A liquid boils when its vapour pressure reaches the pressure of its surroundings. Lower the outside pressure – on a mountain, in a vacuum distillation flask or inside a freeze-dryer – and bubbles form at a lower temperature; raise it, as in a pressure cooker or a steam boiler, and the liquid must get hotter before it boils. This is why water boils at about 100 °C at sea level but near 90 °C at 3,000 m.
The Clausius–Clapeyron equation links the two pressures and temperatures through one property of the liquid: its molar enthalpy of vaporization ΔHvap, the energy needed to turn one mole of liquid into vapour. Substances with strong intermolecular forces (hydrogen-bonded water, alcohols) have a large ΔHvap and their boiling point shifts slowly with pressure; weakly bound liquids such as methane or hydrogen shift quickly.
Pick a substance to use its normal boiling point at 1 atm and its tabulated ΔHvap as the reference point, or choose Custom to enter any known boiling point, its pressure and your own enthalpy of vaporization. The result is most accurate when the new pressure is not too far from the reference, because ΔHvap itself changes slowly with temperature.
How to use the Boiling Point Calculator
- Substance
- Enthalpy of vaporization ΔHvap
- Known boiling point T₁
- Pressure at the known boiling point P₁
- New pressure P₂
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Convert the reference boiling point to kelvin, take the natural logarithm of the pressure ratio P₂/P₁, multiply by R/ΔHvap and subtract the product from 1/T₁. The reciprocal of that number is the new boiling point in kelvin, which the calculator converts to the unit you choose.
Worked example
At about 3,000 m the air pressure is near 70 kPa. For water, T₁ = 99.97 °C = 373.12 K at P₁ = 101.325 kPa, and ΔHvap = 40,650 J/mol. ln(70/101.325) = −0.3699, so 1/T₂ = 1/373.12 − 8.314 × (−0.3699)/40,650 = 0.0026801 + 0.0000757 = 0.0027558 K⁻¹. T₂ = 362.88 K = 89.7 °C, about 10 °C below the sea-level boiling point.
Reference data used for the presets
| Substance | Normal boiling point (1 atm) | ΔHvap at the boiling point |
|---|---|---|
| Water | 99.97 °C | 40.65 kJ/mol |
| Ethanol | 78.29 °C | 38.56 kJ/mol |
| Methanol | 64.7 °C | 35.21 kJ/mol |
| Acetone | 56.05 °C | 29.10 kJ/mol |
| Benzene | 80.09 °C | 30.72 kJ/mol |
| Toluene | 110.63 °C | 33.18 kJ/mol |
| n-Hexane | 68.73 °C | 28.85 kJ/mol |
| Diethyl ether | 34.5 °C | 26.52 kJ/mol |
| Chloroform | 61.17 °C | 29.24 kJ/mol |
| Ammonia | −33.33 °C | 23.33 kJ/mol |
| Methane | −161.48 °C | 8.19 kJ/mol |
| Propane | −42.1 °C | 19.04 kJ/mol |
| n-Butane | −0.5 °C | 22.44 kJ/mol |
| Hydrogen | −252.87 °C | 0.90 kJ/mol |
Values are the normal boiling points and enthalpies of vaporization at the boiling point as listed in the CRC Handbook of Chemistry and Physics and the NIST Chemistry WebBook.
When to use it
Use this page to predict boiling points for vacuum distillation, rotary evaporation, pressure cooking or storage of liquefied gases such as propane and butane. For water at different altitudes, the boiling point at altitude calculator works out the air pressure for you; to go the other way – vapour pressure at a given temperature – use the vapor pressure calculator.
Tips
- Use kelvin for every temperature in thermodynamic formulas – the unit menus convert for you.
- Watch the units: ΔH is usually in kJ/mol and ΔS in J/(mol·K).
- Tabulated values are for standard conditions (1 bar, usually 25 °C).
- Everything is calculated in your browser.
Common mistakes to avoid
- Mixing kJ and J between ΔH and ΔS.
- Assuming ΔH and ΔS do not change with temperature over large ranges.
- Using Celsius in ln(T₂/T₁).
