How it works
This phase diagram of a solution lays the dashed curves of a solution over the unbroken curves of the pure solvent. Along the 1 atmosphere line the solution freezes lower and boils higher. Study the names, then quiz yourself.
Read the full guide
Reading the two sets of curves
A dissolved solute that does not evaporate lowers the vapor pressure of the solvent, so the solution's vapor pressure curve lies below the solvent's. It meets the sublimation curve at a colder triple point. The solution's melting curve rises from that point, parallel to the solvent's.
Reading the 1 atmosphere line
Follow the level dashed line from the left. It crosses the solution's melting curve, then the solvent's, then the solvent's vapor pressure curve, then the solution's. Those crossings are the two freezing points and the two boiling points.
- The chart is drawn far out of scale so that the small shifts can be seen.
Questions and answers
Why is the solution's curve below the pure solvent's?
A solute that does not evaporate lowers the vapor pressure of the solvent at every temperature. Each point on the solution's vapor pressure curve is therefore lower than the solvent's.
Why does a solution boil at a higher temperature?
A liquid boils once its vapor pressure reaches the pressure outside it. The solution's vapor pressure is lower, so it has to be heated further to reach 1 atmosphere.
Why does a solution freeze at a lower temperature?
The solution's vapor pressure curve meets the solid's curve at a colder triple point. The solution's melting curve passes through that point, so its freezing point at 1 atmosphere is lower.
Can I practice here for “label the phase diagram of pure solvent and a solution”?
Yes, for practice. That sentence is the instruction of a labeling exercise. This chart is our own drawing and not a copy of any course figure, so your figure may arrange its labels differently.