Molal boiling point elevation constant is the elevation in boiling point when the molality of solution is unity i.e., 1 mol of solute is dissolved in 1 kg (1000 g) of the solvent. The units of molal elevation constant is degree molality^–1 i.e., K molal^–1 or ºC molal^–1 or K kg mol^–1. It is denoted by K_b . The cause of elevation in boiling point is the vapor pressure of the solution is lower than that of pure solvent. Hence, when the non-volatile solute is added to pure solvent, the solution has to be heated more to make vapor pressure equal to the atmospheric pressure. Molal freezing point elevation constant is the depression in freezing point when the molality of the solutions is unity i.e., 1 mol of the solute is dissolved in 1000 g (1 kg) of the solvent. It is denoted by K_f . The unit of K_f is degree molality^–1 i.e., K molal^–1 or K kg mol^–1 . The cause of depression in freezing point is the vapor pressure of the solution is less than that of pure solvent. Freezing point of a substance is the temperature at which the solid and the liquid forms of the substance are in equilibrium i.e., the solid and the liquid forms of substance have the same vapor pressure. Therefore, for the solution, this will occur at a lower temperature.