The freezing point of a liquid is changed due to the addition of some foreign material which dissolves in the solvent. Like salt in water, ethanol in water etc. The freezing point of a solvent is the temperature at which the vapor pressure of the liquid solvent become equal to the vapor pressure of the solid solvent. Now due to addition of some foreign material like salt, ethanol etc. the vapor pressure of the liquid state of the solvent changes at a particular temperature. However the vapor pressure of the liquid in solid phase remain same. Thus there will be change of freezing point due to addition of some other material in a particular liquid.