Phase Diagrams - the Solid Liquid LineThe solid/liquid line is nearly vertical on a phase diagram. However, nearly vertical means not actually vertical. As such, the line can either lean to the right or to the left.
Drawn in a very exagerated way then the graph can either look like this
or like this
Why does this matter? Look at what happens when a vertical line is placed on the two graphs.
Starting at the bottom of the line and going up on the first graph would change a gas into a liquid and then into a solid. On the second graph the gas would turn into a solid first and then a liquid.
Why would different materials behave so differently? Remember that increasing pressure forces the particles closer together, so that the material goes into a denser phase (Pressure and phase changes). The difference between the two graphs depends on the densities of the various phases of the materials.
For substances that have a phase diagram like the one of the left, the density is highest in the solid phase, so increasing pressure will eventually force the material into a solid.
For substances that have a phase diagram like the one on the right, the liquid has a higher density than the solid, so tat increasing pressure will force the material into the liquid phase.
So...what kind of substance does that?