[richawda]

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

[Corribus]

I would say that any solution can undergo supercooling. A solute can be present and offer no nucleation points. Only if the solute is precipitated will there be a nucleation surface (aside from the vessel walls). I mean, if you want to think practically... what is a "pure solvent"? Does it exist?

To understand the dip=rise effect you describe, it would be good to know a bit more about your experimental protocol and your data. It could be merely inhomogeneity in the sample, particularly if your sample is, as your instructor offered, not stirred sufficiently. In a freezing experiment it may not even be possible to stir sufficiently, a solid being formed and all. The final temperature then represents an equilibrium point.

[Borek]

it rises back up to ~-2 celsius before it freezes

Before or _while_ it freezes?

Hint: latent heat of freezing is involved here.

[Enthalpy]

Up to now, I believe that pure water can be supercooled too. In clouds, water droplets are rather pure, and can be cold enough that they deposit thick ice on aeroplanes very quickly.

Quite the opposite, purity eases supercooling. Impurities help nucleation and hamper supercooling.

Wiki has the same opinion :
https://en.wikipedia.org/wiki/Supercooling
"The process of supercooling requires that water be pure"