Editing Freezing-point depression (section)

{{Short description|Drop in freezing temperature of a solvent due to the addition of solute}}
{{About|the phenomenon caused by solutes|the phenomenon in pure fluids|supercooling}}

[[File:Salt truck Milwaukee.jpg|thumb|Workers spreading salt from a salt truck for deicing the road]]

[[File:Gelato ice cream.jpg|thumb|Freezing point depression is responsible for keeping ice cream soft below 0&nbsp;°C.<ref>{{Cite journal |date=2021-03-18 |title=Controlling the hardness of ice cream, gelato and similar frozen desserts |journal=Food Science and Technology |doi=10.1002/fsat.3510_3.x |s2cid=243583017 |issn=1475-3324|doi-access=free }}</ref>]]
'''Freezing-point depression''' is a drop in the maximum temperature at which a substance [[freezing|freezes]], caused when a smaller amount of another, non-[[Volatility (chemistry)|volatile]] substance is added. Examples include adding salt into water (used in [[ice cream maker]]s and for [[De-icing#Chemical de-icers|de-icing roads]]), [[Alcohol (chemistry)|alcohol]] in water, [[Ethylene glycol|ethylene]] or [[propylene glycol]] in water (used in [[antifreeze]] in cars), adding [[copper]] to molten [[silver]] (used to make [[Solder#Hard_solder|solder]] that flows at a lower temperature than the silver pieces being joined), or the mixing of two solids such as impurities into a finely powdered drug.

In all cases, the substance added/present in smaller amounts is considered the [[solute]], while the original substance present in larger quantity is thought of as the [[solvent]]. The resulting liquid solution or solid-solid mixture has a lower [[Melting point|freezing point]] than the pure solvent or solid because the [[chemical potential]] of the solvent in the mixture is lower than that of the pure solvent, the difference between the two being proportional to the [[natural logarithm]] of the [[mole fraction]]. In a similar manner, the chemical potential of the vapor above the solution is lower than that above a pure solvent, which results in [[boiling-point elevation]]. Freezing-point depression is what causes [[sea water]] (a mixture of salt and other compounds in water) to remain liquid at temperatures below {{convert|0|C|F}}, the freezing point of pure water.