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ChemistryofChangingSeasons:MeltingSnowandIce

Inthewintertime,duringthesnowfalland/orafterthesnowfall,wehavewitnessed

thatroadsarecoveredwithsalt(suchNaClorCaCl2)andsandmixturetomeltthesnowandthe

icetomaketheroadsclearandpassable.Themixtureofsaltandsandhasdualeffects:thesalt

meltsthesnowandtheice,andthesandincreasesthecoefficientoffrictionbetweentheroad

andthetiresoncarsthatinessencereducesslipperyconditionsmeaningimprovestire

traction.However,aftertheroadsaredry,sandcreateshazardousconditionsformotorists,

especiallyformotorcyclistsandbicyclist.Thesaltworksbyloweringthemeltingorfreezing

pointofwater[Inreality,themeltingandfreezingpointsarebasicallythesame.Forexample,

theicemeltsat320F(0

0C)orwaterfreezesat32

0C(0

0F)]dependinguponhowwesayit.We

saywaterfreezes(goingfromliquidstatetosolidsate)andicemelts(goingfromsolidstateto

liquidstate].Whensaltisadded,thefreezingpointofwaterdecreasesthatisknownas

freezingpointdepression,whichofcoursedependsontheamountofsolute(salt)added.

Howthefreezingpointdepressionworks?Whenthesaltisaddedtowater,thesalt

dissolvesinwaterbecausebothsaltandwaterarepolarinnature.Thisprocessintroduces

foreigndissolvedparticlesinwaterthatlowersthefreezingpointofwater.Ingeneral,solution

freezesatlowertemperaturethanpuresolventofthesamesolution.Theloweringoffreezing

pointcontinuesuntiltheaddedsaltstopsdissolving.Thistemperatureis-60F(-21

0C)fortable

salt(NaCl)inwaterundercontrolledlaboratoryconditions.However,intherealworld,on

roadsandsidewalks,NaClcanmelticedownto150F(-9

0C)(Seethetableattheendformore

chemicals).

TheoryBehindFreezingPointDepression

Freezingpointdepressionisacolligativeproperty,whichdependsonthenumberof

dissolvedparticlesinasolvent.Itissdirectresultoftheloweringofthesolventvaporpressure

bythesolute.Allliquidsolventswhensolutesaredissolvedexhibitthisproperty.Other

colligativepropertiesarevaporpressurelowering,boilingpointelevation,andosmotic

pressure.Thefollowingphasediagramillustratestheconceptoffreezingpointdepressionof

aqueoussolution:

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Thisdiagramindicatesthatloweringthevaporpressureofthesolutionlowersthe

freezingpointofsolution.LetTf0bethefreezingpointofthepuresolventandTfbethe

freezingpointofthesolution.Then,thefreezingpointdepression,Tf,isexpressedas

Tf=Tf0-Tf

Further,theTfisproportionaltomolalconcentration(m)ofthesolution[molality(m)

isthenumberofmolesofsolutedissolvedin1kg(1000g)ofsolvent]andhence

Tfm

or Tf=Kfm

whereKfisthemolalfreezing-pointdepressionconstantthatvariesfromsolventtosolvent.

Followingtablelistssomeofthecommonliquids.

Solvent NormalFreezingPoint(0C)# Kf(0C/m)

Water 0 1.86

Benzene 5.5 5.12

Cyclohexane 6.6 20.0

Aceticacid 16.6 3.90

Ethanol(Ethyl

Alcohol)

-117.3 1.99

#Measuredat1atm.

Notethatthesameprinciple,depressioninfreezingpoint,alsoappliesinusingtheantifreezein

carsandtrucksinthewintertimeandaddingsalttomakeicecream.

Qualitativelythefreezingpointdepressioncanbeexplainedintermsoforder-disorder

state.Ingeneral,freezinginvolvesatransitionfromthedisorderedstate(solvent)tothe

orderedstate(solid)(fore.g.waterice).Forthistotakeplace,theenergymustbeexpelled

fromthesystem:

DisorderedstateOrderedstate+energy

Asolutionhasgreaterdisorderthanthesolvent.Hence,moreenergyneedstobeexpelledfrom

ittocreateanorderedstatecomparedtopuresolvent.Thisisthereasonwhythesolutionhas

alowerfreezingpointthanthesolvent.

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ChemicalsUsedtoMeltSnowandIce

Therearenumberof chemicals inusagetomelt the iceandSnow in thewintertime.

FollowingistheinformationtakenfromAbout.com

Name Chemical

Formula

Lowest Practical

Temperature

Pros Cons

SodiumChloride

(RockSalt&

Halite)

NaCl 150F(-9

0C) Keepssidewalks

dry

Corrosive,

damages

concrete&

vegetation

Potassium

Chloride

KCl 200F(-7

0C) Fertilizer Damages

concrete

CalciumChloride CaCl2 -200F(-29

0C) Meltsicefaster

thanNaCl

Attracts

moisture,surfacesbecome

slipperybelow

00F(-18

0C)

Magnesium

Chloride

MgCl2 50F(-15

0C) Meltsicefaster

thanNaCl

Attracts

moisture

Potassium

Acetate

CH3COOK 150F(-9

0C) Biodegradable Corrosive

Calcium

Carbonate,

Magnesium

Carbonate,&

AceticAcid

(CMA)

CaCO3,MgCO3,&

HC2H3O2

150F(-9

0C) Safestfor

concrete&

vegetation

Worksbetterto

preventre-icing

thanasice

remover

Ammonium

Sulfate

(NH4)2SO4 200F(-7

0C) Fertilizer Damages

concrete

Urea NH2-CO-NH2 200F(-7

0C) Fertilizer Agricultural

gradeis

corrosive

MahadevKumbar,ChemistryDepartment,NCC