Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent. It is directly proportional to the molal concentration of the solution according to the equation:

freezing-point depression = i * K_f * molality

• K_f, the freezing-point depression constant is a Colligative property.
• i is the i factor or the van 't Hoff i factor see van 't Hoff

i is the factor that takes into account the presence of ions in a solution, it indicates the number of particles formed.

K_f for water is 1.86 K.Kg/mole which means that per mole solute dissolved in a kilogram of water the freezing point depression is 1.86 Kelvin.

Examples:

• i = 1 for sugar in water
• i = 2 for NaCl in water
• i = 3 for CaCl₂ in water
• i = 2 for HCl in water (complete dissociation)
• i = 1 for HCl in benzene (no dissociation)