Apologies in advance as I am not a science person by any definition. By most, I'm probably not a person but we'll let that be.

I am studying the notion of "change" in things and focusing on chemical change. Could someone please let me know if:

  1. the change in the color of a photogray (photosensitive, photochromic) lens is a "chemical" change?

  2. the change in a baby's diaper when it gets wet is a chemical change (and if it is similar in any nature to the change in #1) and if that is the same as the change in a pregnancy test or a urinalysis stick

  3. if the change in a thermochromic/temperature sensitive item (a beer can that changes color in the cold, a color changing thermometer or spoon or mood ring ) is the same type of chemical change

Sorry if this is too much for one question, or if it can't be explained to a lay person, but I appreciate any responses. I'm too newbie on this board to post all the links as links so the questioned items are connectable through the list of urls.




Pregnancy test

Blood detection

Coors Light Cold-Activated Bottles

Color changing thermometer


Mood ring -wiki

-----edit--------------- The question of "chemical" change would be, in my mind different from "physical" but I don't know if that distinction comes into play here, or is an issue when the change is permanent or temporary.

  • $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, you can ‎visit the help center or take a ‎‎tour of the website.‎ $\endgroup$ – It's Over Apr 17 '15 at 13:08

For starters, I think most chemists would agree that a chemical change is different from a physical change by involving the formation or breaking of chemical bonds, or changes to the charge, coordination, or bonding state of some of the atoms involved. A chemical change can be reversible, however, so reversibility is not a determining factor. Either a physical change or a chemical change could result in a change of color, or not--as the case may be.

In your examples, the absorption of water by a disposable diaper, as well as the color changing of liquid crystal-based displays (like the beer temperature indicator, thermometer, and mood ring) are clearly physical changes. The liquid crystals change their optical properties due to the physical alignment of the molecules, and not because of making or breaking of bonds. On the other hand, the pregnancy test and blood detection test probably involve chemical changes to produce a color readout (sometimes initiated by a relatively loose binding between an antigen and an antibody, or formation of a metal complex). The photochromic lenses turn dark as a result of a chemical change induced by light (according to Wikipedia, anyway--this is not my area of expertise).


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