π§ Colligative Properties Calculator
Boiling point elevation and freezing point depression.
How to Use This Calculator
Pick a solute type (or custom van t Hoff factor), enter molality, and choose a solvent. You get the new freezing and boiling points.
The Formulas
Boiling point elevation: ΞTb = i x Kb x m
Dissolved particles disrupt the orderly crystal of freezing and make it harder for solvent molecules to escape as vapor. Both effects scale with particle count, which is why electrolytes punch above their weight.
Worked Example
1 molal NaCl in water: ΞTf = 2 x 1.86 x 1 = 3.72 C, so it freezes at -3.72 C. ΞTb = 2 x 0.512 x 1 = 1.02 C, so it boils at 101.02 C.
Why Salt Melts Ice
Dissolved particles disrupt the orderly crystal lattice ice needs to form, so the freezing point drops. Road salt exploits this: a concentrated brine can keep roads ice-free well below 32 F. But there is a limit; below about -6 F for sodium chloride, even saturated brine freezes, which is why northern cities switch to calcium chloride or sand in extreme cold.
The van't Hoff Factor
Table salt splits into two ions (Na+ and Cl-), so one mole of NaCl gives nearly two moles of particles and about twice the effect of sugar. The van't Hoff factor i captures this: 1 for sugar, about 2 for NaCl, about 3 for CaCl2. In reality ion pairing makes measured values slightly lower than ideal, especially at high concentrations.
Antifreeze in Your Car
Engine coolant is ethylene glycol and water, typically 50/50, which drops the freezing point to about -34 F and raises the boiling point to 265 F under the radiator cap's pressure. Both effects are colligative: they depend on particle count, not on glycol's chemistry. Never use pure antifreeze; the mixture outperforms it.
Frequently Asked Questions
What are colligative properties?
Properties that depend only on the number of dissolved particles, not their identity: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Salt and sugar affect them equally per particle.
Why does salt melt ice?
Dissolved salt lowers the freezing point of water. A concentrated brine can push it below -20 C. The ice melts because it is now above its new freezing point, and melting dilutes the brine until equilibrium.
What is the van t Hoff factor?
The number of particles a solute splits into. NaCl has i = 2, CaCl2 has i = 3, sugar has i = 1. It multiplies the effect: CaCl2 melts ice better per mole than NaCl.
How much does 1 molal NaCl lower the freezing point?
Delta Tf = i x Kf x m = 2 x 1.86 x 1 = 3.72 C. So the solution freezes at -3.72 C. Real solutions deviate slightly because ions pair up at high concentration.
Why do cars use antifreeze instead of just water?
Ethylene glycol both lowers the freezing point (to about -37 C at 50/50) and raises the boiling point (to about 106 C), protecting the engine in winter and summer. Pure water would freeze and burst the block.