Phase Diagram Of WaterEdit
Water is famous for its unusual behavior, and its phase diagram is the map that captures how the stable forms of H2O change as temperature and pressure vary. In most standard diagrams, temperature is plotted along one axis and pressure along the other, revealing where ice, liquid water, and water vapor exist as the prevailing phase. The diagram also marks the three major boundaries that separate these regions, the special conditions where all three phases meet, and the more exotic crystalline forms that appear at high pressures. Water Phase diagram Thermodynamics
The phase diagram of water is not just a curiosity of pure science; it underpins meteorology, engineering, planetary science, and our understanding of how water behaves under extreme conditions. Its features arise from the fundamental balance of intermolecular forces, most notably hydrogen bonding, and the way these interactions govern the density, structure, and mobility of molecules across phases. The density anomaly of water—the fact that it is most dense near 4 °C rather than at lower temperatures—plays a crucial role in shaping the solid–liquid boundary and the overall shape of the diagram. Hydrogen bonding Density anomaly of water
Major features
The axes and what is plotted
A typical water phase diagram uses temperature and pressure as the two coordinates. The regions colored or labeled on the diagram correspond to stable ice, liquid water, and water vapor (gas). The diagram also encodes how pressure and temperature influence the equilibrium between phases and which phase minimizes the Gibbs free energy under given conditions. Readers often encounter the diagram as a three-way partition of the P–T plane into solid, liquid, and gas regions, with detailed sub-structures at high pressure. Phase diagram Thermodynamics
Phase boundaries: solid–liquid, liquid–vapor, and solid–vapor
Solid–liquid boundary (melting line): This line separates ice from liquid water. A distinctive feature for water is a negative slope at the ordinary ice–water coexistence region: ice is less dense than liquid water near the melting point, so increasing pressure at a fixed temperature can favor the solid phase or the liquid phase depending on the exact conditions. This explains why ice can float on water and why ice formation can respond counterintuitively to pressure changes. The melting line is typically described using the Clapeyron relation, which connects the slope to changes in entropy and volume during the phase transition. Ice Liquid water Clapeyron equation
Liquid–vapor boundary (boiling curve): This boundary marks where liquid water is in equilibrium with its vapor. Above this line, water becomes steam; below it, water remains liquid (assuming the phase is stable). The slope and position of this boundary determine boiling behavior at different elevations and in engines, turbines, and climate systems. The point where this boundary ends in the pressure–temperature plane is the critical point. Water vapor Critical point
Solid–vapor boundary (sublimation line): At low pressures, ice can transition directly to vapor without passing through a liquid phase. This boundary is of particular importance for understanding frost, high-altitude weather, and cometary or planetary ices. Ice Sublimation (and Phase diagram context)
The triple point and the critical point
Triple point: The unique set of conditions where ice, liquid water, and water vapor coexist in equilibrium. For pure water, this occurs at a temperature of 0.01 °C and a pressure of about 0.00604 atmospheres (roughly 611 pascals). The triple point is a fundamental reference because it fixes the equilibrium between all three phases for a pure sample. Triple point Water
Critical point: The temperature and pressure above which liquid and vapor phases become indistinguishable. For water, the critical point lies at about 374 °C and 22.064 MPa. Beyond this point, water becomes a supercritical fluid with properties between those of a liquid and a gas, highly responsive to changes in pressure and temperature. Critical point Supercritical fluid
High-pressure ices: a family of crystalline forms
At elevated pressures, water forms several crystalline ices, each with its own structure and stability range. These high-pressure ices—labeled Ice II, Ice III, Ice V, Ice VI, Ice VII, Ice VIII, and others—exist in distinct regions of the P–T plane and are of interest for understanding planetary interiors such as the icy moons and large icy planets. The precise boundaries between these phases are subjects of ongoing experimental and theoretical work, due to the challenges of recreating extreme conditions in the laboratory. Ice II Ice III Ice V Ice VI Ice VII Ice VIII Phase diagram
Metastable regions and non-equilibrium phenomena
Real systems can exhibit metastable behavior: supercooled liquid water below the normal freezing point, superheated steam, or transient mixtures that do not perfectly align with the equilibrium boundaries. These states matter in weather events, materials processing, and experiments that push the limits of pressure and temperature. Understanding metastability often requires kinetic considerations in addition to thermodynamic phase boundaries. Supercooling Metastability Phase diagram
Reading a phase diagram
A phase diagram tells a concise story: where each phase is thermodynamically stable, where phase transitions occur, and how changes in pressure and temperature move a sample from one region to another. The slopes of the boundaries, the location of the triple point, the existence of a critical point, and the appearance of high-pressure ice phases together convey why water behaves in ways that are both familiar and surprising. The negative slope of the solid–liquid boundary at the ice–water coexistence region, the density maximum of liquid water, and the emergence of polar crystal structures all trace back to the same intermolecular physics. Phase diagram Thermodynamics Hydrogen bonding
Implications and applications
Phase diagrams of water are not only of academic interest; they underpin climate models, frost formation, high-altitude engineering, desalination and steam systems, and the geology of icy worlds. They inform how ice sheets grow and melt under changing temperatures and pressures, how weather systems transfer moisture, and how the internal structure of planets with oceans and ice shells might organize themselves under different conditions. The diagram also serves as a benchmark for theories of water’s anomalous properties and for computational models that seek to reproduce those properties from first principles. Water Phase diagram Hydrogen bonding Density anomaly of water