Inputs explained
| Field | What to enter |
|---|---|
| Substance | Choose one: Custom – enter your own values, Water (bp 99.97 °C, ΔHvap 40.65 kJ/mol), Ethanol (bp 78.29 °C, ΔHvap 38.56 kJ/mol), Methanol (bp 64.7 °C, ΔHvap 35.21 kJ/mol) … |
| Enthalpy of vaporization ΔHvap | Units: kJ/mol, J/mol, kcal/mol, cal/mol, eV (per particle) |
| Known boiling point T₁ | Units: °C, K, °F |
| Pressure at the known boiling point P₁ | Units: kPa, atm, bar, hPa (mbar), mmHg (Torr), psi … |
| New pressure P₂ | Units: kPa, atm, bar, hPa (mbar), mmHg (Torr), psi … |
What each result means
- Change from the reference boiling point = [Boiling point at P₂] – ([tref] – 273.15)
- Reference boiling point T₁ = [tref] – 273.15
- Reference pressure P₁ = [pref]
- Enthalpy of vaporization used = [Hv]
- Pressure ratio P₂/P₁ = [New pressure P₂]÷[pref]
Formula
ln(P₂/P₁) = −(ΔHvap/R) · (1/T₂ − 1/T₁) ⇒ T₂ = 1 / (1/T₁ − R·ln(P₂/P₁)/ΔHvap) · R = 8.314 J/(mol·K), temperatures in kelvinWorked example
Inputs — Substance: Water (bp 99.97 °C, ΔHvap 40.65 kJ/mol); New pressure P₂: 70 kPa.
Results — Boiling point at P₂ = 89.728 °C; Change from the reference boiling point = -10.242 °C (= K); Reference boiling point T₁ = 99.97 °C; Reference pressure P₁ = 101.325 kPa; Enthalpy of vaporization used = 40.65 kJ/mol; Pressure ratio P₂/P₁ = 0.6908; Direction = Lower pressure – the liquid boils at a lower temperature.
Key terms
- Boiling point
- The temperature at which the vapour pressure of a liquid equals the surrounding pressure.
- Normal boiling point
- The boiling point at exactly 1 atm (101.325 kPa).
- Enthalpy of vaporization (ΔHvap)
- The heat absorbed when one mole of liquid turns into vapour at constant temperature and pressure.
- Clausius–Clapeyron equation
- A relation between vapour pressure and temperature derived from the Clapeyron equation, assuming an ideal-gas vapour and a constant ΔHvap.
Frequently asked questions
What is the boiling point of water at 70 kPa?
About 89.7 °C by Clausius–Clapeyron with ΔHvap = 40.65 kJ/mol; steam tables give 89.9 °C, so the estimate is within 0.2 °C.
Why does the boiling point drop when pressure drops?
A liquid boils when its vapour pressure equals the outside pressure. At lower outside pressure, a lower vapour pressure – and therefore a lower temperature – is enough.
What temperature does water reach in a pressure cooker?
A typical cooker adds about 100 kPa (15 psi) of gauge pressure, so the absolute pressure is about 200 kPa and water boils at roughly 120 °C.
How accurate is the Clausius–Clapeyron equation?
It assumes ΔHvap is constant and the vapour is an ideal gas. Within ±30 °C of the reference point the error is usually below 1 °C; far from it (or near the critical point) the error grows.
Which value of ΔHvap should I use?
Use the enthalpy of vaporization at or near the known boiling point. Water's is 40.65 kJ/mol at 100 °C but 44.0 kJ/mol at 25 °C.
Can I use any pressure unit?
Yes – only the ratio P₂/P₁ enters the equation, and each field has its own unit menu, so mixing kPa and mmHg is fine.
Why do the presets use 1 atm?
Tabulated "normal boiling points" are defined at exactly 1 atm (101.325 kPa); the newer "standard boiling point" uses 1 bar, which is about 0.4 °C lower for water.
What formula does the Boiling Point Calculator use?
ln(P₂/P₁) = −(ΔHvap/R) · (1/T₂ − 1/T₁) ⇒ T₂ = 1 / (1/T₁ − R·ln(P₂/P₁)/ΔHvap) · R = 8.314 J/(mol·K), temperatures in kelvin.