Ask A Scientist Chemistry Archive

name         Peter G.
status       educator
age          40s

Question -   Hello I am teaching grade 9 Science, the topic Chemical
and Physical Change.
The class began a discussion on what constitutes a chemical/physical change.
We discussed the primary indicators of chemical change and the question of
pouring cream into coffee arouse.  There is a color change, was the response
so how do we prove it is not a chemical change.  I realize that the cream
becomes soluble in the hot liquid, but is there a chemical change in the
cream
itself and the proteins and fat in the cream when added to a hot liquid.
Is there some method with which to support the hypothesis that pouring
cream into
coffee results in a chemical change.
-----------------------
Peter,

As you already know, in a chemical change, new substances with different
chemical identities (properties) are formed. In the situation you describe,
to prove a chemical change occurred, one would need to know the quantity and
identity of all substances (compounds) present in the coffee AND cream
before they were mixed. After mixing the two, one would again need to know
the quantity and identity of all substances (compounds) present. If the
before and after lists were not identical, one may conclude that a chemical
change had occurred.

Your observation of a possible (chemical) change in "the proteins and fat in
the cream when added to a hot liquid" is astute and not without merit.
Uncertain as we are regarding the exact before/after situation, one could
make a defensible case that a chemical change might have occurred.

Regards,
ProfHoff 521
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Trying to put all changes in substances into the categories of "chemical
change" and "physical change" is always going to run into some gray areas --
like the case you mention. The best criterion for a chemical change is:
"Does the change result in the formation of a new chemical substance that
was not present when the reactants were combined?" Color changes, formation
of precipitates etc. may or may not be a reliable indicator of this. Under
"usual conditions" mixing coffee and cream would be considered a physical
change; however, if the temperature is kept hot enough, long enough
obviously reactions involving components in the milk/cream and water/coffee.
Pouring cream into coffee is pretty complicated if you look at it too
closely!! The cream is an emulsion (that's why the coffee with cream looks
tan) which scatters light.
     It is not inappropriate to discuss cases where the answer in not clear
cut -- even with 9th graders. That is the real world of chemistry sometimes.

Vince Calder
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