Freezing Point Depression Calculator
The freezing point of a solution — the depression a dissolved solute causes, and how much you need for a target temperature.
ΔTf = i × Kf × m, where Kf is the solvent's cryoscopic constant (1.
How the freezing point depression calculator works
ΔTf = i × Kf × m, where Kf is the solvent's cryoscopic constant (1.86 °C·kg/mol for water), m is the molality and i is the number of particles each formula unit produces.
The van 't Hoff factor i is what makes salt so effective on roads: sodium chloride gives two ions, calcium chloride three. It is also why the effect eventually stops — you cannot dissolve more salt once the solution is saturated.
Formula: ΔTf = i Kf m
Worked examples
| Inputs | Freezing point depression | Note |
|---|---|---|
| 100 g of salt in 1 kg of water | 6.3655 °C | freezes at −6.4 °C |
| 1 molal sugar | 1.86 °C | −1.86 °C |
| Calcium chloride | 5.0279 °C | three ions per unit |
FAQFrequently asked questions
Why does salt melt ice?
It lowers the freezing point of water. At the road surface the mixture stays liquid below 0 °C, so ice cannot form or persist.
How cold can salt work?
Down to about −21 °C, the eutectic point of the salt–water system. Below that no amount of sodium chloride helps.
Why is calcium chloride better in deep cold?
It releases three ions rather than two and works to about −29 °C — and it dissolves exothermically, which helps it start.
What is the van 't Hoff factor?
The number of particles each dissolved formula unit produces. Sugar is 1, sodium chloride 2, calcium chloride 3.
Does the identity of the solute matter?
Only through the particle count. Colligative properties depend on how many particles there are, not what they are.
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.