Pressure and temperature control states of matter
A phase diagram plots pressure (vertical) against temperature (horizontal) and shows which phase (solid, liquid, gas) is stable at each combination. For water, the solid-liquid boundary slopes slightly backward (unusual compared to most substances): ice at higher pressure actually melts at lower temperature, which is why ice skating works (pressure under the blade melts the ice slightly). The liquid-gas boundary curves steeply, showing that higher pressures allow the liquid to exist at higher temperatures. Above the critical point (22 MPa, 374 K for water), no distinction between liquid and gas exists: the substance becomes a supercritical fluid with properties of both. At the triple point (611 Pa, 273.16 K), all three phases coexist in equilibrium. The phase diagram is a map of thermodynamic stability: the line separating two phases represents conditions where they are in equilibrium (both phases can exist simultaneously).
Industrial and natural significance
Geothermal systems at depth in Earth experience high pressures and temperatures, placing water in the liquid state kilometers underground where zero-degree conditions exist at the surface. This explains why geysers erupt: superheated water at depth rises, pressure drops, and the water flashes explosively to steam. Supercritical water extraction uses high temperature and pressure to dissolve organic compounds that would not dissolve under normal conditions, useful for extracting valuable compounds from biomass. Cloud formation occurs when water vapor cools to saturation on particles in the atmosphere. Understanding water's phase diagram is central to predicting weather, designing steam power plants, and explaining phenomena from ice ages to submarine geothermal vents.