Start with 0.956 units of pure liquid ethanol and 0.044 units
of pure liquid water. Initially separated. Maintain both at
78.4 C using a massive heat bath. Maintain constant 1 ATM
pressure.
Then let them come into contact. They will evaporate. Make
sure the temperature remains constant. This will be a bit
of a struggle, because of the latent heat of evaporation.
The volume will increase by roughly a factor of 1000.
That's a lot.
All this is at constant temperature and constant pressure.
There are no chemical reactions involved, at least none of
the usual(*) kind. Specifically, there are no water-ethanol
bonds in the pure water, no water-ethanol bonds in the pure
ethanol, and no water-ethanol bonds in the mixed vapor, for
all practical purposes.
This demonstrates that the garbanzo model (filling the
interstices) is not wrong but also not the whole story.
==============
Tangential remarks: There are other systems that have an
azeotrope that is closer to 50/50 concentration, which
would make for a nicer demonstration, but I chose this
system for safety and environmental reasons. People have
been abusing alcohol for a very very long time, and the
risk profile is pretty well known.
As a preliminary step, you can dewater the alcohol
using calcium chloride or some such. (Obviously you
can't do it by distillation, azeotropes being what
they are.)
All this can be understood in terms of the condensed-matter
physics of water plus statistical thermodynamics. However,
that sort of thing gives people nightmares at the grad-school
level, and would be wildly inappropriate at the high-school
level.
In particular: There cannot possibly be a simple explanation,
because the /simple/ theory gives Raoult's law, which is
clearly violated here. https://en.wikipedia.org/wiki/Raoult's_law
(*) You could perhaps say something about water-water hydrogen
bonding, but it's easier to start that story than finish it.
Suffice it to say that the very existence of liquid water
at ordinary pressures and temperatures is predicated on lots
of hydrogen bonding. That gets real complicated real fast.