Freezing Point Depression Calculator
Find how much a solute lowers the freezing point.
Ions per formula unit; 1 for non-electrolytes
Water = 1.86
Results update as you type.
How it works
Dissolving a solute in a liquid lowers its freezing point, and this calculator determines by how much using the colligative property equation. You enter the van 't Hoff factor, the solvent's cryoscopic constant Kf, and the solution's molality, and it returns the freezing point depression in degrees Celsius. Like boiling point elevation, this effect is colligative, depending only on the number of dissolved particles rather than their identity. The van 't Hoff factor accounts for solutes that dissociate into ions: salt yields about two particles per formula unit, so it lowers the freezing point roughly twice as much as an equal molality of a non-dissociating solute. Kf is specific to each solvent; for water it is 1.86 degrees Celsius per molal. This principle is behind some of the most familiar chemistry in daily life: spreading salt on icy roads and sidewalks lowers water's freezing point so ice melts, and antifreeze in a car's radiator keeps the coolant liquid far below zero. Chemists also use it to determine molar masses experimentally. Subtract the calculated depression from the pure solvent's normal freezing point to find the solution's freezing point. Enter the three values to compute the drop.
ΔTf = i · Kf · m
Freezing point depression is a colligative property that depends on the number of dissolved particles. The drop in freezing point equals the van 't Hoff factor, which counts particles produced per formula unit, times the cryoscopic constant of the solvent, times the molality. Multiplying these gives how far the freezing point falls below that of the pure solvent, which you subtract from the solvent's normal freezing point.
Source:Chemistry (colligative properties)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter van 't hoff factor (i), kf (cryoscopic constant) and molality.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Freezing point drop (ΔTf) updates instantly as you type — no button to press.
Frequently asked questions
What is freezing point depression?
Freezing point depression is the lowering of a liquid's freezing point when a solute is dissolved in it. It is a colligative property, so it depends on the number of dissolved particles, not their identity. The equation is ΔTf equals the van 't Hoff factor times the cryoscopic constant times the molality, and you subtract the result from the pure solvent's freezing point.
Why does salt melt ice?
Salt dissolves into the thin layer of liquid water on ice and lowers its freezing point through freezing point depression. Because salt dissociates into ions, it depresses the freezing point strongly. Once the freezing point drops below the surrounding temperature, the ice can no longer stay frozen and melts. This is why salt is spread on winter roads and sidewalks.
How does antifreeze work?
Antifreeze, usually ethylene glycol, is added to a car's water-based coolant to lower its freezing point well below zero, preventing the coolant from freezing and cracking the engine in cold weather. It also raises the boiling point, protecting against overheating. Both effects are colligative properties governed by the same principles as this freezing point depression calculation.
What is Kf?
Kf is the cryoscopic constant, a value unique to each solvent that measures how much its freezing point drops per unit of molality. For water it is 1.86 degrees Celsius per molal. Multiplying Kf by the molality and the van 't Hoff factor gives the total freezing point depression. Use the correct Kf for whichever solvent your solute is dissolved in.
Can this determine molar mass?
Yes. Historically, freezing point depression was a common way to find an unknown's molar mass. By measuring how much a known mass of solute lowers a solvent's freezing point, you can work backward through the equation to find the molality, then the moles, and finally the molar mass. This technique, called cryoscopy, links a measurable temperature change to molecular size.