Boiling Point Calculator
Predict how a liquid's boiling point changes with pressure using enthalpy of vaporization and a reference boiling point.
100 C for water corresponds to 373.15 K.
Boiling point decreases at pressures below the reference.
Approximate value for water near 100 C.
Predicted boiling point
368.21 K
95.06 C
How to Use This Calculator
Record reference data
Use a known boiling point and pressure for the substance, such as water at 100 C and 101.325 kPa.
Measure target pressure
Enter the pressure at which you need the new boiling point (altitude or vacuum conditions).
Provide enthalpy of vaporization
Use tabulated DeltaHvap values near the reference temperature. For water near 100 C, use about 40.65 kJ/mol.
Review adjusted boiling point
The calculator outputs the new boiling temperature in Kelvin and Celsius.
Formula
ln(P2/P1) = (-DeltaHvap / R) * (1/T2 - 1/T1)
Rearranged: T2 = 1 / [(1/T1) - (R/DeltaHvap) ln(P2/P1)]. Temperatures in Kelvin, pressures in consistent units.
Example
For water at 85 kPa: T2 = 1 / [(1/373.15) - (8.314/40650) ln(85/101.325)] is about 365.4 K, which equals 92.2 C.
Full Description
The Clausius-Clapeyron equation relates the vapor pressure of a liquid to temperature via enthalpy of vaporization. Using a reference boiling point and pressure, you can estimate boiling behavior at other pressures.
This calculator is useful for cooking at altitude, process engineering, or vacuum distillation planning, providing quick estimates without detailed phase diagrams.
Frequently Asked Questions
Why use Kelvin?
Thermodynamic equations use absolute temperature to maintain correct proportional relationships.
How accurate is this approximation?
Very accurate for small pressure changes near the reference. Large changes may require more detailed vapor pressure data.
Where can I find DeltaHvap values?
Chemistry handbooks and databooks list enthalpies of vaporization at specified temperatures.
Can this handle mixtures?
No. Clausius-Clapeyron applies to pure substances. Mixtures need phase equilibrium data.
Does unit choice matter for pressure?
Use consistent units for P1 and P2 (kPa, atm, and so on). They cancel in the ratio.