<P> Freezing - point depression is the decrease of the freezing point of a solvent on addition of a non-volatile solute . Examples include salt in water, alcohol in water, or the mixing of two solids such as impurities into a finely powdered drug . In the last case, the added compound is the solute, and the original solid is thought of as the solvent . The resulting solution or solid--solid mixture has a lower freezing point than the pure solvent or solid . This phenomenon is what causes sea water, (a mixture of salt (and other things) in water) to remain liquid at temperatures below 0 ° C (32 ° F), the freezing point of pure water . </P> <P> The freezing point is the temperature at which the vapor pressures of liquid solvent and solid solvent are equal . When a non-volatile solute is added to a volatile liquid solvent, the solution vapor 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 . </P>

What can lower the freezing point of water