MadSci Network: Chemistry

Re: How can two solids in a heterogenous mixture separated?

Date: Thu Sep 30 08:29:17 1999
Posted By: Michael Weibel, Battelle Chemist
Area of science: Chemistry
ID: 937971712.Ch

Hi Dave.
With three items like that, there are 3 permutations of the order in which 
to separate them:

1) water first 
2) sand first 
3) salt first

choice 2 is the easiest, especially with readily available equipment. I'm 
also assuming that no requirement exists for necessarily collecting each 
material (water vapor, for example, can be collected during distillation 
by recondensation).  

choice 1 can be accomplished first by drying, then laboriously separating 
the "white" salt from the "brown" sand.  As with any separation process, 
the degree of efficiency is never 100%.  In this case, once you've 
separated the two materials with a pair of tweezers and magnifying glass, 
it becomes easier to see specks of the other in the "pure" pile.  You 
could improve your separation in this way, and also by re-immersing the 
sand in water, and drying it off to recrystallize any residual salt that 
was bound to the sand.

choice 3 can (in theory) be accomplished electrochemically.  If you put a 
potential across the solution, current will flow, and you'll get a redox 
cell.  You may then be able to reduce the sodium + ions to sodium metal, 
and oxidize the chlorine - ions to Cl2 gas.  What will probably happen is 
that as you produce sodium metal, it will react with the water to reform 
sodium + and OH-.  Unfortunately, it is difficult to get sodium to form a 
covalent bond, which would assist in making a condensable material 
containing the sodium.  

As for your suggestion of heating, I expect that if you heated high enough 
to make either molten (a liquid), you'd get significant reaction:

       (SiO2)n + NaCl  --->  NaO + SiCl4
             (unbalanced rxn)

Hope this helps.
Please feel free to email if you have further questions 

Best Regards,


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