MadSci Network: Chemistry

Re: Will beer freeze as a mixture (ethanol/H2O), or individually (only the H2O)

Date: Thu Sep 15 07:22:41 2005
Posted By: Calvin Cole, Faculty, Engineering Physics, Northeastern State University
Area of science: Chemistry
ID: 1125521101.Ch

	It is certainly possible for a the two components of a mixture to freeze
out of solution together as intimately mixed at the molecular level as they
were in the liquid state.  The resulting solid is called a solid solution.
Typical examples would be alloys of Gold and Platinum or Cobalt and
Chromium at all concentrations or Silver and Copper which forms  two
different solid solutions over a narrower range of concentrations.  On the
other hand most  mixtures such as Lead and Tin or Cadmium and Bismuth do
what Water and Ethanol does when frozen.  One pure material or the other
freezes out first depending on its concentration in the liquid being
frozen.  Even when the temperature is reduced to the point that any of
these latter three mixtures becomes completely solid the result is a
mixture, be they ever so small, of pure crystals of each individual
constituent.  For a reference check just about any university level
Physical Chemistry text.   The details of how the freezing is done, rate of
temperature decrease, stirring or not, etc. may affect the relative shapes,
sizes, stratification, and so on of these crystals but they are still
crystals of the pure constituents.  A look at the description of the
process used by the folks at Labatt ( uses the word
“proprietary” at about this point.  Growing crystals, especially of
specific sizes and shapes, is a bit of an art.
	Solid solutions may occur when the atoms or molecules involved are similar
in size, electronic structure, and form the same crystalline structure.  In
the case of Water and Ethanol, though both are polar molecules and can
hydrogen bond, the Ethanol molecule just doesn’t fit well into the ice
structure due to its much larger relative size and shape if nothing else.  
	Since beer is a dilute Ethanol solution the water freezes out first and as
pure water.  Whether it takes the proteins, polyphenols, or particulate
matter along by simple mechanical entrainment or by growing around some or
all of them as nucleation sites for crystal formation is apparently a
beneficial, (from the consumers point of view) side effect.

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