We are beginning Thermodynamics, and I showed them some P vs. T plots, showing the 3 phases (solid, liquid, gas), the triple point, etc. -- mentioning that water was unique for having a transition line between solid and liquid with a negative slope.
The question arose asking if anything has a transition line that is vertical or nearly vertical. I admitted to never having seen one, and did not think it was possible -- but I am unsure of that last point.
What would a vertical, or nearly vertical, transition line have as a physical significance? Any insights would be most welcome!