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

The short answer is that chemical and physical changes are two distinctly different things, and they do have actual, real meaning. They're not vague and obsolete concepts, although they are often poorly understood. See if this make sit any clearer:

A chemical change results when new molecules are formed. Here are some examples of chemical changes:

burning coal (C and O2 combine to form CO2)
iron rusting (Fe and O2 combine to form Fe2O3)
acid-base reactions (HCl + NaOH --> NaCl + H2O)

Here are some examples of physical changes:

ice melting (H2O solid becomes H2O liquid -- but it's still H2O)
water boiling (H2O liquid becomes H2O gas)
salt dissolving in water (NaCl solid becomes NaCl aqueous -- in this case the Na-Cl bonds break, but if you evaporate away all the water, the original NaCl is still there. The NaCl never changed its *chemical* identity, only its *physical* state.)

Breaking bonds between molecules is a physical change (solid to liquid to gas, for example). Breaking bonds within a molecule and forming new bonds is a chemical change -- the identity of the chemical itself changes.

With that said, let's look at the questions you raised:

michalchik@xxxxxxx wrote:

I team taught a summer chemistry camp last month and one of the other
teachers took the kids through a lab on physical vs chemical changes.
Though the activities we took the kids through were pretty cool, I
found myself having a little problem rationalizing which changes were
chemical and which physical.

For example, crushing a piece of chalk was called a physical change as
was melting ice and disolving sugar. But in all cases you are using
energy to break chemical bonds. Even bending a piece of wire uses
energy to rearrange atoms.

You're not breaking any chemical bonds in these examples. Chemical bonds exist within the chalk molecules (CaCO3), the sugar molecules (sucrose, or table sugar, is C12H24O12), and a paperclip is made up not of molecules, but of Fe atoms (steel is more than just Fe, but let's keep it simple). Before crushing, the chalk is CaCO3. After crushing, it is still CaCO3. Therefore no chemical change has occurred. The same is true for the sugar moelcules and for the metal wire -- the chemical identity of the substance remains the same, so the change is physical, not chemical.

If you were to drip acid on that chalk dust, however, the following reaction would occur:

CaCO3(s) + 2 HCl(aq) --> CaCl2(aq) + H2O(l) + CO2(g)

Now that CaCO3 molecule is gone -- part of it is in a water molecule, part of it has bubbled away as CO2, and part of it is dissolved as Ca+2 ions along with the Cl- ions. The molecule has changed, so it's a chemical reaction.

The curriculum likes to say that physical changes tend to be easily
reversable while chemical changes require extraordinary means like
chemical reactions, aside from being tautological this is a
quantitative distinction and seems to have so many exceptions that it
can't really be called a rule.

That's a really bad explanation. No wonder you're confused.

Is it easier to reassemble a shattered
vase than it is to recharge a battery? Many chemical changes exist in
dynamic equilibriums that shift one way or another depending on
factors like temperature.

Well, it's only easy to recharge a battery because all the ugly chemical dirty work is done for you and is hidden from view. I really think whoever wrote that line in the curriculum was thinking about simple changes of state (solid to liquid to gas), compared with simple chemical reactions (like burning coal). The changes of state are easily reversed, whereas the chemical reaction is not. But the fact remains that that explanation is a bad one that applies only in select, simple cases.

I suppose you could also try to make a distinction based on whether
you were breaking intermolercular bonds or intramolecular bonds but
that has plenty of problems too. Stick a macromolecule like dna or
starch in a blender and you will certainly break many covalent bonds.
On top of this I always though that the intra versus inter bond types
were rather arbitrary. Just really a quantitative difference in energy
of the bond.

Intramolecular bonds are very different from intermolecular bonds, and not just in terms of the energy of the bond. A shared electron covalent bond is a very different thing from the momentary flicker of dipole that results in van der Waals forces. Generally speaking, breaking intermolecular bonds is a physical change, and breaking intramolecular bonds is a chemical change -- but only if a new compound is formed. In your example of putting DNA in a blender, if you can break a covalent bond simply by applying enough physical force, that's still a physical change, because the identity of the molecule hasn't changed. But if those broken covalent bonding sites then pick up H atoms and form new covalent bonds, then you have a different molecule, and a chemical reaction has occurred.

About the only one of the lab activities we did which does not seem
like a chemical change in some sense was the heating of a nichrome
wire to incandescence. I can't yet see any way that can be framed as a
chemical reaction.

Right, because the chemical makeup of the wire was the same before and after, so no chemical change occurred.

Overall though, it just seems like physical change versus chemical
change is a really flawed and out of date concept. Perhaps if we
restrict physical change to things like change in temperature and
velocity it would make more sense, but I don't know what kids get from
this concept in the long run.

One of the main purposes of understanding physical and chemical changes is that differences between the physical and chemical properties of different substances are used to separate them. Separation science is incredibly improtant in medicine, environmental science, and any field where what you're interested in is part of some chemical or biochemical soup. The entire field of chromatography is based on physical and chemical separations.

I hope that helps!
.

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