Editing Freezing-point depression (section)

==Uses==
The phenomenon of freezing-point depression has many practical uses. The radiator fluid in an automobile is a mixture of water and [[antifreeze|ethylene glycol]]. The freezing-point depression prevents radiators from freezing in winter. 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 {{convert|-21|C|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]], and [[potassium acetate]] are used instead.

[[File:2018-07-26 08 09 30 View north along New Jersey State Route 23 at Scenic Lake Road in Hardyston Township, Sussex County, New Jersey.jpg|thumb|Pre-treating roads with salt relies on the warmer road surface to initially melt the snow and make a solution; Pre-treatment of bridges (which are colder than roads) does not typically work.<ref name=":0">{{Cite web |last=Pollock |first=Julie |title=Salt Doesn't Melt Ice&mdash;Here's How It Makes Winter Streets Safer |url=https://www.scientificamerican.com/article/salt-doesnt-melt-ice-heres-how-it-makes-winter-streets-safer/ |access-date= |website=Scientific American |language=en}}</ref>]]
[[File:Les feuilles toutes givrées du petit pommier.jpg|thumb|Dissolved solutes prevent sap and other fluids in trees from freezing in winter.<ref>{{Cite news|last=Ray|first=C. Claiborne|date=2002-02-05|title=Q & A|language=en-US|work=The New York Times|url=https://www.nytimes.com/2002/02/05/science/q-a-461229.html|access-date=2022-02-10|issn=0362-4331}}</ref>]]
Freezing-point depression is used by some organisms that live in extreme cold. Such creatures have [[evolution|evolved]] means through which they can produce a high concentration of various compounds such as [[sorbitol]] and [[glycerol]]. This elevated concentration of solute decreases the freezing point of the water inside them, preventing the organism from freezing solid even as the water around them freezes, or as the air around them becomes very cold. Examples of organisms that produce antifreeze compounds include some species of [[arctic]]-living [[fish]] such as the [[rainbow smelt]], which produces glycerol and other molecules to survive in frozen-over estuaries during the winter months.<ref>{{cite journal|last1=Treberg|first1= J. R. |last2=Wilson |first2=C. E.|last3= Richards|first3= R. C. |last4=Ewart|first4= K. V. |last5=Driedzic|first5= W. R. |year=2002 |journal= The Journal of Experimental Biology |volume=205 |pages=1419–1427 |title=The freeze-avoidance response of smelt ''Osmerus mordax'': initiation and subsequent suppression 6353 |issue=Pt 10 |doi= 10.1242/jeb.205.10.1419 |pmid= 11976353 |url=http://jeb.biologists.org/content/205/10/1419.long|url-access=subscription }}</ref> In other animals, such as the [[spring peeper]] frog (''Pseudacris crucifer''), the molality is increased temporarily as a reaction to cold temperatures. In the case of the peeper frog, freezing temperatures trigger a large-scale breakdown of [[glycogen]] in the frog's liver and subsequent release of massive amounts of [[glucose]] into the blood.<ref>L. Sherwood et al., ''Animal Physiology: From Genes to Organisms'', 2005, Thomson Brooks/Cole, Belmont, CA, {{ISBN|0-534-55404-0}}, p.&nbsp;691–692.</ref>
With the formula below, freezing-point depression can be used to measure the degree of [[dissociation (chemistry)|dissociation]] or the [[molar mass]] of the solute. This kind of measurement is called '''cryoscopy''' ([[Ancient Greek|Greek]] ''cryo'' = cold, ''scopos'' = observe; "observe the cold"<ref>Bioetymology – Biomedical Terms of Greek Origin. [http://bioetymology.blogspot.com/2011/06/cryoscopy.html cryoscopy]. bioetymology.blogspot.com.</ref>) and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the [[van 't Hoff factor]] ''i'' by first determining ''m''<sub>B</sub> and then comparing it to ''m''<sub>solute</sub>. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing ''m''<sub>B</sub> with the amount of solute dissolved. In this case, ''i'' must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was, but it was included in textbooks at the turn of the 20th century. As an example, it was still taught as a useful analytic procedure in Cohen's ''Practical Organic Chemistry '' of 1910,<ref>{{cite book|first =Julius B. |last =Cohen|url =https://archive.org/details/PracticalOrganicChemistry |title =Practical Organic Chemistry|date = 1910|publisher = MacMillan and Co.|location = London}}</ref> in which the [[molar mass]] of [[naphthalene]] is determined using a ''Beckmann freezing apparatus''.

===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>