Re: Is there really a qualitative difference between physical and chemical changes?
- From: "michalchik@xxxxxxx" <michalchik@xxxxxxx>
- Date: Wed, 12 Sep 2007 22:28:52 -0700
On Sep 12, 9:07 pm, BZ <n...@xxxxxxxx> wrote:
Bob wrote:
The problem is that it is not always clear whether new substances are
formed, and it may even be open to interpretation. A classic case is
of salt dissolving. BZ makes the case for that being physical. But as
soon as you get to discussing solutions, one learns that a better
description is NaCl(s) --> Na+(aq) + Cl-(aq). Obviously a chemical
change. Students will pick up on this. I think the best answer is that
either view is correct -- that phys/chem is not always right vs wrong,
but a shorthand for how you view what is happening.
The breakup of the NaCl "molecule" when it dissolves in water leads to
confusion for a lot of people. But think about it for a moment -- what
exactly *is* a "molecule" of NaCl? It's an ionic solid, right?
Therefore it's really nothing more than a collection of Na+ ions and Cl-
ions. When you dissolve solid NaCl in water, you still have a
collection of Na+ and Cl- ions, they're just farther apart. Dissolving
NaCl in water is strictly a physical change, and in no way a chemical one.
I find bizarre BZ's claim that shearing a DNA molecule to smaller
molecules is not a chemical change. The molecular weight changed, and
MW is a property of a chemical. (But BZ gave his explanation, and I
follow his point -- just don't particularity agree with it.)
I was envisioning a "fracture" in the DNA molecule in his blender, not a
wholesale shredding of it, but that doesn't change the result. If the
"blender" is an electron impact ionizer and the DNA is torn to pieces,
that is a physical destruction. There is no chemistry going on. It
might help to think of it like this: if all the pieces are still there
and all you have to do is glue them back together, then it's a physical
change. Whether it's a vase or a molecule, physically shattering it
with a blast of force of some kind is a physical change, not a chemical one.
Tearing a piece of paper is most commonly considered physical, yet
almost certainly involves breaking covalent bonds.
When you tear a piece of paper (or chop a piece of wood, or break a
ceramic cup, or shatter a window), you're tearing between the molecules,
not tearing the molecules themselves apart. A piece of paper looks like
one solid piece to us, but it's not one gigantic molecule.
> So, I find phys/chem "useful", but with limitations. Make sure how you
> intend the terms to be used, and don't become a slave to rigid
> definitions.
Bob, science is *built* on rigid definitions! Any change in a substance
is either physical or chemical -- it's one or the other, not both, and
it's never a matter of opinion or preference. The intuitive answer may
not be the definitive answer, but intuition in these cases is often
wrong, which is why so many people have problems discerning the two.
BZ, I think your opinion is well thought out and I think I understand
where and how you draw the lines. I think if we were given a test that
required us to identify physical and chemical changes we would pick
the same answers. Nevertheless, I don't think I agree with you that we
can still use these concepts rigidly. They are at best approximations
that have in some cases hindered our understanding of things. Surface
chemistry and microclusters are two areas I can think of where the
notion that a substance is still just a substances until you break the
intramolecular bonds has betrayed us. A cluster of ten atoms of a pure
element behaves neither like a large mass of the element or as the
sing atom. The same is probably true of molecules. Our definitions
need to be clear but they need to be based on the world around us
including the exotic and exceptional cases. I am reminded of one of my
professors of biology discussing the intellectual framework of
evolution. He believed and I agree with him, that evolution probably
could not have been discovered had not the Greeks started classifying
and categorizing forms of life, but he also feels that our
understanding of evolution was hampered and is still hampered today by
our notion of species as bounded categories instead of attractors in
dynamic vector spaces of gene flow.
I think that inter and intra molecular forces are are also more
analogous than chemical and nuclear reactions. There is a qualitative
difference in the fact that different sub-atomic particles and
fundamental forces are involved in nuclear reactions while chemical
reactions are exclusively eletromagnetic. Is there really a
qualitative difference between ionic bonds, polar bonds, dipole-
induced dipole, and momentary dipole induced dipole? I'll grant you
that covalent bonds and hydrogen bonds are kind of odd men out.
As far as Ice, Water, Steam all being the same chemical substance...
We really need to go into the definition of what a chemical substance
is. If you want to say it is the ratio of the constituent elements,
then fine they are the same. But if you look at conductivity, density,
solvation, viscosity, optics, reactivity, and on and on They appear
to be different substances.
As far as sugar dissolved in water, you say that it tastes sweet just
like sugar does, but in fact in both cases you are dissolving it in
water (saliva). I don't think you can use taste as a unifying
principle.
Things get screwier with NaCl. I think the key difference between
Solid Salt and dissolved salt is that the ions charges are neutralized
by association with completely different atoms. NaCl plasma is nasty
stuff and that is what you get when you use lots of heat to just
spread out the ions.
Anyway, lest you think I am too nitpicky, I think you made a lot of
good points. The chemical dirtywork is done for you in recharging
batteries and there is meaning to the idea that a little piece of
chalk is still chalk and behaves like chalk. I just think that is not
because there was something qualitatively different from a chemical
reaction done to it, just that the change was such that it did not
effect the properties we normally associate with chalk.
.
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