For example, the heat capacity of a container filled with ice will change abruptly as the container is heated past the melting point. The curves on the phase diagram show the points where the free energy (and other derived properties) becomes non-analytic: their derivatives with respect to the coordinates (temperature and pressure in this example) change discontinuously (abruptly). 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. The simplest phase diagrams are pressure–temperature diagrams of a single simple substance, such as water. The green lines mark the freezing point and the blue line the boiling point, showing how they vary with pressure. The solid green line applies to most substances the dotted green line gives the anomalous behavior of water. Types 2-dimensional diagrams Pressure vs temperature Ī typical phase diagram. Working fluids are often categorized on the basis of the shape of their phase diagram. There may be a gap between the solidus and liquidus within the gap, the substance consists of a mixture of crystals and liquid (like a " slurry"). The liquidus is the temperature above which the substance is stable in a liquid state. The solidus is the temperature below which the substance is stable in the solid state. For example, the water phase diagram has a triple point corresponding to the single temperature and pressure at which solid, liquid, and gaseous water can coexist in a stable equilibrium ( 273.16 K and a partial vapor pressure of 611.657 Pa).
Triple points mark conditions at which three different phases can coexist. Triple points are points on phase diagrams where lines of equilibrium intersect. Metastable phases are not shown in phase diagrams as, despite their common occurrence, they are not equilibrium phases. Phase transitions occur along lines of equilibrium. Common components of a phase diagram are lines of equilibrium or phase boundaries, which refer to lines that mark conditions under which multiple phases can coexist at equilibrium.
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