I'm having troubles with colligative properties and how to relate them to binary liquid-vapour and solid-liquid phase diagrams.

First of all, in my book it is indicated that colligative properties are defined in the presence of a non-volatile solute, so much so that it simultaneously determines a lowering of the vapor pressure, an ebullioscopic raising and a cryoscopic lowering of the solution with respect to what happens in the ambit of the pure solvent. But this does not seem entirely correct. In fact, even a water-ethanol mixture can undergo cryoscopic lowering, i.e. a eutectic is reached, although both constituents are volatile (therefore the solute is volatile, whether it is water or alcohol). The same argument can be made, for the same volatile mixture, in the context of boiling point:

to the right of the azeotrope, an ebullioscopic rise is observed even though both compounds are volatile.

So, actually, when do these two colligative properties manifest themselves? Is it really necessary for the solute to be volatile?

The other doubt, as already mentioned, concerns about the correlation between state diagrams and the same colligative properties.

This state diagram refers to two substances that are miscible in the liquid state, but immiscible in the solid state, and which reach a eutectic, so the graph is based on the cryoscopic lowering. If freezing point lowering is defined only in the presence of non-volatile solutes, how does this graph hold true even in the presence of a volatile solute? (as in the case before, I mention the water+ethanol mixture, which reaches a eutectic)

This other graph, on the other hand, is a state diagram which refers to two miscible substances both in the liquid state and in the solid state. In this case a eutectic is not achieved and the graph, at least at first glance, is not based on the freezing point lowering, given that starting from A and progressively adding B, the melting/freezing temperature of the mixture increases.

Why is there this difference?

Thanks for every help, because i'm stuck.

  • $\begingroup$ When we say that nonvolatile solutes cause lowering of vapour pressure, boiling point rise or cryoscopic effect, it doesn't mean ONLY nonvolatile solutes cause it. It just happens that for nonvolatile solutes, there are simple equations to calculate those effects. For an arbitrary solute, opposite effects may occur like an increase in vapour pressure. Quite naturally, it depends on whether the solute is more volatile itself than the solvent. $\endgroup$ Jan 12, 2023 at 12:09
  • $\begingroup$ @ViníciusGodim So what you mean is that, in the presence of a non-volatile solute, these colligative properties ALWAYS appear (freezing cryoscopic lowering, boiling point raising, vapor pressure lowering). In the presence of a volatile solute, on the other hand, both these colligative properties or opposite events can occur and this always depends on the characteristics of the solute: for example, ethanol is more volatile than water, so by adding it progressively, an increase in vapor pressure will be observed and so we Will see a decrease in boiling point of the solution. $\endgroup$ Jan 12, 2023 at 12:40
  • $\begingroup$ @ViníciusGodim At the same time, a cryoscopic lowering of the solution occurs, given that ethanol has a lower freezing point than that of water. However this does not explain why by progressively adding water to ethanol (so in this case the ethanol acts as a solvent), a cryoscopic lowering is still observed although water has a higher freezing point than that of the ethanol. $\endgroup$ Jan 12, 2023 at 12:43
  • $\begingroup$ Nobody knows why a mixture of two compounds has or does not have a phase diagram with or without an eutectic or an azeotrope. Anyway these strange points may disappear when changing the pressure. Eutectic mixtures are not understood. They are simply here ! $\endgroup$
    – Maurice
    Jan 13, 2023 at 13:40
  • $\begingroup$ @Maurice oh, perfect. At this point I Guess that the colligative properties are defined in the presence of non-volatile solutes precisely for this reason: in the presence of a volatile solvent and a non-volatile solute, the cryoscopic lowering, the boiling point raising and the lowering of the vapor pressure ALWAYS occur. In the presence of a volatile solute, on the other hand, it is not possible to exactly predict the behavior: a cryoscopic lowering can occur, as well as an elevation. Same for vapor pressure and boiling point. $\endgroup$ Jan 13, 2023 at 14:15


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