During depression of freezing point in a solution the following are in equilibrium : 

Liquid and solid solvents are in equilibrium as a solution's freezing point depresses. Only the solvent freezes out during the freezing of a solution, and the solvent's solid and liquid states are in equilibrium. The vapor pressure of the liquid and frozen forms of the solution must equal that of the liquid form at the freezing point for the system to be in equilibrium. As the vapor pressure decreases, the temperature at which this equilibrium will be achieved is also reduced.