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I came across a situation in which my chemistry textbook gives phase diagram of a pure solvent and a solution with the same solvent.I am considering 2 different , say beakers, one containing some amount of pure solvent , and the other contains a solution formed by a non volatile solute and the same amount of solvent. In this, the author has shown the part of the graph corresponding to solid solvent , and solid form of solution to be identical. It's natural for me to think that a solid obtained by freezing a solution would be different from that obtained by freezing a pure solvent. So they must not have identical graphs. Is my argument correct ? Or is it like this- when the solution is frozen , since the solvent molecules are brought together for making a solid, we simply don't have solvent molecules to solvate our solute, and the solid is more or like a frozen pure* solvent with solute particles trapped in it?

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  • $\begingroup$ In you first sentence, do you mean a pure solvent in contact with a solution containing a solute in the same solvent but which do not mix? Then solid solvent in contact with the solid solution and/ or liquid solution ? $\endgroup$ – porphyrin Mar 6 '17 at 15:48
  • $\begingroup$ I am considering 2 different , say beakers, one containing some amount of pure solvent , and the other contains a solution formed by a non volatile solute and the same amount of solvent. $\endgroup$ – Pyro Recorcinol Mar 6 '17 at 15:50
  • $\begingroup$ The phase diagram for say pure water and a NaCl/water mixture would be different. The NaCl would lower the freezing point of water. Also the salt would tend to concentrate in the liquid until some lower freezing point was achieved. $\endgroup$ – MaxW Mar 6 '17 at 15:50
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If you were to freeze the solution in a normal manner, using something similar to a home freezer, a more and more concentrated solution will form as the majority of the solvent freezes. The solute will be found in a small amount of solution that did not freeze, since the solute allows for freezing point depression. So, if you put both of your beakers into the freezer, one beaker will have a completely frozen block of solvent and the other will have a frozen block of solvent with some volume of highly concentrated solution.

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