Supersaturated Solution

Most students find it easy to give definitions for different solution, such as:

  1. An unsaturated solution
  2. A saturated solution, and
  3. A supersaturated solution

However, most students do not realize why a supersaturated solution is so important.

Briefly, most solutes dissolve to a certain extend in a solvent.  When you can add more solute which continues to dissolve, your solution is said to be unsaturated.  When you cannot dissolve any more solute at a given temperature, then your solution is now saturated. For most chemicals, increasing the temperature of the solution will facilitate more solute to dissolve.  When a solution that has had more solute dissolved at a higher temperature is now cooled, it becomes supersaturated at the lower temperature.  Because that supersaturated solution holds more solute than is stable at the lower temperature, crystals start to form.  As a result, some of the solute comes out of solution when crystals form.  Please understand that not all of the solute comes out of solution to form crystals. The liquid that remains behind is actually now saturated for that particular solute.  Hence, you cannot recover all of your solute via re-crystallization because some of the solute simply stays dissolved in the Mother Liquor. So, it is not advantageous to use too much solvent because more of your solute will stay in solution instead of coming out of solution as crystals. 

The most important thing to remember, however, is that you cannot form crystals unless you have a supersaturated solution. By definition, the solution would be either saturated or unsaturated. Crystal formation can only occur when you have an unstable supersaturated solution. Examples are honey or syrup, which if allowed to stand, crystals will form.  But for either unsaturated or saturated solutions, no crystals can form.  This is the importance of a supersaturated solution.