Triple PointIf each line on a phase diagram represents a transition between phases, what does the intersection of all three lines represent?
This is a point where all three phases are present and all 6 phase changes are occuring simultaneously. This is called the triple point.
Now, the question is, how do you get there?
The easiest way to achieve a triple point is to start with a liquid and decrease the pressure until the liquid boils.
The boiling process requires heat a was discussed before. Since there is no external heat source (like the fire under a pot on the stove) the heat must come from the molecules of the liquid. In other words, the molecules that are leaving, do so by taking heat away from the molecules left behind. As a result, the temperature of the liquid left behind drops.
But, of course if the pressure continues to drop, the liquid will continue to boil until the liquid left behind gets so cold that it freezes.
This can be seen in the video here.