Phase Diagrams of pure substances Moving from solid to vapour: There's just one more line to look at on the phase diagram. This is the line in the bottom left-hand corner between the solid and vapour areas. That line represents solid-vapour equilibrium. If the conditions of temperature and pressure fell exactly on that line, there would be solid and vapour in equilibrium with each other - the solid would be subliming. (Sublimation is the change directly from solid to vapour or vice versa without going through the liquid phase.) Once again, you can cross that line by either increasing the temperature of the solid, or decreasing the pressure. The diagram shows the effect of increasing the temperature of a solid at a (probably very low) constant pressure. The pressure obviously has to be low enough that a liquid can't form - in other words, it has to happen below the point labelled as T. You could read the sublimation temperature off the diagram. It will be the temperature at which the line is crossed.
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