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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