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FREEZING-POINT DEPRESSION is the process in which adding a solute to a solvent decreases the freezing point of the solvent. Examples include salt in water, alcohol in water, or the mixing of two solids such as impurities in 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 in water) to remain liquid at temperatures below 0 °C (32 °F), the freezing point of pure water.

CONTENTS

* 1 Uses * 2 Of a solvent and a nonvolatile solute * 3 Calculation * 4 See also * 5 References

USES

Ice on a road.

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.

Freezing-point depression
Freezing-point depression
is used by some organisms that live in extreme cold. Such creatures have evolved means through which they can produce 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. 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.

With the formula below, freezing-point depression can be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called CRYOSCOPY (Greek _cryo_ = cold, _scopos_ = observe "observe the cold" ) 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_B and then comparing it to _m_solute. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing _m_B 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, in which the molar mass of naphthalene is determined using a _Beckmann freezing apparatus_.

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

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 used in the dairy industry to ensure that milk has not had extra water added. Milk with a FPD of over 0.509 °C is considered to be unadulterated.

OF A SOLVENT AND A NONVOLATILE SOLUTE

Consider the problem in which the solvent freezes to a very nearly pure crystal, regardless of the presence of the nonvolatile solute. This typically occurs simply because the solute molecules do not fit well in the crystal, i.e. substituting a solute for a solvent molecule in the crystal has high enthalpy . In this case, for low solute concentrations, the freezing point depression depends solely on the concentration of solute particles, not on their individual properties. The freezing point depression thus is called a colligative property.

The explanation for the freezing point depression is then simply that as solvent molecules leave the liquid and join the solid, they leave behind a smaller volume of liquid in which the solute particles can roam. The resulting reduced entropy of the solute particles thus is independent of their properties. This approximation ceases to hold when the concentration becomes large enough for solute-solute interactions to become important. In that case, the freezing point depression depends on particular properties of the solute other than its concentration.

CALCULATION

If the solution is treated as an ideal solution , the extent of freezing-point depression depends only on the solute concentration that can be estimated by a simple linear relationship with the cryoscopic constant ("Blagden 's Law"): _ΔT_F = _K_F · _b_ · _i_,

where:

* _ΔT_F, the freezing-point depression, is defined as _T_F (pure solvent) − _T_F (solution). * _K_F, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. (Note: When conducting experiments, a higher _K_F value makes it easier to observe larger drops in the freezing point. For water, _K_F = 1.853 K ·kg/mol. ) * _b_ is the molality (moles solute per kilogram of solvent) * _i_ is the van \'t Hoff factor (number of ion particles per individual molecule of solute, e.g. i = 2 for NaCl, 3 for BaCl2).

This simple relation doesn't include the nature of the solute, so this is only effective in a diluted solution. For a more accurate calculation at a higher concentration, for ionic solutes, Ge and Wang (2010) proposed a new equation: T F = H T F fus 2 R T F ln a liq 2 C p fus T F 2 R ln a liq + ( H T F fus ) 2 2 ( H T F fus T F + C p fus 2 R ln a liq ) . {displaystyle Delta T_{text{F}}={frac {Delta H_{T_{text{F}}}^{text{fus}}-2RT_{text{F}}cdot ln a_{text{liq}}-{sqrt {2Delta C_{p}^{text{fus}}T_{text{F}}^{2}Rcdot ln a_{text{liq}}+(Delta H_{T_{text{F}}}^{text{fus}})^{2}}}}{2left({frac {Delta H_{T_{text{F}}}^{text{fus}}}{T_{text{F}}}}+{frac {Delta C_{p}^{text{fus}}}{2}}-Rcdot ln a_{text{liq}}right)}}.}

In the above equation, _T_F is the normal freezing point of the pure solvent (273 K for water, for example); _a_liq is the activity of the solution (water activity for aqueous solution); Δ_H_fusTF is the enthalpy change of fusion of the pure solvent at _T_F, which is 333.6 J/g for water at 273 K; Δ_C_fusp is the differences of heat capacity between the liquid and solid phases at _T_F, which is 2.11 J/(g·K) for water.

