Talk:Freezing-point depression

Confused

I am confused. How does this explain the mixture/solution of water and alcohol whose freezing temperature is only lower than water, but not alcohol? I am sure I am wrong, but why? Can we not deem alcohol as the solvent and water the solute? Racoon.zip (talk) 00:15, 28 November 2007 (UTC)

Unfortunately not. When you have two liquids, one acting as the solute, you can only depress the freezing point of whichever substance has the lower freezing point in most cases. The reason for this, is water at ethanol's freezing point will not be a solute, since it will not be dissolved. Water will be a solid, and thus cannot act as a solute. While colligative properties says any solute will work, it does not account for solubility and phase (especially at low temperatures)... this part is assumed!

Mistake???

From the article:

ΔTf = i · Kf · activity

   * activity is in units of mol/kg, and is equal to an activity coefficient times the molality

...

   * i is the i factor or the van 't Hoff factor (see van 't Hoff), accounts for the number of individual ions formed by a compound in solution.

Eehm, isn't i the activity coefficient? I'm a bit puzzled, but it seems to me that it is used twice here.

—The preceding unsigned comment was added by 83.98.253.62 (talk) 09:05, 6 February 2007 (UTC).

No, i isn't the activity factor, and neither the activity factor or the activity of the solute is really applicable in the case of colligative properties, where the solute plays the passive role of diluting the solvent (which is why molality is the relevant concentration measure). The standard phrase "concentration, or to be more precise activity..." does not apply to colligative properties! The article has now been changed to reflect this. The activity (or better, the chemical potential) of the solvent is another matter, but the ΔT_f formula is the final stage in a derivation where the chemical potential has been expressed in those other terms on the right hand of the equation. Tomas e 20:59, 20 October 2007 (UTC)

Osmometry

a good addition to this article would be the clinical uses for freezing point depression, such as osmometry!

From the article:

Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent.

It is a well-known fact that salt can cause freezing-point depression. The electrolyte article mentions that sodium and chloride are both electrolytes. Maybe the article should mention electrolytes instead of nonelectrolytes?

Freezing-point depression is the difference between the freezing points of the pure solvent and the solution. It occurs in both electrolyte and non electrolyte solutions. However, the van't Hoff factor kicks in when you have an electroyte solution because it takes into account how the ions disassociate.

Table of constants?

Anyone willing to compile a table of freezing point depression constants for a variety of solvents? I can do it if no one else does, but right now I'm busy.Quaxmonster 23:23, 22 May 2006 (UTC)

I compiled one from Atkins Physical Chemistry & German Wikipedia. Feel free to add more solvents! Tomas e 20:59, 20 October 2007 (UTC)

75.139.65.41 (talk) 06:57, 4 November 2020 (UTC)I don't see the table in the article. It would be useful to have a table as mentioned.

Thank you for pointing this out. I searched the article history and found that the table was deleted on 19 May 2010 (!) by a numbered user with no explanation. Normally such deletions are reverted quickly but not this time. I have now restored the original table. Dirac66 (talk) 13:03, 4 November 2020 (UTC)

Theory

I think it would be wise to include a section on what exactly causes freezing point depressions. I would also be glad to add this section if there are no objections--Helgers7 05:57, 15 May 2007 (UTC)

I've added a section on this, based on a diagram of chemical potential against temperature. It's correct, but it can probably be made more pedagogical. However, this is a phenomenon where I've often seen erroneous explanations (and I just removed one from the end of the previous version of the article). Many people are tempted to find "direct" molecular explanations, which tend to be nonsense in the case of colligative properties (and in many other cases where more subtle entropic phenomena are to be explained). Tomas e 20:59, 20 October 2007 (UTC)

Why isn't it there, then??? 134.197.153.254 (talk) 19:54, 21 September 2010 (UTC)

And lo and behold someone did exactly that. They did a Crystal Defect explanation which is utterly wrong. We now have an article explaining this based on someone's intuition rather than fact. Entropy is the only good description to be used here. Crystal defect should be removed entirely. 66.114.154.25 (talk) 18:08, 17 April 2023 (UTC)

Pointless entry

"In Cohen's Practical Organic Chemistry of 1910 [1] the molar mass of napthalene is determined in a so-called Beckmann freezing apparatus at 128 g/mole."

