Re: [Phys-L] Volume non-conservation example

[John Denker <jsd@av8n.com>]

uOn 5/10/25 16:54, Scott Goelzer via Phys-l wrote:
> The classic from chemistry is 50 ml of ethanol and 50ml of water
> poured into a grad cylinder. Produces ~90ml of mixture.

1) Agreed, that's a classic, for good reason.

Also:

2) Start with one cup of dried garbanzo beans, measured in the
usual way. Add one cup of water. That makes a whole lot less
than two cups, and the reason is obvious.

Garbanzos are preferred over most other beans, because they are
close enough to spherical that you can easily estimate the
packing fraction. Ditto for unsplit garden peas. Compare theory
to experiment.

That makes the idea of non-conservation plausible. It is a good
model for some *but not all* volume non-conservation.

3a) Expose the bimetallic strip from a dial-type thermometer, or
old-timey house thermostat, or portable electric heater thermostat.
Blow on it with warm air and it will move a lot. Visible from a
considerable distance.

  https://www.youtube.com/watch?v=uDcBbrmHQpQ

3b) A liquid-in-glass thermometer also works, but may not be so
easily visible.

4) You can make a heat engine using a wheel with rubber bands
for spokes. This is amusing because the rubber band gets
*tighter* when you heat it, opposite to what you might expect.

Feynman I-45-2 : https://www.feynmanlectures.caltech.edu/I_45.html
one-minute video: https://www.youtube.com/watch?v=OW6aEmOsXv0

5) Instead of garbanzos, use salt crystals. At first glance
people will think it's just like garbanzos, as the water fills
the interstices between the crystals. That's true, but there's
more to the story.

Add a small amount of water to the salt crystals, so that you
have undissolved crystals with barely enough water to cover
them. So now there is no air between the crystals. Then add
more water. Then add more water, causing some of the undissolved
salt to dissolve. You get slightly less than the sum of the
volumes. The density of salt water is less than you would get
by averaging the density of the salt and the water.

It's a small effect, only a few percent, but it serves to
demonstrate that "filling interstices" is not the whole story.

6) Partially baked idea: One could imagine cracking one mole
of butane to produce two moles of ethylene plus one mole of
hydrogen. Make sure there's no oxygen involved. I imagine
using a hot platinum wire to provide energy and catalysis
(700 C = dull red heat). I don't know whether it is safe
or even possible to do this with simple equipment, but it's
fun to think about.

Oil companies carry out cracking on an eeeeenormous scale.