Salting Pasta Water Raises the Boiling Point by Two Tenths of a Degree

Altitude moves it far more, and downward

Two tenths of a degree from a generous handful of salt is nothing beside what the barometer does: water boils several degrees below 100 in a mountain kitchen, and the effect is a change in pressure rather than in composition. The two act independently and can be added, which means a cook at altitude who salts heavily is still boiling well below sea-level temperature. If a recipe genuinely depends on the temperature of the water — a custard, a sugar syrup, an egg timed to the second — the lever that matters is the altitude, and salt is not a correction for it.

Molality, not molarity, and the reason matters

The formula asks for moles per kilogram of solvent rather than per litre of solution, and the choice is deliberate: a litre of anything expands when heated, so a concentration expressed per litre changes as the pot approaches the boil, while a concentration expressed per kilogram does not. For dilute salt water at room temperature the two figures are close enough to swap without noticing; for anything hot, concentrated or measured carefully, they are not, and using the wrong one introduces an error that grows exactly when the calculation is being taken seriously. All four colligative properties are defined on molality for the same reason.

Frequently asked questions

Does salted water boil faster or slower?

Marginally slower, because it has to reach a marginally higher temperature, and the difference is far too small to notice against everything else going on. Salt also very slightly lowers the specific heat of the water, which pulls the other way, and the two effects are of the same negligible order. What actually decides how long a pan takes is the burner's power, the amount of water, the lid and the shape of the pan — a lid is worth minutes, and salt is worth seconds at most.

Why does salt work so well on icy roads then?

Because the freezing end of the scale is far more sensitive than the boiling end. Water's cryoscopic constant is 1.86 °C per mole per kilogram against 0.512 for boiling, so the same dissolved amount pushes the freezing point down more than three and a half times as far as it pushes the boiling point up. Road salting also uses concentrations no cook would contemplate, which multiplies a coefficient that was already the larger of the two. The direction is the same physics; only the sensitivity differs, and that difference is the whole reason one use is a winter industry and the other is a kitchen myth.