What Is a Phase Diagram? A Map of What a Substance Does at Each Temperature and Pressure

Each region of the diagram marks the conditions under which one phase is stable, so a point in the liquid region means the substance will be liquid if held there. The lines between regions mark conditions where two phases coexist in equilibrium, so the line between liquid and gas is the set of temperature and pressure combinations at which boiling occurs, which immediately shows that boiling point depends on pressure rather than being a fixed property. The line between solid and liquid shows the same for melting. Crossing a line means a phase change, which absorbs or releases energy without changing temperature, since the energy goes into rearranging the substance rather than heating it. That is why a diagram explains cooking at altitude, pressure cooking and freeze drying in the same picture.

Two points on such a diagram have particular significance:

• •The triple point, where all three phases coexist, which occurs at exactly one temperature and pressure for a given substance
• •That point is reproducible enough that the triple point of water was used to define the temperature scale until 2019
• •The critical point, where the liquid and gas lines end, beyond which the distinction between them disappears
• •Above the critical point the substance is a supercritical fluid, with the density of a liquid and the flow of a gas
• •Below the triple point pressure a solid sublimes directly to gas without melting, which is what dry ice does
• •Some substances have several solid forms, which adds further regions and lines

The diagram for water has a feature that almost no other substance shares and that has substantial consequences. The line between solid and liquid slopes backwards, meaning that increasing pressure lowers the melting point rather than raising it, which follows from ice being less dense than liquid water, which in turn follows from the open structure hydrogen bonding produces in the solid. The consequence is that ice floats, which is unusual, and that lakes freeze from the top rather than from the bottom, which allows aquatic life to survive winters and which would not be the case for a normal substance. Water also has an unusually high number of distinct solid forms at high pressure, over a dozen identified, which occupy a region of the diagram nobody encounters ordinarily and which occur inside icy moons.

Several features of these diagrams cause predictable confusion. The axes are pressure and temperature, not composition, so a diagram for a pure substance says nothing about mixtures, which require a different kind of diagram with composition on an axis. The lines mark equilibrium, meaning the conditions where two phases can coexist indefinitely, and a substance can persist in the wrong region temporarily if the transition is slow, which is why supercooled water exists below freezing and why diamond persists where graphite is stable. The diagram says what is stable and nothing about how fast a change happens. Pressure on the axis is the pressure on the substance, which for a liquid in an open container is atmospheric and for a liquid in a sealed one is its own vapour pressure. And the scales are frequently logarithmic, which compresses enormous ranges and misleads a casual reading.

The diagrams are working tools rather than teaching aids. Metallurgy depends on them, with diagrams for alloys showing which solid arrangements form at which compositions and temperatures, and heat treatment schedules are read directly from them. Chemical engineering uses them for separation and for designing processes at pressure. Extraction with supercritical carbon dioxide depends on knowing exactly where the critical point sits. Food processing uses freeze drying, which works by taking a frozen material below the triple point pressure so the ice sublimes and leaves the structure intact. Geology uses diagrams for minerals to determine the conditions rocks formed under, since a mineral assemblage that is only stable in a particular region indicates the pressure and temperature at depth. And pharmaceutical manufacture uses them because different solid forms of a drug behave differently.

Regions show which phase is stable and lines show where two coexist, so the liquid to gas line is every pressure and temperature at which boiling happens. All three meet at the triple point, and beyond the critical point liquid and gas become indistinguishable. Water's solid to liquid line slopes backwards because ice is less dense, which is why it floats and lakes freeze from the top.