ChemBench

Freezing Point Depression Calculator

The chemistry of why salt melts ice.

Van 't Hoff factor (i)

Cryoscopic constant Kf (°C·kg/mol)

Molality (mol/kg)

Freezing point depression

3.72 °C

What you entered

Van 't Hoff factor (i): 2Cryoscopic constant Kf (°C·kg/mol): 1.86Molality (mol/kg): 1

1
ΔTf = i × Kf × m
2 × 1.86 × 1= 3.72 °C

Result

Freezing point depression: 3.72 °C

This solution's freezing point drops by 3.72°C below pure solvent — this is why salt melts ice.

Freezing point depression for solutions in water

Delta Tf equals i times Kf times molality. Water's Kf is 1.86 C per molal, more than three times its boiling constant.

Solute	van't Hoff i	Molality	Depression	Freezes at
Glucose	1	1 m	1.860 C	-1.860 C
Glucose	1	2 m	3.720 C	-3.720 C
NaCl	2	0.5 m	1.860 C	-1.860 C
NaCl	2	1 m	3.720 C	-3.720 C
NaCl	2	3 m	11.160 C	-11.160 C
CaCl2	3	1 m	5.580 C	-5.580 C

This is why roads are salted rather than sugared: NaCl releases two particles per formula unit and CaCl2 three, so calcium chloride works at lower temperatures and is used when it is very cold.

The same principle as boiling point elevation, in reverse

Dissolved particles disrupt the solvent's ability to form an ordered solid, so a lower temperature is needed to freeze — this is exactly why salted roads and antifreeze work.

Why salt spreads on icy roads

Road salt (NaCl) dissolves into water on the road, lowering the freezing point below the ambient temperature so ice melts and doesn't easily re-form — this calculator quantifies exactly how many degrees that shift is.

Frequently asked questions

How much does 1 mol of NaCl per kg of water lower the freezing point?

ΔTf = i × Kf × m = 2 × 1.86 × 1.0 = 3.72°C. The freezing point drops from 0°C to about -3.72°C.

How is this different from boiling point elevation?

Same formula, opposite direction. Freezing point depression uses Kf (1.86°C·kg/mol for water) instead of Kb (0.512°C·kg/mol). Both use the molality calculator's output. The freezing point effect is about 3.6 times larger than the boiling point effect for water.

Why does road salt stop working below about -15°C?

NaCl can only depress water's freezing point to about -21°C at saturation. Below roughly -10 to -15°C, the practical effect weakens because less salt dissolves in the remaining liquid water. CaCl2 (i = 3) works to about -29°C.

Does sugar lower the freezing point as much as salt?

No. Sugar (sucrose) does not dissociate, so i = 1 versus NaCl's i = 2. For the same molality, sugar lowers the freezing point by half as much as NaCl.