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I'm doing a textbook problem that shows a "molecular level" view of some matter, little colored balls, before and after, and there are, among the four questions, two that say: 1) Did a physical change occur? and 2) Was there a change in state?

I wonder, and the textbook doesn't seem to explain it, does the consensus meaning of "physical change" in a "doing-science" context include changes that are not state changes? It seems safe to say that state change implies physical change, but does physical change not imply state change? Is breaking a solid candy bar into two solid candy bar pieces or molding a piece of clay considered to be a physical change?

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"Physical change" is meant to be a self-explanatory term but depending on the context, it may impose slightly different conditions, sometimes stronger, sometimes weaker. However, all the changes you mention are surely considered physical changes in any context. On the other hand, your term "common consensus" and your interest in this bizarre term is completely unphysical. Science doesn't work in this way, the search for consensus belongs to politics and religion, and and it's not even possible to determine things like "consensus on the meaning of the term physical change". It's just nonsense. – Luboš Motl Sep 28 '12 at 16:44
Thank you. Though didn't scientists vote whether or not to let Pluto be considered (by H. sapiens) a planet or not? Or "conventional current" in the field of electronics? – Pete Sep 28 '12 at 17:02
No, @Pete, a organization of astronomers held a vote. Perhaps the distinction is subtle, but organizations are political. Additionally the particular organization that held the vote was one that didn't include a lot of planetologists; many of whom disagree with the inherently astronomical definition that was the subject of the vote, which points out the importance of context in the meaning of such thing. Pluto has the planetological characteristics of a planet, but not the orbit-clearing, region dominating ones. So who is right? Depends what you want to do with the word. – dmckee♦ Sep 28 '12 at 17:43

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Originally, a notion of a physical change is a change in system that does not change its chemical composition, however, this includes the idea that some physical property of the system changed as a result of the physical change. This however is often related to macroscopic properties of the system. The physical change, in this context might best be understood as a change in physical phase (phase transition). The phases are also described as different states.

In this usage it is implied that these are in fact synonymous. However, one area of confusion is that microstates and macrostates are two different things. A good example would be the difference between changing the order of some colored balls vs the number of colored balls. I might equate a macrostate to the number of colored balls I have (2 red, 3 blue and 2 yellow balls) (RRBBBYY). If I change the order of the balls from RRBBBYY to RBBRYBY, I have changed the microstate of the balls, but the macrostate has not changed because I still have 2 red, 3 blue and 2 yellow balls. However if I change the color of the balls so now I have 3 red, 1 blue and 3 yellow, I have change some macroscopic property such that no reordering (or changes in microstate) of the balls will bring me back to the 2R,3B,2Y macrostate.

Based on the information provided, since this is a questions of molecules, a change in the balls is definitely a physical change, and a change in microstate, however it might not be a change in the macrostate of the system.

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