Editing Freezing-point depression (section)

===Laboratory uses===
Freezing-point depression can also be used as a purity analysis tool when analyzed by [[differential scanning calorimetry]]. The results obtained are in mol%, but the method has its place, where other methods of analysis fail.

In the laboratory, [[lauric acid]] may be used to investigate the [[molar mass]] of an unknown substance via the freezing-point depression. The choice of lauric acid is convenient because the melting point of the pure compound is relatively high (43.8&nbsp;°C). Its [[cryoscopic constant]] is 3.9&nbsp;°C·kg/mol. By melting lauric acid with the unknown substance, allowing it to cool, and recording the temperature at which the mixture freezes, the molar mass of the unknown compound may be determined.<ref>{{Cite web |url=http://faculty.sites.uci.edu/chem1l/files/2015/04/Freezing-Point-Depression.pdf |title=Archived copy |access-date=2019-07-08 |archive-date=2020-08-03 |archive-url=https://web.archive.org/web/20200803132047/http://faculty.sites.uci.edu/chem1l/files/2015/04/Freezing-Point-Depression.pdf |url-status=dead }}</ref>{{Citation needed|reason=This source is from a college-level general chemistry lab and does not provide evidence that actual chemists use lauric acid for this use.|date=February 2020}}

This is also the same principle acting in the melting-point depression observed when the melting point of an impure solid mixture is measured with a [[melting-point apparatus]] since melting and freezing points both refer to the liquid-solid [[phase transition]] (albeit in different directions).

In principle, the boiling-point elevation and the freezing-point depression could be used interchangeably for this purpose. However, the [[cryoscopic constant]] is larger than the [[ebullioscopic constant]], and the freezing point is often easier to measure with precision, which means measurements using the freezing-point depression are more precise.

FPD measurements are also used in the dairy industry to ensure that milk has not had extra water added. Milk with a FPD of over 0.509&nbsp;°C is considered to be unadulterated.<ref>{{cite web
 |title       = Freezing Point Depression of Milk
 |publisher   = Dairy UK
 |year        = 2014
 |url         = http://www.dairyuk.org/component/docman/doc_download/3940-freezing-point-depression-of-milk
 |archive-date = 2014-02-23
 |archive-url  = https://web.archive.org/web/20140223151245/http://www.dairyuk.org/component/docman/doc_download/3940-freezing-point-depression-of-milk
 |url-status     = dead
}}</ref>