Reading the diagram
Each line is a coexistence boundary. The blue vaporization curve is exactly the saturation vapor pressure of water — the same quantity the main calculator returns — running from the triple point up to the critical point, where it ends. Below freezing, the cyan sublimation curve gives the frost point, where ice turns directly to vapor. The steep dashed melting curve divides ice from liquid water.
Pick any point in the plane and the phase is whichever region it falls in. On a boundary, two phases coexist — which is why water boils at 100 °C only at about one atmosphere, and at a lower temperature on a mountaintop where the pressure is lower. The marker traces that boiling (or frost) point as you change the temperature.
Frequently asked questions
What does the water phase diagram show?
It maps which phase of water — solid, liquid, or vapor — is stable at each combination of temperature and pressure. The lines are the boundaries where two phases coexist: the vaporization curve (liquid ⇌ vapor) is the saturation vapor pressure curve, the sublimation curve (solid ⇌ vapor) lies below freezing, and the steep melting curve (solid ⇌ liquid) separates ice from liquid water. Pressure is drawn on a logarithmic scale because it spans more than five orders of magnitude.
What are the triple point and critical point?
The triple point (0.01 °C, 611.657 Pa) is the single temperature and pressure at which solid, liquid, and vapor all coexist — it is the fixed point that anchors the temperature scale. The critical point (373.946 °C, 22.064 MPa) is where the liquid–vapor boundary ends: above it, liquid and vapor become indistinguishable and water is a supercritical fluid.
Why does the melting line of water tilt to the left?
Almost uniquely, water's solid–liquid boundary has a slightly negative slope: ice is less dense than liquid water, so increasing the pressure lowers the melting temperature. The effect is small (about −13.5 MPa per kelvin), so the line is nearly vertical; on this diagram it is drawn schematically.
Where do the curves come from?
The vaporization curve is computed from the IAPWS-95 saturation equation, the international reference standard, and the sublimation curve from the Murphy–Koop (2005) equation over ice — the same verified engine used across this site. The melting line is schematic. Move the temperature marker to read the exact saturation pressure at any point.
References
Every formula on this page is implemented from, and validated against, the following primary standards and papers.
- IAPWS R6-95(2018) / Wagner & Pruss 2002 — International Association for the Properties of Water and Steam — the reference standard
- Review of the vapour pressures of ice and supercooled water — Murphy & Koop 2005, QJRMS 131:1539
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