Last updated: July 3, 2026
Boiling Point Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The boiling point calculator has two strategies. Pressure mode estimates water's boiling point from absolute pressure with the Clausius–Clapeyron equation anchored at 100 °C and 101.325 kPa. Elevation mode estimates aqueous solution boiling-point elevation with ΔTb = i Kb m, using Kb(water) = 0.512 °C·kg/mol.
Water boils at 100 degrees Celsius at 101.325 kilopascals. Lower pressure lowers the boiling point, while dissolved solute raises it by delta T b equals i times K b times molality.
Key Takeaways
- Pressure mode uses 1/Tb = 1/T0 − (R/ΔHvap) ln(P/P0) for water.
- At 101.325 kPa, the formula returns the normal boiling point: 100 °C.
- At 84.5 kPa water boils at about 94.9 °C; at 70 kPa it boils at about 89.8 °C.
- Elevation mode uses ΔTb = i Kb m with Kb(water) = 0.512 °C·kg/mol.
- A 1 molal ideal NaCl solution raises water's boiling point by 1.024 °C at 1 atm.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Pressure: 1/Tb = 1/T0 − (R/ΔHvap) ln(P/P0); Elevation: ΔTb = i Kb m and Tb = 100 + ΔTb
Where:
- Tb=Boiling point(K or °C)
- T0=Normal boiling point of water(373.15 K)
- P=Absolute pressure(kPa)
- P0=Standard pressure(101.325 kPa)
- R=Gas constant(8.314 J/(mol·K))
- ΔHvap=Enthalpy of vaporization of water(40660 J/mol)
- i=van't Hoff factor(dimensionless)
- Kb=Ebullioscopic constant of water(0.512 °C·kg/mol)
- m=Molality(mol/kg)
Worked Examples
Normal boiling point of water
At standard atmospheric pressure, water boils at 100 °C.
- 1Use T0 = 373.15 K and P0 = 101.325 kPa.
- 2ln(P/P0) = ln(1) = 0, so 1/Tb = 1/T0.
- 3Tb = 373.15 K = 100 °C.
Water near 1500 m altitude
A pressure of about 84.5 kPa gives a lower boiling point than sea level.
- 1Insert P = 84.5 kPa into the Clausius–Clapeyron relation.
- 2Compute 1/Tb = 1/373.15 − (8.314/40660) ln(84.5/101.325).
- 3Convert from kelvin to Celsius: Tb ≈ 94.9 °C.
Water at 70 kPa
Reduced pressure makes water boil around 90 °C.
- 1Use P/P0 = 70/101.325 in the logarithmic pressure term.
- 2Solve for absolute temperature Tb in kelvin.
- 3Tb ≈ 362.91 K = 89.76 °C.
Pure water in elevation mode
No dissolved solute means no colligative boiling-point elevation.
- 1Use ΔTb = i Kb m.
- 2With m = 0, ΔTb = 1 × 0.512 × 0 = 0 °C.
- 3Tb = 100 + 0 = 100 °C at 1 atm.
1 molal NaCl solution
Ideal NaCl produces about two dissolved particles per formula unit.
- 1Use Kb(water) = 0.512 °C·kg/mol.
- 2ΔTb = i Kb m = 2 × 0.512 × 1 = 1.024 °C.
- 3Tb = 100 + 1.024 = 101.024 °C.
0.5 molal glucose solution
Glucose is a nonelectrolyte, so i = 1.
- 1Set i = 1 for glucose.
- 2ΔTb = 1 × 0.512 × 0.5 = 0.256 °C.
- 3Tb = 100 + 0.256 = 100.256 °C.
Introduction
The boiling point calculator estimates how water's boiling temperature changes when pressure changes or when a dissolved solute raises the boiling point. In pressure mode it uses an integrated Clausius–Clapeyron approximation anchored at water's normal boiling point. In elevation mode it uses the colligative-property equation ΔTb = i Kb m for aqueous solutions at 1 atm. For solution preparation, pair it with the molarity calculator or concentration calculator. The thermodynamic background follows standard phase-equilibrium treatments such as NIST Chemistry WebBook and Chemistry LibreTexts.
What this boiling point calculator solves
This general tool deliberately covers two common boiling-point effects in one place. Pressure mode answers: at what temperature will pure water boil when the external pressure is P? Elevation mode answers: how much does a dissolved solute raise water's normal boiling point at 1 atm? The first is a vapor-pressure equilibrium problem; the second is a colligative-property problem.
Pressure mode: pure water, pressure in kPa, result from Clausius–Clapeyron.
Elevation mode: aqueous solution at 1 atm, molality and van't Hoff factor.
Primary output is always boiling point in °C.
Use a dedicated altitude calculator when pressure must be inferred from elevation and weather.
