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Q & A: solutes and freezing points

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Most recent answer: 01/20/2013
Q:
Do solutes lower the freezing point of substances that don't contain hydrogen bonds? Would salt lower the freezing point of oil?
- Jenna Cangialosi (age 24)
Sacramento, CA, USA
A:
Yes, the details of the types of bonds in the liquid are pretty much irrelevant. Here's the basic logic of when and why stable solutions have lower freezing points.

 Look at a cup of the solution and a cup of the pure solvent at the freezing point of the pure solvent. Which one is more stable?  If the pure solvent were more stable, then the solution would spontaneously form a region of pure solvent. But that would mean that the solution wasn't stable. So the solvent molecules, given a choice, go to the solution. We say that the solvent has lower chemical potential in the solution.

 Let's assume that the solute simply doesn't fit in the frozen crystal. This is almost always true, since the constraints to fit in an ordered crystal are much tighter than those for fitting in a jumbled liquid. So even if more stuff freezes, it all forms just the same sort of crystal as formed from the pure solvent.

Now put a crystal of the frozen solvent in the solution. Will it grow or melt? The chemical potential in the crystal is the same as in the pure solvent at the freezing point. But that's higher than in the solution. So the material flows from the crystal "down" to the solution. The crystal melts.

As for salt in oil, not much dissolves. So it only lowers the freezing point a tiny bit.

Mike W.

(published on 01/20/2013)

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