Re: solutions and temperature

[palmer@sfu.ca (Leigh Palmer)]

>   What is the actual mechanism by which a solution of sugar and water
> (for example) holds more solute at higher temperatures?  The space
> between molecules doesn't seem to be increasing appreciably- a cup of
> coffee doesn't significantly shrink as it cools. We're talking about a
> true solution, not just a suspension here, so increased Brownian motion
> wouldn't seem to be the reason.  What is?  Increased stirring at higher
> temperatures on a molecular level?  Any ideas or references will be
> appreciated!

The problem lies in expecting that there is a simple, er, solution to
the problem. If your simple explanation is to hold water (there I go
again!) you must be able to apply it consistently to explain why other
solutes become less soluble with increasing temperature. Gadolinium
sulfate comes to mind, as do carbon dioxide and air, but there are
many others as you will see if you start looking through the rubber
bible.

This business of looking for a "reason" which can be conveyed to one's
students while protecting them from the real (and well understood)
physics has lead to many ideas in their heads that later on confuse
them - and make them distrust their teachers - when they are ready to
learn the more conventional explanations. There are many examples of
such erroneous high school "explanations" that would be better off
left as mysteries*, or perhaps shown to be so when they come up. You
can best answer your student's question by pointing out to her that
the answer is too complex to discuss usefully in terms of naive models
of the phenomena (pl.) of solution. Give her the counterexamples I
gave you to prove your point; that should thoroughly satisfy - and
motivate - a good student. Then teach her all the neat things she can
comprehend, but which for some strange reason have been omitted from
the curriculum.

Leigh

*Perhaps the worst is the association of entropy with disorder. It is
difficult to pry the student loose from this idea later on. I get to
teach thermo next semester; I'll hit this barrier again. Some students
will get over it; some others will quietly go away after the course
not having a clue what entropy is - and I'll pass them anyway.

(deep sigh)