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Q & A: Does mixing cause disorder?

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Most recent answer: 06/16/2012
Q:
Mixing is categorically considered to be a process of disorder. 2 places where I have seen it described as such in no uncertain terms are Hawking's unauthorized collection of essays, The Theory of Everything, and James Gleick's Chaos. Yet, for example, the mixing of hydrogen and oxygen is required to get water. Is this mixing then also considered to be disorder?
- Mike (age 47)
Albuquerque, New Mexico, USA
A:
This is a semantic question. We know what goes on in a typical mixing process. Different types of particles, initially separate, get scrambled in together with each other. Sometimes that's followed by a chemical reaction, as can happen with H2 and O2, sometimes things just stay mixed up.  Whether you use the word "disorder" or not doesn't change what you know is going on.

There are specific problems (say in the formation of crystals with different types of molecules) where we do have a clear meaning for "disorder".  In that case, an ordered crystal would be one where the atoms form a regular pattern. A disordered crystal would be one where two or more types (say K+ and Na+) occupy some type of site in an irregular, unpredictable pattern.

In more general cases the word disorder isn't especially useful. What is useful is the word "entropy", used to denote a measure of the number of available quantum states. Often, but far from always, a high entropy state has a more disordered appearance. Entropy is useful because we have a law about it. The second law of thermodynamics says that in a nearly isolated system entropy never decreases, and tends to increase. Whether that leads to more or less disordered appearance depends on the system and the eye of the beholder.

Mike W.

(published on 06/16/2012)

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