Phase Diagrams and Phase ChangesOn a phase diagram, the lines that separate the phases are transition points. In other words, if you start at point A and go to point B (by increasing the temperature), the solid will melt when you cross the line. Likewise, if you start at point B and go to point A (by decreasing the temperature), the liquid will freeze when you cross the line.
Even more interesting, if you hold the temperature and pressure so that the substance remains on the line, it will both melt and freeze at the same time. The result is that the amount of solid will not change, nor will the amount of liquid.
We can also cause a phase change by changing the pressure.
If we move from point B to point C (by reducing the pressure, perhaps in a vacuum pump) the liuid will boil when we cross the line. Likewise, if we move from point C to point B (by increasing the pressure) the gas will condense when we cross the line.