Question

The figure shows two phase diagrams, one for a pure liquid (black line) and the other for a solution made using the liquid as the solvent (red line). What does point B represent?

Answer 1

Consider the following general phase diagram:

Now, consider the pure substance, whose phase diagram was represented by the black curves.

The normal freezing point of the pure substance at constant pressure is indicated by `B`, and its reduced normal freezing point (due to addition of nonvolatile solute to the pure liquid substance) is `A`, since `Tdarr` leftwards.

`DeltaT_f = T_f - T_f^"*" = -iK_fm`

is the freezing point depression equation, where:

• `i` is the van't Hoff factor (the effective number of particles in solution per solute formula unit).
• `K_f` is the freezing point depression constant.
• `m` is the molality of the solution.
• `T_f` is the freezing point, and `"*"` indicates the pure solvent.

Now consider the solution made using the pure liquid substance as the solvent, with the phase diagram indicated by the red curves.

Similarly, the normal boiling point of the pure substance at constant pressure is indicated by `C`, and the raised normal boiling point of the solution with the pure liquid as the solvent (again, due to addition of nonvolatile solute to the pure liquid substance) is indicated by `D`, since `Tuarr` rightwards.

`DeltaT_b = T_b - T_b^"*" = iK_bm`

is the boiling point elevation equation, where:

• `i` and `m` are as defined before.
• `K_b` is the boiling point elevation constant.
• `T_b` is the boiling point, and `"*"` indicates the pure solvent.