I see absolutely no point in this entry. I will delete it unless someone objects —The preceding unsigned comment was added by Helgers7 (talk • contribs).

• The statement is vital: nowadays nobody uses freezing point depression as an analytical tool but in 1910 it was pretty standard stuff. Please do not delete, See also User:V8rik#On_deletions V8rik 15:16, 28 May 2007 (UTC)

• Actually, depending on the equiptment you have available, some chemists do use it as an analytical tool, I know I have. However, this still doesn't justify this statement. It is simply a pointless fact that has no real right to be on this page. It would be acceptable to add "however, this technique is not as common as it once was" to the statement "or to determine molar mass of the solute." —The preceding unsigned comment was added by Helgers7 (talk • contribs).

I have no problem with it being there (I tried to integrate it a little better), but V8rik, please don't call things "vandalism" in your edit summaries unless they're clearly vandalism. That word has a specific meaning and doesn't apply to Helgers7's edits because they were made in good faith. —Keenan Pepper 18:14, 31 May 2007 (UTC)

- Spam, useless information, added to the page under section "Freezing point depression of a solute vs a solvent" as seen here - http://d.pr/dP0T - this was obviously done with no regard for people who use Wikipedia as a source of information, and needs to be corrected

Typo

"This is due to solute molecules disrupting the ability of the solute to form crystals during the freezing process."

Since it has been a while since first year chem, I thought I would check...

The second "solute" should be "solvent"... right?

—carnage_joe 6:35, 8 July 2007 (UTC)

Difficult to say, since it is totally wrong as an explanation of why the freezing point depression takes place. This sentence is now removed. Tomas e 20:59, 20 October 2007 (UTC)

Exactly Tomas! There are academic texts that state otherwise, but "blocking" or "disrupting" is not the cause of these colligative properties.

unit of equations

I suggest to change the unit of temperature into kelvin.

Another suggestion to change the molality for molar fraction.

Both of them preferred by IUPAC documents for units and quantities. Regards, --ZJ (talk) 13:42, 22 February 2010 (UTC)

Not a hard thing. You can multiply only by the molar mass of the compound --ZJ (talk) 21:21, 26 February 2010 (UTC).

Math formula improvement

$\Delta T_{f}={\frac {\Delta H_{TF}^{fus}-2RT_{F}\log {a_{liq}}-{\sqrt {2\Delta C_{P}^{fus}T_{F}^{2}R\log {a_{liq}}(\Delta H_{TF}^{fus})^{2}}}}{2(\Delta H_{TF}^{fus}/T_{F}+0.5\Delta C_{P}^{fus}-R\log {a_{liq}})}}$

I did not verify it being correct —Preceding unsigned comment added by 130.237.74.66 (talk) 16:21, 8 December 2010 (UTC)

It should be mentioned that the shown formula already appeared in numerous much earlier textbooks and publications - although in different arrangements. See K.S. Pitzer (1991) "Activity coefficients in electrolyte solutions" CRC Press, Boca Raton, for example. The principal relationship between water activity and freezing point depression has already been extensively discussed by Debye and Hückel in 1923 (Phys. Zeitschr. Vol. 24 p. 185-206). — Preceding unsigned comment added by AkademijaNauk~enwiki (talk • contribs) 15:52, 15 December 2017 (UTC)

Not so pure Pure Water

The article claims the freezing point of pure water to be zero degree C. However the article Properties of Water states that in pure form it can be supercooled to -42 deg C.

I suggest that Pure Water in this article be exchanged with Ordinary Water or similar. —Preceding unsigned comment added by 60.241.91.114 (talk) 07:19, 19 December 2010 (UTC)

Rewrite

This page needs a bit of a rewrite. Using chemical potentials as an explanation is not an explanation at all. It is simply making something needlessly more difficult so that the reader becomes too confused to question what is really going on beyond potentials. Potentials are a result of what is happening, not the cause. Molecules don't float around asking what their potential is before doing something. Someone should put in a more appropriate entropic description!

