It is very hard to have homogeneous nucleation in common situation. Pure water with no nuclei at all will not freeze above -40oC. The presence of impurity particles allow the so-called heterogeneous nucleation to occur so that we can see water freeze at 0oC. It may be reasonable to guess at what temperature between -40oC and 0oC will water freeze depends on the amount of impurities in water. But we never see water freeze above 0oC at atmospheric pressure. Can the freezing point be defined in the following way? Freezing point is the temperature at which further increase of impurities cannot increase the temperature for the substance to freeze.