Boiling Point Elevation Calculator
Find how much a dissolved substance raises the boiling point and lowers the freezing point of water — ΔT = i × K × m — from the molality and the van't Hoff factor.
How much salt or sugar changes water's boiling and freezing points.
How the boiling point elevation calculator works
Dissolved particles raise a solvent's boiling point and lower its freezing point in proportion to their molality (moles per kilogram of solvent) and how many particles each formula unit makes (the van't Hoff factor: 1 for sugar, 2 for NaCl, 3 for CaCl₂). For water K_b is 0.512 °C·kg/mol and K_f is 1.86.
Enter the solute mass, molar mass and solvent mass; the calculator finds the molality and both changes. Real electrolytes deviate a little from the whole-number factor at high concentration.
Formula: ΔT_b = i K_b m · ΔT_f = i K_f m · m = moles ÷ kg solvent
Worked examples
| Inputs | Boiling point rises by | Note |
|---|---|---|
| 58.44 g NaCl in 1 kg water (1 mol) | 1.024 °C | boils at 101.02 °C, freezes at −3.72 |
| 342 g sucrose in 1 kg water | 0.512 °C | half the effect of NaCl |
| A teaspoon of salt (6 g) in a 2 L pot | 0.053 °C | boils 0.05 °C higher — pasta water is a myth |
FAQFrequently asked questions
Does salting pasta water make it boil faster?
No — a teaspoon in two litres raises the boiling point by about 0.05 °C and slightly lengthens the time to boil. Salt is for flavour.
Why does antifreeze work?
Ethylene glycol dissolved in the coolant lowers its freezing point (and raises its boiling point) exactly by this mechanism — a 50/50 mix freezes near −37 °C.
What is the van't Hoff factor?
The number of particles a formula unit produces in solution: NaCl → Na⁺ + Cl⁻ gives 2; CaCl₂ gives 3; sugar stays whole, 1. Real electrolytes give slightly less because ions pair up.
Can I use another solvent?
Yes — enter its K_b and K_f in Detailed mode (benzene 2.53 / 5.12; ethanol 1.22 / 1.99) and read the change relative to its normal boiling and freezing points.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.