Editing Freezing-point depression (section)

==Explanation==
===Using vapour pressure===
The freezing point is the temperature at which the liquid solvent and solid solvent are at equilibrium, so that their [[vapor pressure]]s are equal. When a non-volatile solute is added to a volatile liquid solvent, the solution vapour pressure will be lower than that of the pure solvent. As a result, the solid will reach equilibrium with the solution at a lower temperature than with the pure solvent.<ref>{{cite book |last1=Petrucci |first1=Ralph H. |last2=Harwood |first2=William S. |last3=Herring |first3=F. Geoffrey |date=2002 |title=General Chemistry |edition=8th |publisher=Prentice-Hall |pages=557–558 |isbn=0-13-014329-4 }}</ref> This explanation in terms of vapor pressure is equivalent to the argument based on chemical potential, since the chemical potential of a vapor is logarithmically related to pressure. All of the [[colligative properties]] result from a lowering of the chemical potential of the solvent in the presence of a solute. This lowering is an [[entropy]] effect. The greater randomness of the solution (as compared to the pure solvent) acts in opposition to freezing, so that a lower temperature must be reached, over a broader range, before equilibrium between the liquid solution and [[solid solution]] phases is achieved. Melting point determinations are commonly exploited in [[organic chemistry]] to aid in identifying substances and to ascertain their purity.

=== Due to concentration and entropy ===

In the liquid solution, the solvent is diluted by the addition of a solute, so that fewer molecules are available to freeze (a lower concentration of solvent exists in a solution versus pure solvent). Re-establishment of equilibrium is achieved at a lower temperature at which the rate of freezing becomes equal to the rate of liquefying. The solute is not occluding or preventing the solvent from solidifying, it is simply diluting it so there is a reduced probability of a solvent making an attempt at freezing in any given moment.  

At the lower freezing point, the vapor pressure of the liquid is equal to the vapor pressure of the corresponding solid, and the chemical potentials of the two phases are equal as well.