Phase Diagram

A phase diagram is a graphical representation that depicts the pressure and temperature at which various phases of a substance are stable.

An area in a phase diagram represents a single phase, whereas lines or phase boundaries specify the equilibrium between two phases.

In the phase diagram of water, the line marked AD is the solid-liquid equilibrium line and has a negative slope, causing the solid to be less dense than the liquid.

The line OA represents the equilibrium between ice and water vapor and shows the vapor-pressure curve of the solid.

In addition, line AC represents the boiling point curve or vapor pressure curve of the liquid. Here, liquid water and water vapor are at equilibrium.

In the phase diagram, the curve AC ends at 374°C and 218 atmospheres pressure. This point is the critical point. Beyond this point, it exists as a supercritical fluid.

The triple point, which specifies the equilibrium between all three phases, happens at 0.01 °C and 0.006 atmospheres pressure for water.