What this calculator does
Dissolving a solute raises the boiling point of the solvent, by an amount proportional to particle concentration. Ten grams of sodium chloride in a kilogram of water raises the boiling point by only 0.175 °C.
That figure is worth noting because of a persistent kitchen myth. Salting pasta water does not meaningfully raise its boiling temperature: a heavily salted pot might gain half a degree. Salt is added for flavour, not for cooking speed, and the effect on cooking time is nil.
The formula
The elevation is the van t Hoff factor multiplied by the ebullioscopic constant of the solvent and the molality. Water has an ebullioscopic constant of 0.512 °C·kg/mol, which is less than a third of its cryoscopic constant of 1.86. That is why freezing point depression is the far more noticeable of the two effects.
| Term | Meaning |
|---|---|
| Ebullioscopic constant (Kb) | A solvent property. Water is 0.512 °C·kg/mol. |
| van t Hoff factor (i) | Particles per formula unit. |
| Molality | Moles of solute per kilogram of solvent. |
| Vapour pressure lowering | The underlying cause: solute reduces vapour pressure, so a higher temperature is needed to reach atmospheric. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass of solute (g) | Mass of solute in grams. |
| Molar mass of solute (g/mol) | Molar mass of the solute. |
| Mass of solvent (kg) | Mass of solvent in kilograms. |
| Ebullioscopic constant Kb (water 0.512) (°C·kg/mol) | Ebullioscopic constant of the solvent. Water is 0.512. |
| Van’t Hoff factor i | van t Hoff factor. About 2 for NaCl, 1 for sugar. |
| Boiling point of pure solvent (°C) | Boiling point of the pure solvent. Water is 100 °C at sea level. |
When to use it
Checking the pasta water claim
The calculation settles a common kitchen argument, and the answer is that the effect is negligible.
Engine coolant behaviour
Antifreeze raises the boiling point as well as lowering the freezing point, which matters for cooling system headroom.
Determining molar mass
The ebullioscopic method is the boiling-point analogue of the cryoscopic one, though less sensitive.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does dissociation raise the boiling point?
The same solution with different particle counts.
| van t Hoff factor | New boiling point | Boiling point elevation ΔTb | Molality |
|---|---|---|---|
| i = 1 | 100.09 °C | 0.088 °C | 0.1711 mol/kg |
| i = 2 | 100.18 °C | 0.175 °C | 0.1711 mol/kg |
| i = 3 | 100.26 °C | 0.263 °C | 0.1711 mol/kg |
Questions
Does salting pasta water make it boil hotter?
Barely. A tablespoon of salt in a large pot raises the boiling point by a fraction of a degree, which has no measurable effect on cooking time. Salt is added for flavour. The often-repeated claim that it speeds cooking does not survive the arithmetic.
Why is boiling point elevation smaller than freezing point depression?
Because the constants differ. For water, Kf is 1.86 and Kb is 0.512, so the same solution depresses the freezing point roughly 3.6 times as much as it raises the boiling point. The constants come from the enthalpies of the two phase changes.
Does altitude affect this?
It changes the starting boiling point, not the elevation. Water boils near 90 °C at 3,000 m, and dissolving solute raises that figure by the same increment it would at sea level. Enter the local boiling point as the starting value.
Why does the boiling point rise at all?
Because solute particles lower the vapour pressure of the solvent. Boiling happens when vapour pressure reaches atmospheric pressure, so a solution with reduced vapour pressure must be heated further to get there. Vapour pressure lowering is the root cause of both this and freezing point depression.
For the larger opposite effect, see the freezing point depression calculator. For the underlying vapour pressure change, see the Raoult’s law calculator.