Re: Is there really a qualitative difference between physical and chemical changes?

michalchik@xxxxxxx wrote:

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.

The notion of specifically defined physical and chemical properties -- and specific definitions for physical and chemical changes -- actually are rigid concepts.

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.

One molecule has the same chemical properties as a million of that same molecule, but it has different physical properties. Surfaces and microclusters similarly have different physical properties from bulk quantities, but the chemical properties are the same. All chemical reactions involving solids take place at the surface.

I think that inter and intra molecular forces are are also more
analogous than chemical and nuclear reactions.

Whoa -- this whole red herring came from Craig, not from me. Don't drag me into that mess.

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.

That's another Craigism. In his defense, he's right that they're all the same chemical substance, just in different physical states. Most of the properties you mentioned are physical properties, which do indeed change when you change the physical state of the compound. When you say that reactivity changes with physical state, really it's the rates of reaction, not the reactivity itself, that changes. But I don't want to get involved any deeper in Craig's arguments, I'll let him handle that.

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.

Talk to Craig about that -- that's another one from his post.

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.

NaCl(s) to NaCl(plasma) is a physical change -- you still have the same Na+ ions and Cl- ions that you started with. There is not a chemical reaction involved.

As I suggested to Bob, it might be a good idea to Google around for definitions of physical and chemical changes, and see if you get a better understanding of the difference that way. This is not a good area to rely on reason and intuition, because in the absence of sufficient facts, our intuition often steers us wrong.
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