Freezing Point Depression Calculator
Enter the van 't Hoff factor, the cryoscopic constant, and the molality to find how far a solution's freezing point drops below the pure solvent — the science behind salting icy roads.
Use molality
Concentration must be molality (mol per kilogram of solvent), not molarity, and Kf must match the solvent — water is about 1.86 K·kg/mol.
What is freezing point depression?
A colligative property of solutions
A freezing point depression calculator works out how far a solution freezes below its pure solvent once a solute is dissolved in it. Freezing point depression is a colligative property: it depends only on how many solute particles are present, not on what they are. Dissolve sugar, salt, or antifreeze in water and the freezing point always drops. This tool turns three numbers — the van 't Hoff factor (how many particles each unit splits into), the cryoscopic constant of the solvent, and the molality — into the temperature drop ΔTf. It is the number behind salting winter roads, antifreeze in a radiator, and how chemists measure molar masses.
Enter the van 't Hoff factor, the cryoscopic constant Kf, and the molality to get the freezing point depression ΔTf instantly.
The freezing point depression equals the van 't Hoff factor times the cryoscopic constant times the molality.
ΔTf = i × Kf × mEach factor pulls in one piece of the chemistry. The van 't Hoff factor i counts the particles a solute releases — 1 for sugar, 2 for NaCl, 3 for CaCl₂. The cryoscopic constant Kf is fixed by the solvent (about 1.86 K·kg/mol for water). The molality m is the moles of solute per kilogram of solvent. Multiply the three and the answer comes back in kelvin, which is also the size of the drop in degrees Celsius. Suppose you dissolve 1 mole of table salt in 1 kg of water: with i = 2, Kf = 1.86, and m = 1, the result is 2 × 1.86 × 1 = 3.72, so the salt water freezes at roughly −3.72 °C instead of 0 °C. Doubling the salt to 2 molal doubles the drop to 7.44 K, which is exactly how road crews keep ice melting well below freezing.
The formula is simple, but it rests on assumptions that matter at higher concentrations.
Dilute, ideal solutions only
ΔTf = i × Kf × m assumes a dilute, ideal solution. In concentrated brines the ions interact, so the effective van 't Hoff factor falls below the ideal value (NaCl behaves more like 1.9 than 2), and the real drop is smaller than the formula predicts. Always use molality (mol per kilogram of solvent), not molarity, and a Kf that matches your solvent — water is 1.86 K·kg/mol, benzene about 5.12. The result is the size of the drop, so subtract it from the pure solvent's freezing point to get the new freezing temperature.