Re: Why do some crystals grow faster than others?

Dear Donna,

There is no simple answer to your questions on crystal formation.  The 
growth of crystals is very much an art.  There are many angles to it (no 
pun intended).

From my limited knowledge in this area I can say that yes the time 
required for crystal growth can depend on the solubility.  I presume that 
your procedure goes something like this:  you make a solution of a 
substance, then allow the solvent (water?) to evaporate (usually slowly).  
If you are heating the solution to dissolve the solute(the dissolved 
substance) then cooling the solution to form crystals, then what I say 
below would be different.

When the water has evaporated to the point that it cannot "hold" all of 
the solute, then some of the solute must come out of the solution in the 
form of crystals.  For a more soluble substance more water will have to 
evaporate and the onset of crystal formation will take longer.  In the 
case I have just described, the time required to see crystals form will 
depend on how much "extra" solvent you added when you made the original 
solution.  Even though you make solutions of equal concentrations, unequal 
solubilities will require that unequal amounts of "extra" solvent 
evaporate.

One trick to apply to this area is to not add any "extra" solvent. Add 
just enough solvent to dissolve the original amount of each substance.  In 
this way you start with a "saturated" solution.  Then, in theory, 
crystallization should begin as soon as the solvent begins to evaporate.

Even if you begin with saturated solutions, the solvent may not evaporate 
from them at the same rate.  This can also effect the rate of crystal 
growth.  Faster evaporation would correspond to faster crystal growth.

You are corret that the type of crystal forming and the nature of the 
attractive forces can also effect the rate of crystal growth.  But I 
cannot give you any specific guidelines here.  There have been books 
written on this subject, but none are at my hand now.

There is one other thing.  It is generally accepted that in order to get 
larger crystals, slow growth is important.  Once crystals begin to form, 
the water must evaporate slowly.  It appears that this gives time for the 
ions or molecules in solution to get well organized to form larger 
crystals.I hope that this has been of some help.

Dr. Jerry Franzen
Chemistry Department
Thomas More College
333 Thomas More Parkway
Crestview Hills, KY 41017

franzenj@thomasmore,edu