<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> <P> The phenomenon of freezing - point depression has many practical uses . The radiator fluid in an automobile is a mixture of water and ethylene glycol . As a result of freezing - point depression, radiators do not freeze in winter (unless it is extremely cold, e.g. − 30 to − 40 ° C (− 22 to − 40 ° F)). Road salting takes advantage of this effect to lower the freezing point of the ice it is placed on . Lowering the freezing point allows the street ice to melt at lower temperatures, preventing the accumulation of dangerous, slippery ice . Commonly used sodium chloride can depress the freezing point of water to about − 21 ° C (− 6 ° F). If the road surface temperature is lower, NaCl becomes ineffective and other salts are used, such as calcium chloride, magnesium chloride or a mixture of many . These salts are somewhat aggressive to metals, especially iron, so in airports safer media such as sodium formate, potassium formate, sodium acetate, potassium acetate are used instead . </P>

What is the cause of freezing point depression
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