Reading the water phase estimate
A pressure–temperature phase diagram divides conditions into regions where a substance is stable as a solid, liquid, gas, or supercritical fluid. Boundary curves mark conditions where two phases can coexist.
Water has an unusual negatively sloped ice-Ih melting boundary at moderate pressures. The estimator includes that behavior, an IAPWS liquid–vapor correlation, and a simplified sublimation boundary.
How to use the water phase diagram calculator
- Enter temperature: Choose °C, °F, or K; the model accepts 200–1,000 K.
- Enter absolute pressure: Choose atm, bar, or Pa and stay within the 200 MPa model limit.
- Estimate: Compare the point with the modeled phase boundaries.
- Check scope: Use reference-quality property software for design, safety, or high-pressure ice work.
Formula and variables
The entered point is compared with approximate sublimation and melting boundaries and the IAPWS liquid–vapor saturation curve.
Phase = f(absolute temperature, absolute pressure)- T — Absolute temperature
- Thermodynamic temperature after unit conversion (K)
- p — Absolute pressure
- Pressure measured relative to vacuum (Pa)
Room-temperature water
Estimate the phase of pure water at 25 °C and 1 atm absolute.
- Temperature
- 25 °C
- Absolute pressure
- 1 atm
- 25 °C = 298.15 K
- 1 atm = 101,325 Pa
- The point lies in the liquid region.
Result: The estimated phase is liquid.
The pressure is above water’s vapor pressure at 25 °C and the temperature is above the ice-Ih melting boundary.
Understanding your results
Boundary results
A boundary label means the entered pressure is within the model tolerance of a coexistence curve. Small input or model changes can place the point on either side.
- The triple point is near 273.16 K and 611.657 Pa.
- The critical point is near 647.096 K and 22.064 MPa.
- Above both critical coordinates, liquid and gas are no longer distinct phases.
Assumptions
- The substance is pure ordinary water at equilibrium.
- Pressure is absolute, not gauge pressure.
- The simplified ice-Ih boundary is adequate within the stated range.
Limitations
- Educational estimator, not a reference-quality thermodynamic property package.
- Does not model every high-pressure ice polymorph, metastable state, dissolved substance, or nucleation effect.
- The sublimation and melting approximations are less precise than the liquid–vapor correlation.
- Restricted to 200–1,000 K and pressures no greater than 200 MPa.
Common mistakes
- Entering gauge pressure instead of absolute pressure.
- Assuming every substance shares water’s phase boundaries.
- Treating a boundary estimate as an exact engineering design value.
- Ignoring salts, gases, impurities, or nonequilibrium supercooling.
Practical use cases
Phase-diagram learning
Explore how water’s stable phase changes with pressure and temperature.
Unit-aware checks
Convert common temperature and absolute-pressure inputs into a phase estimate.
Frequently asked questions
Why does the calculator require absolute pressure?
Thermodynamic phase boundaries are defined using absolute pressure. Gauge pressure omits local atmospheric pressure.
What is supercritical water?
Above the critical temperature and pressure, liquid and vapor are no longer separated by a first-order phase boundary.
Can water remain liquid below 0 °C?
Pressure shifts the equilibrium melting boundary, and water can also supercool as a metastable liquid; this calculator reports an equilibrium estimate.
Sources and review
- Water Thermochemistry and Phase-Change Data — NIST Chemistry WebBook. Accessed 2026-07-13.
- IAPWS-95 Formulation for Ordinary Water Substance — International Association for the Properties of Water and Steam. Accessed 2026-07-13.
- Pressure along the Melting and Sublimation Curves of Water — International Association for the Properties of Water and Steam. Accessed 2026-07-13.
Reviewed 2026-07-13.