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Name: Rebecca W.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2/9/2004


Question:
Some of my Honors Physical Science students thought it would be fun to play "stump the teacher." We are discussing the differences between physical and chemical changes right now. The question is, "with the fact that lizards can change their color, is this a physical or chemical change?" I said it can technically be considered both because it can go back to the original state of color, unchanged but yet chemical reactions occur hormonally to produce the change in color. I guess I just need a little ammo to give them tomorrow since I told them I wouldd research it and come back with something.


Replies:
Chemical changes typically include physical changes, and those things we observe help us to know that chemical reactions have taken place. I am not a biologist, but I think the best answer is that it is a chemical change with associated physical changes that give you evidence of the chemical change. While reversibility or lack thereof is one way that we teachers help clarify these concepts for students, there are in fact many chemical changes that are reversible, but are chemical changes nonetheless (energy in and out is one factor to consider in chemical reactions, but is not often considered at the same time as physical and chemical changes are covered in class). I think it is wonderful that your bright students are asking these questions. Perhaps you should turn the tables on THEM and make THEM investigate this via experiments, research, etc. You could have a class debate on this! What an exciting opportunity to have student interests drive the activities!!!!

Pat Rowe


Seems like you are ok to me. The chemicals in the lizards body change around, but the result is that light is absorbed or reflected differently. Color at the chemical level is just answering the question "ok, here is a bunch of molecules, each of which will resonate a bit to this wavelength of light. So we call this the color they absorb, leaving the other colors to reflect".

So the color changes, and we just call that a "physical" change, because our eyes can see it. I guess I would not be too tied up in the words chemical versus physical, past the point of understanding these concepts. Nature does not draw a tight line between "chemistry" and "physics".

Steve Ross


Dear Rebecca,

In my opinion, this is certainly a chemical change. Reversibility is not the issue; the issue is whether there is an underlying chemical transformation that accompanies the macroscopic manifestation.

There are lots of reversible chemical changes in nature. You can melt a solid into liquid, then freeze it back into a solid. Both melting and freezing are chemical changes, not physical ones. The microscopic state of the solid is very different at the molecular level from that of the liquid. However, if you use a hammer to shatter a solid block into tiny pieces, that would be a physical change. The microscopic state of the pieces is the same as the microscopic state of the original solid block.

Hope this helps,

Prof. Topper


Color can be used an example of both a physical property and a chemical property. Color can be a characteristic property (something that identifies it as such) if it has a natural color. For example, copper compounds tend to be blue and elemental sulfur yellow. But if it changes color that is evidence of a chemical change. So when a lizard or chameleon changes color that is evidence of a chemical change. Some kind of cell signal, whether it be hormonal or enzymatic, etc. is triggering the skin cells to begin producing a different pigment, which is evidence of a chemical reaction in those cells.

vanhoeck



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