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In some parts of the phase diagram for water the solid-liquid phase boundary has a negative slope (especially the portion corresponding to standard pressure). This is due to the solid phase having a higher density than the liquid, so that increasing the pressure increases the melting point the temperature at which a substance melts. Noteworthy is that the solid-liquid phase boundary in the phase diagram of most substances has a positive slope.

However, the solid-liquid phase boundary can only end in a critical point this way if the solid and liquid phases have the same symmetry group. Thus, the liquid and gaseous phases can blend continuously into each other. When going from the liquid to the gaseous phase, one usually crosses the phase boundary, but it is possible to choose a path that never crosses the boundary by going to the right of the critical point. The existence of the liquid-gas critical point reveals a slight ambiguity in the above definitions. This reflects the fact that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable, in what is known as a supercritical fluid. Instead, it terminates at a point on the phase diagram called the critical point. In the phase diagram, the phase boundary between liquid and gas does not continue indefinitely. The phases are separated by lines of non-analyticity, where phase transitions occur, which are called phase boundaries. The open spaces, where the free energy is analytic, correspond to the phases. The green lines mark the freezing point and the blue line the boiling point, showing how they vary with pressure.The markings on the phase diagram show the points where the free energy is non-analytic. The dotted line gives the anomalous behaviour of water. The picture shown above is a typical phase diagram. The phase diagram shows, in pressure-temperature space, the lines of equilibrium or phase boundaries between the three phases of solid, liquid, and gas. The axes correspond to the pressure and temperature. A phase diagramThe simplest phase diagrams are pressure-temperature diagrams of a single simple substance, such as water.