Pressure mode: Clausius–Clapeyron equation
For pressure changes near ordinary boiling temperatures, the calculator uses 1/Tb = 1/T0 − (R/ΔHvap) ln(P/P0), with T0 = 373.15 K, P0 = 101.325 kPa, R = 8.314 J/(mol·K), and ΔHvap = 40660 J/mol for water. Lower pressure makes the logarithm negative and lowers Tb; higher pressure raises Tb.
This is much better than a fixed linear rule, but it is still an approximation because ΔHvap changes with temperature.
Elevation mode: ΔTb = i Kb m
Boiling-point elevation is colligative: it depends mainly on the number of dissolved particles, not their identity. For water, Kb = 0.512 °C·kg/mol. A 1 molal ideal NaCl solution with i ≈ 2 raises the boiling point by 1.024 °C, while 0.5 molal glucose with i = 1 raises it by 0.256 °C. Use the mole calculator when converting solute mass to amount before molality work.
Boiling point benchmarks
These values are useful checks for classroom and lab calculations.
| Situation | Input | Expected boiling point |
|---|---|---|
| Standard pressure | 101.325 kPa | 100.00 °C |
| Moderate altitude pressure | 84.5 kPa | ≈ 94.90 °C |
| Reduced pressure | 70 kPa | ≈ 89.76 °C |
| Pure water, elevation mode | m = 0 | 100.00 °C |
| Ideal 1 m NaCl | i = 2, m = 1 | 101.024 °C |
| 0.5 m glucose | i = 1, m = 0.5 | 100.256 °C |
Assumptions and limits
The pressure model is calibrated for water and is most reliable near the normal boiling region, not near the critical point or deeply cryogenic pressures. The elevation model assumes an ideal dilute aqueous solution at 1 atm. Electrolytes can have non-ideal van't Hoff factors, especially at higher concentration, so measured boiling points may differ from the ideal result.
Use absolute pressure, not gauge pressure. If a gauge reads 0 kPa at atmospheric pressure, the absolute pressure is still about 101.325 kPa.
Choosing the van't Hoff factor
For nonelectrolytes such as glucose or sucrose, use i = 1. For an ideal fully dissociated salt, i is approximately the number of ions per formula unit: NaCl ≈ 2, CaCl2 ≈ 3, and AlCl3 ≈ 4. Real solutions often have smaller effective i values because ions interact. Check high-precision work against activity-coefficient data or measured boiling points from references such as the CRC Handbook.
Quick Reference Card
Boiling Point — Quick Reference
Quick reference • Boiling Point Calculator
Pressure: 1/Tb = 1/T0 − (R/ΔHvap)ln(P/P0); Elevation: ΔTb = iKbmValid range: Water near ordinary boiling temperatures; ideal dilute aqueous solutions at 1 atm for elevation mode
Common Values
⚠ Watch Out
- •Use absolute pressure in kPa, not gauge pressure.
- •The pressure equation is for water and assumes a nearly constant ΔHvap.
- •Do not apply water's Kb to other solvents.
- •Real electrolytes can have effective van't Hoff factors below ideal integer values.
Pro Tips
- →Check standard pressure first: 101.325 kPa should return exactly 100 °C.
- →Use molality, not molarity, for boiling-point elevation.
- →For glucose or sucrose, set i = 1 because they do not dissociate appreciably.
- →For approximate altitude problems, convert altitude to pressure before using pressure mode.
FAQs
How does pressure affect the boiling point of water?
Water boils when its vapor pressure equals the external pressure. Lower pressure means that equality is reached at a lower temperature; higher pressure requires a higher temperature.
Why use Clausius–Clapeyron instead of a simple linear rule?
The pressure-temperature relationship is logarithmic, not exactly linear. The Clausius–Clapeyron approximation captures that curvature and gives realistic checks such as about 94.9 °C at 84.5 kPa and about 89.8 °C at 70 kPa.
What is boiling-point elevation?
Boiling-point elevation is the rise in a solvent's boiling point caused by dissolved solute particles. For water at 1 atm, the ideal equation is ΔTb = i Kb m, where Kb = 0.512 °C·kg/mol.
What van't Hoff factor should I use for NaCl?
For an ideal dilute sodium chloride solution, use i ≈ 2 because NaCl dissociates into Na+ and Cl−. In concentrated real solutions the effective factor can be lower.
Can I combine pressure and solute effects in one result?
This calculator keeps the two textbook strategies separate for clarity: pressure mode for pure water at pressure P, and elevation mode for solutes at 1 atm. Combining both accurately requires vapor-pressure lowering data or more advanced solution thermodynamics.
Is this calculator valid for every liquid?
No. The constants are for water. Other liquids need their own normal boiling point, vaporization enthalpy or Antoine coefficients, and ebullioscopic constant.