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When a substance is dissolved or mixed with water in high concentrations, the activity of water drops from 1 to a value that depends on the substance and its concentration. Is it correct to say that water solutions with the same activity have the same colligative properties irrespective of what the added substance is?

To give an example, let us say that dissolving 100 mg/ml of NaCl in a water sample decreases its activity to 0.9. and that dissolving 50 mg/ml of CaCl2 in water will decrease the water activity to 0.9. Will the two solutions behave similarly? Examples of what I mean by behavior are physical properties such as the freezing point, viscosity, and solubility of certain gases? Is there a rule of thumb to say what properties will stay the same and what properties will be different?

Please note that the used numbers in the question are arbitrary, and are used to make the question easier to understand.

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    $\begingroup$ If by "behave" you mean "have the same water vapor pressure" yes. You need to clarify what you mean by this. It depends on the property. The conductivity would not be the same. And the term "colligative property" is typically introduced in the context of ideal solutions. At high concentrations you observe non-ideal behavior which might include temperature-dependent changes in water-solute interactions and in the self-association of the solute. $\endgroup$
    – Buck Thorn
    Jul 19 at 11:03
  • $\begingroup$ @BuckThorn Thanks, I updated the question. From your comment, I think the simple answer is no. Is there a rule of thumb to say what properties will stay the same and what properties will be different? $\endgroup$ Jul 19 at 11:17

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