How this calculator works
Boiling occurs when equilibrium vapour pressure equals the surrounding absolute pressure. The integrated Clausius-Clapeyron approximation uses a constant vaporization enthalpy and ideal vapour. Presets use rounded near-normal boiling data; their enthalpy is held constant, so accuracy worsens away from the reference. Custom mode accepts a measured pressure-temperature pair. Preset estimates outside the liquid range are rejected. Mixtures, dissolved solutes and critical-region behaviour require a more complete model.
Formula
ln(P/Pref) = -(Hvap/R)(1/T - 1/Tref). T = 1/[1/Tref - (R/Hvap) ln(P/Pref)].
Worked example
A custom substance with Tref = 350 K at 100 kPa still boils at 350 K when pressure is 100 kPa. The water preset gives 373.15 K at its 101.325 kPa reference.
Advertisement
Frequently asked questions
Do I enter gauge pressure?
No. Add local atmospheric pressure to gauge pressure to obtain absolute pressure.
Why is water not exactly 100 C everywhere?
The 100 C reference applies near one atmosphere; lower surrounding pressure lowers the boiling temperature.
Are presets exact equations of state?
No. They are constant-enthalpy estimates. Use measured vapour-pressure correlations for precision work.
Sources
- OpenStax Chemistry 2e: phase transitions and Clausius-Clapeyron
- NIST: 2022 CODATA fundamental physical constants
- NIST WebBook: water phase-change data
- NIST WebBook: acetone phase-change data
- NIST WebBook: ammonia phase-change data
- NIST WebBook: butane phase-change data
- NIST WebBook: ethanol phase-change data
- NIST WebBook: hydrogen phase-change data
- NIST WebBook: methane phase-change data
- NIST WebBook: methanol phase-change data
- NIST WebBook: propane phase-change data
- NBS Monograph 168: selected properties of hydrogen