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My teacher told me that the expressions for calculation of colligative properties like lowering in freezing point are only applicable if the solute is non volatile liquid or a solid. We cannot apply the formula for gases or volatile liquids. Is this true? If so then why/ why not?

I have not been able to come up with a satisfying reasoning explaining this statement. Also I have not been able to find evidence regarding this statement online. Although one of my books does say this without an explanation but I am not convinced.

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  • $\begingroup$ Well, if somethin's gonna boil off before your solvent then it can hardly influence b.p. like salt will... $\endgroup$ – Mithoron Jul 18 at 19:28
  • $\begingroup$ @Mithoron Solute doesn`t necessarily need to have a boiling point lower than solvent. Also what about gas in liquid mixtures. $\endgroup$ – Manit Agarwal-El psy congroo Jul 20 at 2:39
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Yes, they are, but consider that colligative properties are often described in terms of equations whose derivation require making various assumptions about the substances involved, most importantly that the solvent and its mixtures behave close to ideally (follow Raoult's law), and that the solute does not affect the chemical potential of the solvent in the second phase (solid in freezing point depression or vapor in boiling point elevation). Textbook descriptions of freezing point depression in general assume that the solute does not partition into the solid, for instance, since partitioning would imply formation of a solid solution with a depressed (altered) chemical potential for the solid solvent, a complication too formidable to tackle in an introductory text (and even more advanced ones).

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  • $\begingroup$ Why would volatility affect the chemical potential of the solvent in the second phase? Also what about gas in liquid mixture? If we assume that the solute doesn`t affect the chemical potential, would these mixtures follow the colligative laws? $\endgroup$ – Manit Agarwal-El psy congroo Jul 20 at 2:38
  • $\begingroup$ @ManitAgarwal-Elpsycongroo If the gas is ideal then different species will not affect each other's potential. The same cannot be said of a solid mixture, even if it is ideal (the solute will still affect the chemical potential of the solid solvent in that case). $\endgroup$ – Buck Thorn Jul 20 at 10:58

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