The solvent activity can be calculated from Pitzer model or modified TCPC model , which typically requires 3 adjustable parameters. For the TCPC model, these parameters are available for many single salts.

SEE ALSO

* Melting-point depression * Boiling-point elevation * Colligative properties * De-ice * Eutectic point * Frigorific mixture * List of boiling and freezing information of solvents * Snow removal

REFERENCES

* ^ Treberg, J. R.; Wilson, C. E.; Richards, R. C.; Ewart, K. V.; Driedzic, W. R. (2002). "The freeze-avoidance response of smelt _Osmerus mordax_: initiation and subsequent suppression 6353". _The Journal of Experimental Biology_. 205 (Pt 10 http://jeb.biologists.org/content/205/10/1419.long): 1419–1427. * ^ L. Sherwood et al., _Animal Physiology: From Genes to Organisms_, 2005, Thomson Brooks/Cole, Belmont, CA, ISBN 0-534-55404-0 , p. 691–692 * ^ BIOETYMOLOGY- Biomedical Terms of Greek Origin bioetymology.blogspot.com * ^ Julius B. Cohen _Practical Organic Chemistry_ 1910 Link to online text * ^ "Freezing Point Depression of Milk". Dairy UK. 2014. Archived from the original on 2016-03-04. * ^ Atkins, Peter (2006). _Atkins' Physical Chemistry_. Oxford University Press. pp. 150–153. ISBN 0198700725 . * ^ Aylward, Gordon ; Findlay, Tristan (2002), _SI Chemical Data 5th ed._ (5 ed.), Sweden: John Wiley & Sons, p. 202, ISBN 0-470-80044-5 * ^ Ge, Xinlei; Wang, Xidong (2009). "Estimation of Freezing Point Depression, Boiling Point Elevation, and Vaporization Enthalpies of Electrolyte Solutions". _Industrial & Engineering Chemistry Research_. 48 (10): 5123–5123. ISSN 0888-5885 . doi :10.1021/ie900434h . * ^ Ge, Xinlei; Wang, Xidong (2009). "Calculations of Freezing Point Depression, Boiling Point Elevation, Vapor Pressure and Enthalpies of Vaporization of Electrolyte Solutions by a Modified Three-Characteristic Parameter Correlation Model". _Journal of Solution
Solution
Chemistry_. 38 (9): 1097–1117. ISSN 0095-9782 . doi :10.1007/s10953-009-9433-0 . * ^ Ge, Xinlei; Wang, Xidong; Zhang, Mei; Seetharaman, Seshadri (2007). "Correlation and Prediction of Activity and Osmotic Coefficients of Aqueous Electrolytes at 298.15 K by the Modified TCPC Model". _Journal of Chemical & Engineering Data_. 52 (2): 538–547. ISSN 0021-9568 . doi :10.1021/je060451k . * ^ Ge, Xinlei; Zhang, Mei; Guo, Min; Wang, Xidong (2008). "Correlation and Prediction of Thermodynamic Properties of Some Complex Aqueous Electrolytes by the Modified Three-Characteristic-Parameter Correlation Model". _Journal of Chemical & Engineering Data_. 53 (4): 950–958. ISSN 0021-9568 . doi :10.1021/je7006499 . * ^ Ge, Xinlei; Zhang, Mei; Guo, Min; Wang, Xidong (2008). "Correlation and Prediction of Thermodynamic Properties of Nonaqueous Electrolytes by the Modified TCPC Model". _Journal of Chemical & Engineering Data_. 53 (1): 149–159. ISSN 0021-9568 . doi :10.1021/je700446q . * ^ Ge, Xinlei; Wang, Xidong (2009). "A Simple Two-Parameter Correlation Model for Aqueous Electrolyte Solutions across a Wide Range of Temperatures†". _Journal of Chemical & Engineering Data_. 54 (2): 179–186. ISSN 0021-9568 . doi :10.1021/je800483q .

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