Does the same formula explain the boiling point?

Yes, with a different constant and the opposite sign — the boiling point rises rather than falls. Water's ebullioscopic constant is much smaller than its cryoscopic one, which is why salting pasta water is a seasoning decision and not a cooking-time decision: the quantity a cook actually adds raises the boiling point by a fraction of a degree, far too little to matter to the pasta. Both effects are colligative, meaning they depend on how many particles are dissolved and not on what they are.

Why does grit work even below the calculated temperature?

Because it is doing something else entirely. Sand and grit do not melt ice; they sit on top of it and give a tyre or a boot something to bite into, which works at any temperature and stops working when fresh snow covers it. Chemical de-icers and abrasives are often spread together for exactly that reason, and the mixture is chosen from the forecast rather than from a formula — below the useful range of the salt, the grit is the only part still contributing.

Why Salt Beats Sugar on an Icy Path

Published 10/6/2026 · 3 min read · Everyday calculators

Marco Bianchi — Home, DIY & motoring writer at OneKitly

Renovation · Materials

Checked against 2 sources

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In short

The depression is the cryoscopic constant times the molality times the van 't Hoff factor. For water the constant is 1.86 °C·kg/mol, so one mole per kilogram of a substance that stays as one particle gives 1.86 °C — and the same molality of common salt, which separates into a sodium ion and a chloride ion, gives 2 × 1.86 = 3.72 °C. That factor is the whole reason salt is spread on roads and sugar is not: the effect counts particles, not molecules, and a salt that splits delivers two for the price of one. Triple the molality to 3 mol/kg and the depression reaches 11.16 °C, which is roughly where a well-treated road sits — and also where the simple formula starts to overstate what really happens.

One mole per kilogram of a salt that splits in two lowers water's freezing point by 3.72 °C. The same amount of something that stays whole manages 1.86 °C — exactly half.

Per mole, not per gram

Molality counts particles by the mole, and a mole of a light substance weighs far less than a mole of a heavy one. That is why the sack in the shed and the arithmetic here disagree about what is efficient: to reach a given molality you need the molar mass of whatever you are spreading, and a substance with half the molar mass gives twice the effect per kilogram. Comparing two de-icers therefore means converting both to moles per kilogram of water before looking at the constant at all — a comparison made in kilograms of product answers a different question, though it is the one the invoice asks.

The formula keeps going after the salt stops

Two things break at high concentration and the calculation notices neither. The first is solubility: there is a limit to how much salt water will hold, and past it the extra sits on the surface doing nothing. Below the temperature at which the mixture freezes as a whole — the eutectic point of the water-and-salt system — spreading more changes nothing at all, which is why grit lorries switch to other products in a hard frost. The second is the ideal-solution assumption behind the formula, which weakens well before that: at road concentrations the ions interact and the effective particle count falls below two, so the real depression is smaller than the arithmetic promises.

Particles per unit

Water, cryoscopic constant 1.86
Solute	Particles per unit	Depression at 1 mol/kg
Sugar (stays whole)	1	1.86 °C
Table salt (two ions)	2	3.72 °C
A salt giving three ions	3	5.58 °C

Worked with our own calculator

Freezing point depression calculator

Given

Cryoscopic constant Kf (°C·kg/mol): 3.72
Molality (mol/kg): 2
van't Hoff factor (i): 4
Pure solvent freezing point (°C): 5

Result

Freezing point depression ΔTf: 29.76 °C
New freezing point: -24.76 °C

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

Does the same formula explain the boiling point?: Yes, with a different constant and the opposite sign — the boiling point rises rather than falls. Water's ebullioscopic constant is much smaller than its cryoscopic one, which is why salting pasta water is a seasoning decision and not a cooking-time decision: the quantity a cook actually adds raises the boiling point by a fraction of a degree, far too little to matter to the pasta. Both effects are colligative, meaning they depend on how many particles are dissolved and not on what they are.
Why does grit work even below the calculated temperature?: Because it is doing something else entirely. Sand and grit do not melt ice; they sit on top of it and give a tyre or a boot something to bite into, which works at any temperature and stops working when fresh snow covers it. Chemical de-icers and abrasives are often spread together for exactly that reason, and the mixture is chosen from the forecast rather than from a formula — below the useful range of the salt, the grit is the only part still contributing.

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