water properties in civil engineering.pdf

It shows the properties of water

1. DEPARTMENT OF CIVIL ENGINEERING
CIV2103: HYDRAULICS & HYDRAULIC
INSTRUCTOR:BYAMUKAMA DAN
[email protected]
0789541607/0705418906

2. INTRODUCTION TO PROPERTIES OF WATER
Water(H20)isthe“universalsolvent”andthemostabundant
surfaceonEarth.Itisalsotheonlycommonsubstancetoexistas
asolid,liquid,andgasnaturally.Watermoleculesformhydrogen
bondsandareextremelypolar.
Thefivemainpropertiesofwaterareitshighpolarity,high
specificheat,highheatofvaporization,lowdensityasasolid,
andattractiontootherpolarmolecules.

3. Polarity and Structure
Oneoxygenatomandtwohydrogenatomsmakeawatermolecule.Ithasabentmolecular
geometrywiththeoxygenhavingtwolonepairsofelectrons.Thedifferenceinelectronegativity
betweentheoxygenandhydrogenscausestheoxygentohaveapartialnegativechargeandthe
hydrogentohaveapartialpositivecharge.Thisdifferenceinchargecausespolarity.Thepartial
positivechargeofthehydrogenofonewatermoleculeattractsthepartialnegativechargeofthe
oxygenofanotherwatermolecule.Thisattractioniscalledhydrogenbonding.
Hydrogenbondingisweakerthanthecovalentbondsbetweentheoxygenandhydrogensofthe
samemoleculebutcausesmanyofwater’suniqueproperties.Forexample,moreenergyis
requiredtobreakhydrogenbonds,sowaterhasahighermeltingandboilingpoint.

4. Universal Solvent
Wateristhesolventoflife.Hydrophilicsubstancesarethosethatdissolveinwater,while
hydrophobicsubstancesdonotmixwellwithwater.Substancescandissolveinwaterif
theycanmatchorovercomethehydrogenbondingbetweenwatermolecules.Ifthey
cannot,thesubstanceformsaprecipitate.Acids,alcohols,andsaltsarerathersolublein
waterwhilefatsandoilsarehydrophobic.
Anionicsolutedissolvedinwaterbecomesseparated.Forexample,NaClseparatesinto
Na+cationsandCl-anionssurroundedbywatermolecules.Waterisamphoteric,meaning
itcanactaseitheranacidorabasedependingonthesolution.ItcanproducebothH+and
OH-ions.

5. Specific Heat Capacity
Waterhasahighspecificheatof4184J/(kgxK)at20ºCandhighheatof
vaporizationbecauseofhydrogenbonding.Thisallowsbodiesofwatertohave
minimalfluctuationsintemperaturetoregulateclimate.
Water’shighheatofvaporizationallowshumanstousesweattocooloff.Sweatis
mostlymadeofwater.Itabsorbsexcessbodyheatasitevaporates.Thisprocessis
knownasevaporativecooling.

6. Density
Water’sdensityis1grampercubiccm.Thisisusedtodefinethegram.Insteadofundergoingthermal
expansion,thedensityriseswithtemperaturetoapeakof3.98ºCandthendecreases.Negativethermal
expansionistheincreaseindensitybetween32and39.16F.Asaresult,iceislessdensethanwater,
whichhasadecreaseindensitybyabout10%.Thisiswhybodiesofwatermayhavealayeroficeon
thesurfacebutcontainliquidunderneath.Thisallowsfishandmarinelifetosurviveundertheice.
Highspecificheatkeepsthetemperatureofthewaterrelativelystablethroughthewintersothat
marinelifecansurvive.
Saltcontentlowersthefreezingpointoftheoceanbyalmost2ºC.Icestillfloatsontheocean,butthe
iceisnearsalt-freewithasimilardensitytoiceonbodiesoffreshwater.Thesaltaddstothesalinity
anddensityoftheremainingwaterwhichsinksbyconvection.Thisprocessiscalledbrinerejection.

7. Compressibility
Compressibilityisaresultofpressureandtemperature.Thecompressibilityofwaterisso
lowthatisoftenassumedtobeincompressible.Lowcompressibilityallowswaterindeep
oceanswithhighpressuretoonlydecreaseby1.8%involume.
Electrical Conductivity
Purewaterisagoodinsulator,butdeionizedwaterisnevercompletelyfreeofions.Water
undergoesaprocesscalledautoionizationasaliquid.Thismeansthattwowatermolecules
canformonehydroxideanion(OH-)andonehydroniumcation(H
30+).

8. Cohesion and Adhesion
Hydrogenbondsbetweenwatermoleculesareconstantlybreakingandreformingwith
otherwatermolecules.Cohesionistheabilityofwatermoleculestosticktogether.
Water’spolarityalsogiveswaterhighadhesion:theabilitytosticktoothersurfaces.
Theadhesiveforcesarestrongerthancohesiveforces.
Strongcohesionandadhesioncausewatertoexhibitcapillaryaction.Capillaryactionis
theprocessofliquidflowingthroughanarrowspacewithoutandoftenagainstgravity.
Wateradherestothewallsofplants’rootsandrisesupintotheplant.Porousmaterials
suchaswateralsoexhibitcapillaryaction.Treescantransportwaterthroughcapillary
actionover100meters.

9. Surface Tension
Thehydrogenbondingalsocauseswatertohaveahighsurfacetensionof71.99mN/m
at25.Thesurfacetensionishighenoughforinsectstowalkonwater.Surfacetension
isaresultofwater’scohesiveproperties.Waterdropletsandwaterrisingabovetherim
ofaglassshowwater’shighsurfacetension.

10. Capillary Action
Evenifyou'veneverheardofcapillaryaction,itisstillimportantinyourlife.Capillary
actionisimportantformovingwater(andallofthethingsthataredissolvedinit)around.It
isdefinedasthemovementofwaterwithinthespacesofaporousmaterialduetotheforces
ofadhesion,cohesion,andsurfacetension.
Capillaryactionoccursbecausewaterissticky,thankstotheforcesofcohesion(water
moleculesliketostayclosetogether)andadhesion(watermoleculesareattractedandstick
toothersubstances).Adhesionofwatertothewallsofavesselwillcauseanupwardforce
ontheliquidattheedgesandresultinameniscuswhichturnsupward.
Thesurfacetensionactstoholdthesurfaceintact.Capillaryactionoccurswhentheadhesion
tothewallsisstrongerthanthecohesiveforcesbetweentheliquidmolecules.

11. Hydraulics,branchofscienceconcernedwiththepracticalapplicationsoffluids,
primarilyliquids,inmotion.Itisrelatedtofluidmechanics,whichinlargepartprovides
itstheoreticalfoundation.Hydraulicsdealswithsuchmattersastheflowofliquidsin
pipes,rivers,andchannelsandtheirconfinementbydamsandtanks.