Is there really a qualitative difference between physical and chemical changes?
- From: "michalchik@xxxxxxx" <michalchik@xxxxxxx>
- Date: Wed, 12 Sep 2007 13:45:22 -0700
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.
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. 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.
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.
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.
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change is a really flawed and out of date concept.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.
.
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