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

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.
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