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Name: Nancy
Status: educator
Grade: K-3
Location: SD
Country: USA
Date: Winter 2010-11

Question:
We are studying states of matter and physical and chemical changes to matter. We would like to know if popping popcorn is a physical or chemical change. We think that water vapor is given off when the corn kernel is popped so does that mean it is a chemical change? Or does the corn basically stay the same chemically and simply change shape? Thanks for your answer to a very interested third grade class!



Replies:
Nancy,

In order to make things simpler and easier to remember, we have categorized "change" as either chemical or physical. We say that if the composition or nature of a substance changes, then it is a chemical change, whereas a change that only involves a change in the state or shape of the substance - without changing the chemical composition of the substance is considered a physical change.

By this definition, if water was in the unpopped corn kernel as a liquid, and then it changes to gas as the kernel is heated, the nature of the substance remained the same (it is still water), the only difference is that it changed state (from liquid to gas), and we should therefore consider the water as undergoing a physical change. The corn on the other hand may be undergoing a chemical change. If some of the chemical bonds are broken, if some of it reacted with air, then it has become a different substance - it underwent a chemical change.

Greg (Roberto Gregorius)
Canisius College


"Physical" and/or "Chemical" changes are arbitrary distinctions, totally depending upon what you want to make of them. The change "Is what it is." What "shoe box" you want to put it in is a matter of your choice; it doesn't change the process. Unfortunately, many texts, in an effort to simplify things, actually end up making more of the distinction than the idea deserves. Making a "big deal" about the distinction between the two categories serves no useful purpose. You can always find examples where the classification is blurred beyond recognition.

A better approach is to avoid the classification completely. The test of the utility of a classification is the answer to the question, "So What!" What do we know more after making the distinction that we did not know before? What have we added to the body of scientific knowledge? The answer in this case is, "Nothing." We just get into hand wringing about arbitrary distinctions. The distinction should be allowed to die -- the sooner the better.

Although beyond the K-3 level, the thermodynamic treatment of chemical reactions and changes of state are identical. No distinction is made. Similarly, if a chemical reaction is multiplied, both product and reactant sides, by an arbitrary constant, the coefficients change value but the chemistry doesn't change.

Vince Calder



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