We as science teachers usually know that a model is just
>that, and are not terribly confused if the model breaks down in other
>situations. Students sometimes lack that skill (sometimes this is a
>good thing!) and are confused by our explanations. No matter what
>explanation is given, the immortal words of Paul Doherty still ring
>true ... "But it is more complicated than that ..."
>
>Hey, we're up to a tropical 10 degrees F! Hope my "ice garden"
>doesn't melt! :-)
>
>Roy Mayeda
>Sauk Centre HS
>Sauk Centre, MN
>("We're in the middle of Minnesota")
>
>
>
>
>
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