🚀 go-pugleaf

In article <5447@ucsbcsl.ucsb.edu>, 6500alh@ucsbuxa.ucsb.edu (Archie Holmes) writes:
|> Hello.
|> 
|> I have been told that it is possible to prove thermodynamically
|> that alcohol can only be made 95% pure.  Do anyone knoe how
|> to do this proof.  Thanks in advance.
|> 
|> Archie Holmes

It is impossible to DISTILL ethanol from water at 1 bar pressure to more than ~95% ethanol.  
There are other ways to separate water from ethanol, however, so one can make a sample of
ethanol as pure as one wishes.


Distillation works in cases where the composition of a gas phase is different from that of the 
liquid phase with which it is in chemical equilibrium.  This is generally the case.  However, 
if the boiling temperature curve as a function of composition for some solution has a maximum, the 
compositions of the vapor and liquid in equilibrium at that temperature are identical.  The ethanol-
water system is such a system.  The boiling point temperature as a function of composition 
has a maximum at ~95% ethanol (at one bar).  This point in known in the thermo trade as an 
"azeotrope", and this particular composition of water+ethanol is called an 
"azeotropic mixture".  Look in any thermo book under these headings for more information.

-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
CIS: 76347,2523                        |
Fidonet: Henry Shaw at 1:161/55        |