Freezing-point depression

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

• molality is in units of mol/kg
• K_f, the freezing-point depression constant is a colligative property.
  • K_f for water is 1.86 K·kg/mol which means that per mole of solute dissolved in a kilogram of water the freezing point depression is 1.86 kelvins.
• 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.

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)