Hi
Recently, I made a solution containing a substance which is well soluble and easily melted under room condition and can be gained through evaporation and drying under vacuum condition.
As it is unstable when it is stored in room condition, I stored it in the refrigerator for a few days.
When it comes out, however, it rapidly forms a crystal with a slight temperature increase.
As I know, the super-cooling is occurred when the substance, e.g. water, is stored under its freezing point. However, the temperature of the refrigerator is between 3 ~ 5 celsius.
That's why I don't think that this is not the super-cooling because the solution was stored over its freezing point.
How can I explain its sudden crystallization?
The equilibrium temperature of melting (crystallization) depend not only on the temperature but solute concentration as well (it depends on diagram of state of the solution). In might be supposed that after storage in refrigerator the solute concentration changed (because of evaporation of water or change of solute concentration -you mentioned that solution is not stable)
We did observe such transition with Flufenamic Acid (FFA), we suggested it was similar to the 'Lindemann vibrational catastrophe'. Try and read up on it.
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