What is freezing point depression?

Freezing point depression is the phenomenon where adding a solute to a pure solvent lowers the freezing point of the resulting solution below that of the pure solvent. It is a colligative property — it depends on the number of dissolved particles, not their identity.

What is the cryoscopic constant (Kf)?

The cryoscopic constant (Kf) is a property of the solvent that quantifies how much the freezing point is depressed per unit molality of solute. For water, Kf = 1.86 °C·kg/mol. Each solvent has its own unique Kf value.

What is the van't Hoff factor (i)?

The van't Hoff factor (i) accounts for the number of particles a solute produces when dissolved. For non-electrolytes (e.g., glucose, urea), i = 1. For strong electrolytes, i equals the number of ions per formula unit: NaCl → i = 2, CaCl₂ → i = 3. Real solutions often show i slightly below the theoretical value due to ion pairing.

What is molality and how is it different from molarity?

Molality (m) is the number of moles of solute per kilogram of solvent (mol/kg). Unlike molarity (mol/L), molality does not change with temperature because it is mass-based, not volume-based. This makes molality the preferred concentration unit for colligative property calculations.

Why does salt lower the freezing point of water?

When NaCl dissolves in water it dissociates into Na⁺ and Cl⁻ ions (i = 2), doubling the effective particle concentration. These additional particles disrupt the crystal lattice formation of ice, so a lower temperature is required before the solution freezes.

Can I use this to find molar mass of an unknown solute?

Yes. If you measure the freezing point depression (ΔTf) experimentally, you can rearrange the formula to find molality (m = ΔTf / (Kf × i)), then use m = moles_solute / kg_solvent to find moles, and finally molar mass = mass_solute / moles_solute. This technique is called cryoscopy.

Freezing Point Depression Calculator

Enter the cryoscopic constant (Kf), molality, and van't Hoff factor to calculate how much a solute lowers the freezing point of a solvent.

Enter Values

Fill in the known values to calculate freezing point depression.

ΔT_f = K_f × m × i

Solve for

Freezing Point Depression (ΔT_f) Molality (m) Cryoscopic Constant (K_f) van't Hoff Factor (i)

Solvent Preset

Cryoscopic Constant (K_f) °C·kg/mol

Molality (m) mol/kg

van't Hoff Factor (i) dimensionless

Freezing Point Depression (ΔT_f) °C

Freezing Point of Pure Solvent °C (optional)

Fill in the known values and click Calculate

Result

All Variables Summary

Symbol	Description	Value

Step-by-Step Solution

Summary

Freezing point depression is a colligative property describing how a dissolved solute lowers the freezing point of a solvent. The formula ΔTf = Kf × m × i relates the depression to the cryoscopic constant (Kf) of the solvent, the molality (m) of the solution, and the van't Hoff factor (i) which accounts for the number of particles a solute dissociates into. This calculator lets you find ΔTf, the actual freezing point of the solution, or solve for any single unknown variable — all client-side with no data sent to any server.

How it works

Choose a preset solvent or select "Custom" and enter your own Kf value.
Enter the molality of the solution (moles of solute per kilogram of solvent).
Enter the van't Hoff factor (i = 1 for non-electrolytes, 2 for NaCl, 3 for CaCl₂, etc.).
Click Calculate — the tool applies ΔTf = Kf × m × i.
Review the freezing point depression and the new freezing point of the solution.
Click Reset to clear inputs and start a new calculation.

Use cases

Calculate how much salt lowers the freezing point of water on icy roads.
Determine the freezing point of antifreeze solutions in automotive coolants.
Solve colligative property problems for general and physical chemistry coursework.
Estimate the molality of an unknown solute from a measured freezing point.
Compare the effectiveness of different de-icing agents.
Prepare for AP Chemistry or university physical chemistry exams.
Model food preservation and frozen dessert formulations.
Understand cryoscopy as a method for measuring molar mass.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu