Freezing Point Depression Calculator
Why road salt works, quantified.
Work out Freezing Point Depression. Why road salt works, quantified. Temperature in kelvin, handled for you.
1.86 for water. It is a property of the solvent, not the solute.
Particles per formula unit: 1 for sugar, 2 for NaCl, 3 for CaCl₂.
Freezing point (°C)
-1.86
Lowered by 1.86 °C
ΔT = i·K·b. The van 't Hoff factor is the part most often left at 1 by mistake: sodium chloride splits into two ions, so per mole it shifts the freezing point twice as far as sucrose does. Colligative properties count particles, not identity — which is why road salt works and why it is spread by mass rather than by chemistry.
How the Freezing Point Depression Calculator works
ΔT = i·Kf·b. For water Kf is 1.86 °C·kg/mol, so a 1 molal solution of sodium chloride — which contributes two particles per formula unit — depresses the freezing point by 3.72 °C rather than 1.86.
Also known as: cryoscopic constant calculator · why does salt melt ice · delta t freezing point · how to calculate freezing point depression
Frequently asked questions
How do I calculate freezing point depression?
Multiply the van 't Hoff factor, the cryoscopic constant and the molality. For water that is 1.86 per molal per particle.
Why does salt melt ice?
It depresses the freezing point of the water film on the ice, so the ice cannot stay solid at that temperature. Calcium chloride works better than sodium chloride because it contributes three particles per formula unit rather than two.
Why does road salt stop working in deep cold?
Because the depression is limited by how much salt will dissolve. Sodium chloride bottoms out near −21 °C at its eutectic point, below which no amount helps.
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