Freezing Point Depression Calculator
Dissolving a solute in a solvent lowers the temperature at which it freezes, which is why salt melts road ice and antifreeze protects engines. The size of the drop depends only on how many solute particles are present, a colligative property, not on what they are. It is set by the molality of the solution, the solvent's cryoscopic constant, and the van 't Hoff factor counting particles per formula unit. This calculator applies Delta Tf equals i times Kf times m and returns both the freezing point depression and the new freezing point.
Freezing point depression formula
Delta Tf = i * Kf * m
new freezing point = pure freezing point - Delta Tf
The depression is proportional to the molality of dissolved particles. The van 't Hoff factor i accounts for solutes that dissociate into multiple ions, multiplying the effective particle count. Subtract the depression from the pure solvent freezing point to get the solution's freezing point.
Freezing point depression facts
- Water's cryoscopic constant is 1.86 degrees C per molal.
- Sodium chloride has a van 't Hoff factor of 2; calcium chloride about 3.
- The effect is colligative: it depends on particle count, not particle identity.
- Road salt and engine antifreeze both exploit freezing point depression.
- Kf and the van 't Hoff factor are user-editable for any solvent and solute.
Freezing point depression: frequently asked questions
What is freezing point depression?
Freezing point depression is the lowering of a solvent's freezing point when a solute is dissolved in it. It is a colligative property, meaning it depends on the number of dissolved particles, not their identity. Adding salt to icy roads and antifreeze to engine coolant both rely on this effect.
What is the freezing point depression formula?
Delta Tf = i * Kf * m, where i is the van 't Hoff factor (the number of particles each formula unit produces), Kf is the cryoscopic (molal freezing point depression) constant of the solvent, and m is the molality of the solution in moles of solute per kilogram of solvent. The new freezing point is the pure solvent value minus Delta Tf.
What is the van 't Hoff factor?
The van 't Hoff factor i is the number of particles a solute releases per formula unit when dissolved. It is 1 for nonelectrolytes like sugar, 2 for sodium chloride (which splits into one sodium and one chloride ion), and 3 for calcium chloride. Real values can be slightly lower than ideal due to ion pairing.
What is the cryoscopic constant Kf?
Kf is a property of the solvent giving the freezing point drop per unit molality. For water it is 1.86 degrees C per molal; for benzene it is about 5.12 degrees C per molal. It is a user-editable input here so you can enter the value for your specific solvent from a reference table.
What units does this calculator use?
Molality is in moles of solute per kilogram of solvent, Kf is in degrees C per molal, and the van 't Hoff factor is dimensionless. The freezing point depression Delta Tf and the resulting new freezing point are reported in degrees Celsius. The pure solvent freezing point defaults to 0 degrees C for water.
Official sources
- NIST Chemistry WebBook: Thermophysical property data.
- NIST: SI units and quantity definitions (SP 811).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.