[iScience]

In gen chem, my professor told me that the principle leading to freezing point depression is similar to the basic idea for the technique of recrystallization, where if an impurity adsorbs onto a surface it is energetically less favorable than for a particle (that is the same constituent as the rest of the crystal) to adsorb onto that surface. So similarly, he stated that when pure water forms into ice, it forms a uniform crystal lattice but then when impurities are introduced such as Na and Cl ions, the crystallizing H2O molecules now have to crystallize around the Na and Cl ions which is energetically unfavorable and therefore there had to be less thermal energy for the crystallization to actually take place. But he stated that the Na+ and Cl- ions do adsorb onto the ice crystal.

I've been told else where that this was not the case, and that freezing point depression comes from the fact that the Na+ and Cl- ions do not adsorb onto the ice crystal, but rather remain in the aqueous phase and their concentrations become higher and higher as more and more water molecules go into the solid phase.

...i don't see how the latter is a reason for why the freezing point of the water solution should be depressed.

[Arkcon]

Briefly, the first paragraph identifies the phenomena well, twice.  However, combining the thermal energy and the energetic "unfavorableness", you gloss over another term.  Can you identify it?  And define it.  It is important to understanding the concept.

But yes, perfect crystals don't actually exist, so NaCl or Na⁺ and Cl^- ions can become embedded into the lattice sooner or later.  And definitely by the time the complete mass freezes.

[iScience]

hmm, no i don't know which term you are referring to.

but the information that the other guy told me (the second paragraph) is wrong right? because his explanation implies that the stuff that freezes is just the water which would imply that by the time the entire water mass freezes, the salt crystals would be in one location and not spread throughout the crystal. (since it would make sense that freezing happens from the outside going inward since the outside part is in contact with the colder environment, and so we end up with a salt crystal center)

[Arkcon]

Yes, that definitely happens.  As things freeze, especially large amounts freezing gradually, the pure substance freezes first, eventually concentrating impurities in some other part.  Given enough time, it all must freeze, locking impurities ... somewhere.  And it is true that a pure crystal isn't always perfect and pure -- things other than water molecules can be part of an ice crystal.  This will deform the ice crystal, but that's ok, no crystal is made of absolutely perfect repeating molecular subunits.  Otherwise -- think about it -- it would never end and have a surface.

[iScience]

but if i'm cooling it fast and not slow (as opposed to a recrystallization technique (slow cooling)), sure, it'd make sense that the solvent would freeze first since it's more likely to, and therefore a minute amount of solute would be more concentrated in the remaining solvent. but would there actually be a significant difference in the concentration of the solute throughout the ice cube? If i'm cooling it fast i'd expect there to be very little difference in the concentrations of salt throughout the ice cube.

[Babcock_Hall]

My understanding is that when one freezes an aqueous solution of a salt slowly that there are regions of almost pure ice and regions where the salt is more concentrated.  People who are concerned about such things include EPR spectroscopists, and protein x-ray crystallographers.  It is one reason why cooling is sometimes done quickly or very quickly.