Phase Changes and Phase Diagrams

A phase diagram puts temperature on the x-axis and pressure on the y-axis, and splits the plane into three regions: solid, liquid, and gas. Pick any point on the diagram and you're picking a pressure and temperature, and whichever region that point falls in tells you the phase. The lines between regions show conditions where two phases coexist in equilibrium — the solid-liquid line is the melting curve, the liquid-gas line is the boiling curve, and the solid-gas line is the sublimation curve. Two special points matter: the triple point, where all three lines meet and solid, liquid, and gas all coexist at once, and the critical point, the far end of the liquid-gas line where the distinction between liquid and gas disappears entirely into a single supercritical fluid.

Trace a horizontal line across the diagram at fixed pressure and you're simulating heating something on a stove or in a sealed container held at that pressure — the sample moves left to right through whatever regions and boundary lines that pressure level crosses, melting where it crosses the solid-liquid line and boiling where it crosses the liquid-gas line. Where that fixed-pressure line sits relative to the triple point decides whether the substance melts before it vaporizes, or skips the liquid entirely.

This connects directly to intermolecular forces: stronger forces between particles push the melting and boiling curves to higher temperatures, since more energy is needed to pull particles apart. It also sets up the next step in the unit, where you use these same phase boundaries to think about vapor pressure and how solutions shift them.