Supersaturation is defined as S=C/C*. What are C and C*. This relates to crystallization process.?

C is the actual concentration in solution of the substance you want to crystallize, and C* is the concentration of that substance in solution when it is in equilibrium with the solid at the temperature and pressure of interest.





In order for a solute to crystallize from solution, the concentration of that solute must exceed it's equilibrium concentration. If C = C*, then nothing happens -- the rate at which molecules of the material are added to the solid is exactly equal to the rate at which molecules spontaneously leave the surface, so the solid neither dissolves nor gains mass.





If C %26lt; C*, the solid will dissolve until C = C*





If C %26gt; C*, then the solute will precipitate (and if there is a pre-existing piece of the solid present, it will most likely precipitate on that preexisting surface) and the concentration in solution will decrease until C = C*. In order for crystallization to continue (i.e., for the solute to continue to precipitate) one needs to constantly adjust the conditions so that C is always greater than C*. This can be accomplished, for instance, by evaporation of the solvent (thus concentrating the solution, i.e., making C bigger), or by changing the temperature of the solution (the solubility of most solids increases with increasing temperature; for such solutes, decreasing the temperature will make C* smaller).

pear