I had a chemistry lab on freezing point depression; however, there is a part on supercooling that is giving me trouble.
Here is the question: Unlike a pure solvent, when cooling a solution to its freezing point, a process called supercooling can occur. Define supercooling.
The lab that this question is part of has a portion where the solution (DI water as solvent and Potassium Iodide as solute) is cooled below zero Celsius down to ~-5 celsius and it rises back up to ~-2 celsius before it freezes. The instructor explained that this dip and rise is due to supercooling and also said that it was due to insufficient stirring of the solution upon it freezing. I completely understand the concept of freezing point depression but am not sure exactly why supercooling causes this dip and subsequent rise in temperature before the solution freezes. Now Wikipedia basically explains supercooling as when there is a lack of a seed crystal or other impurity that can form as the nucleus for the crystal nucleation. This I understand and seems logical. However, in my instructor's question, she stipulates that supercooling can't happen in a pure solvent but can in a solution. This seems completely contradictory to what Wikipedia just said. Wouldn't a solution have dissolved ions or solutes that could act as the seed crystal for nucleation, And therefore only a pure solvent could undergo supercooling?
If you could explain why supercooling causes this dip and then rise in temperature observed during the lab and also why this would occur in a solution but not a solvent, I would greatly appreciate it.
Thank you