ΔTf = i × Kf × m
Freezing point depression is a colligative property — it depends only on the number of solute particles, not their identity. The formula ΔTf = i × Kf × m relates the depression to the van't Hoff factor (i, number of particles per formula unit: 1 for non-electrolytes, 2 for NaCl, 3 for CaCl₂), the cryoscopic constant Kf (solvent-specific), and molality m (mol solute/kg solvent). Water has Kf = 1.86 °C·kg/mol. This property is used to: determine molar masses of unknown solutes, formulate antifreeze (ethylene glycol depresses water's FP to about −37 °C at 60% concentration), de-ice roads with salt, and make ice cream (salt-ice baths). The depression occurs because solute particles disrupt the solvent's crystal lattice formation.