Also, the line about (Explanations claiming that the solute molecules somehow "prevent" the solvent molecules from forming a solid are thus wrong.) is absolutely critical to this article, and should not be removed. There are countless textbooks, instructors, and websites that use this INCORRECT explanation - and it should be addressed as it is totally false. In fact the last entry in this discussion shows that junk like this was once present here. —Preceding unsigned comment added by 168.156.39.231 (talk) 22:34, 9 January 2009 (UTC)

I agree with the criticism above. The current 'explanation' starts with a false statement, that the kinetic energy doesn't change in the phase transition. That appears to be a garbled and inappropriate attempt to apply equipartition. There follows a confused and nearly meaningless discussion of different energy terms, neglecting entropy, and conflating kinetic questions with the equilibrium topic at issue here. I'll try to write a replacement, if nobody else does.Mbweissman (talk) 23:14, 22 January 2011 (UTC)

"(number of solute particles per mol, e.g. i = 2 for NaCl)." This is misleading and should be rephrased. Either particles of solute in solution per particle solute out of solution, or change both bolded terms to mols. — Preceding unsigned comment added by 129.255.228.203 (talk) 19:50, 6 October 2011 (UTC)

Enthalpy of Fusion?

Under "Uses" the section talks about adding salt to ice in order to cool beer. Would I be right in thinking that this is not about freezing point depression but due to enthalpy of fusion? Of course the freezing point of the ice is lowered but the actual temperature is lowered by the removal of heat to change the molecular arrangement of the water molecules. I'm not sure so I thought I'd ask before changing. Matthewcgirling (talk) 09:27, 11 February 2012 (UTC)

Hmm, tricky. You're right, the cooling effect comes from the enthalpy of fusion. However, if there was no freezing point depression then there would be no melting and thus no cooling. --RolfSander (talk) 12:54, 11 February 2012 (UTC)

Binary mixed electrolytes

How is this concept of fpd applied to a mixture containing two electrolytes and water/solvent? Can the mixture of at least two electrolytes be considered a pseudo-component and the mixture thus a pseudo-binary mixture?--5.15.50.146 (talk) 11:44, 26 January 2016 (UTC)

I don't really know what the definition of a pseudo-component is, but since freezing point depression basically depends on the dilution of the solvent and nothing else, summing up the dilution resulting from the various solutes should be correct. Tomas e (talk) 12:06, 28 February 2016 (UTC)

It is defined at Apparent_molar_property#Multicomponent_mixtures_or_solutions--82.137.9.165 (talk) 20:28, 28 November 2017 (UTC)

Solvent chemical potential and vapor pressure are both lowered

@ChemMater:

The discussion of chemical potential added to the intro last week is incorrect. Actually the chemical potential of the solvent in liquid solution is lowered BOTH near the freezing point and near the boiling point. Near the freezing point this stabilizes the liquid phase relative to the (pure) solid so the freezing point is depressed; near the boiling point the liquid is stabilized relative to the (pure) vapor (if the solute is nonvolatile) so the boiling point is raised. See the diagram at Boiling-point elevation, which could be copied into this article.

Also the two explanations in terms of vapor pressure and chemical potential are equivalent, since chemical potential of a vapor is logarithmically related to pressure. I would mention both in the Explanation section (not the intro) - first the vapor pressure one which is given in general chemistry textbooks such as the one cited, and then the chemical potential one which is more advanced and related to thermodynamics. Dirac66 (talk) 01:57, 17 September 2019 (UTC)

@Dirac66:

Yes, you are absolutely correct! All of the colligative properties result from a lowering of the chemical potential of the liquid solvent in the presence of a solute. Thanks for pointing that out. I will make the recommended changes.

Garbage

Explanations are garbage. Solute prevents freezing? Salt doesn't "fit" into the pure crystal? Someone wrote this with no understanding of the subject. Further there's been a lot of previous versions that mention enthalpy and cooling.

The only viable explanation here is entropy and concentration. Everything else needs to be nuked. You can use chemical potential but that's already been discussed here as annoying.

Use this: In the liquid solution, the solvent is diluted by the addition of a solute, so that fewer molecules are available to freeze. Re-establishment of equilibrium is achieved at a lower temperature at which the rate of freezing becomes equal to the rate of liquefying. At the lower freezing point, the vapor pressure of the liquid is equal to the vapor pressure of the corresponding solid, and the chemical potentials of the two phases are equal as well.

66.114.154.25 (talk) 18:13, 17 April 2023 (UTC)

Unfortunately, the word "entropy" does not occur at all in the subsection titled "Due to concentration and entropy". 73.50.103.119 (talk) 01:35, 23 August 2023 (UTC)

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