Freezing Point Depression Calculator
Dissolving a solute lowers the freezing point of the solvent, the effect behind road salt and antifreeze. The depression depends only on the number of dissolved particles, making it a colligative property. This calculator returns the freezing point depression and the new freezing point from the van't Hoff factor, the solvent's cryoscopic constant, the molality, and the pure solvent freezing point. The constants are inputs because they depend on the solvent and solute you choose.
Freezing point depression formula
delta Tf = i * Kf * m
New freezing point = pure freezing point - delta Tf
i is the number of particles each formula unit yields (1 for a non-electrolyte like sucrose, about 2 for NaCl), Kf is the solvent constant, and m is the molality. The depression is subtracted from the pure solvent's freezing point.
Worked example
One molal NaCl in water with i = 2 and Kf = 1.86: delta Tf = 2 * 1.86 * 1 = 3.72 degrees Celsius. Starting from water's 0 degrees Celsius, the new freezing point is 0 - 3.72 = -3.72 degrees Celsius.
Freezing point depression: frequently asked questions
What is freezing point depression?
Adding a solute lowers the freezing point of a solvent. The drop, delta Tf, is proportional to how many dissolved particles are present. It is a colligative property: it depends on the number of particles, not their identity.
What is the formula for freezing point depression?
Delta Tf = i times Kf times m, where i is the van't Hoff factor (particles per formula unit), Kf is the solvent's cryoscopic constant, and m is the molality of the solution in moles of solute per kilogram of solvent.
Why are Kf and the van't Hoff factor inputs?
The cryoscopic constant Kf is specific to each solvent (for water it is about 1.86 degrees Celsius per molal) and the van't Hoff factor depends on how the solute dissociates. Because both depend on the chemistry you choose, they are entered from a reference table rather than assumed.
Sources
- NIST: SI units, including the mole and kelvin.
- The colligative law delta Tf = i Kf m is standard; Kf and i are solvent and solute specific and are entered from a reference, never assumed.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.