Liquids and SolidsLiquids and Solids

Use the Force Luke!Use the Force Luke!

Learning objectivesLearning objectives

Describe origins of surface tension and Describe origins of surface tension and meniscusmeniscus

Describe different types of cohesive forceDescribe different types of cohesive force Identify type of cohesive force based on Identify type of cohesive force based on

molecular formulamolecular formula Describe origin of hydrogen bondingDescribe origin of hydrogen bonding Explain unique properties of waterExplain unique properties of water Determine concentrations of solutions Determine concentrations of solutions

using the common concentration scalesusing the common concentration scales

Liquids and solids exist because of forcesLiquids and solids exist because of forces

CohesiveCohesive forces are attractive forces forces are attractive forces between molecules of the between molecules of the samesame substance substance

AdhesiveAdhesive forces are attractive forces forces are attractive forces between molecules of between molecules of differentdifferent substances substances

Surface tension results from Surface tension results from cohesive forcescohesive forces

Molecules on surface are drawn inwardsMolecules on surface are drawn inwards Tendency to form sphereTendency to form sphere Surface behaves like a shrink-wrap filmSurface behaves like a shrink-wrap film Liquids form into spherical dropsLiquids form into spherical drops Denser objects can “float” on the surface Denser objects can “float” on the surface

tensiontension

Consequences of surface tension: Consequences of surface tension: What will these hands ne’er be clean?What will these hands ne’er be clean?

Cleaning requires wettingCleaning requires wetting Surface tension prevents wettingSurface tension prevents wetting

Beading on a waxed carBeading on a waxed car Detergent acts as go-betweenDetergent acts as go-between

Cohesive forces and meniscusCohesive forces and meniscus

Adhesive forces pull HAdhesive forces pull H22O molecules to O molecules to

maximize coveragemaximize coverage Cohesive forces between HCohesive forces between H22O molecules O molecules

drag liquid updrag liquid up Gravity pushes liquid downGravity pushes liquid down

Solid: strong interactionsSolid: strong interactions

Fixed shapeFixed shape Not compressibleNot compressible RigidRigid DenseDense

SolidsSolids

Atoms in solids rotate and vibrate but do not Atoms in solids rotate and vibrate but do not translate translate

Melting occurs when the translational energy of Melting occurs when the translational energy of the atoms is sufficient to break free of the latticethe atoms is sufficient to break free of the lattice

Usually this is a very well defined pointUsually this is a very well defined point With amorphous solids it can be smeared out – With amorphous solids it can be smeared out –

softening of fatssoftening of fats

Liquid: medium interactionsLiquid: medium interactions

LiquidLiquid Not rigidNot rigid Assumes shape of containerAssumes shape of container Not compressibleNot compressible DenseDense

Vapour pressure and boilingVapour pressure and boiling

Molecules do not all have the same Molecules do not all have the same energyenergy

Evaporation: High energy molecules Evaporation: High energy molecules escape the liquid – vapour pressureescape the liquid – vapour pressure

When vapour pressure = atmospheric When vapour pressure = atmospheric pressure boiling occurs – all liquid pressure boiling occurs – all liquid becomes gasbecomes gas

Sublimation is direct transition of solid to Sublimation is direct transition of solid to gas (dry ice)gas (dry ice)

Name of forceName of force OriginOrigin StrengthStrength

Ion-dipoleIon-dipole Between ions and Between ions and moleculesmolecules

Quite strong Quite strong

Dipole-dipoleDipole-dipole Between Between permanent dipolespermanent dipoles

WeakWeak

Hydrogen bondsHydrogen bonds Polar bonds with H Polar bonds with H and (O,N)and (O,N)

Quite strongQuite strong

London London dispersion dispersion forcesforces

Fluctuating dipoles in Fluctuating dipoles in non-polar moleculesnon-polar molecules

WeakWeak

The Four Forces of the ApocalypseThe Four Forces of the Apocalypse

Dispersion forceDispersion force

Arises from fluctuations in electron clouds Arises from fluctuations in electron clouds in atomsin atoms

Only force present in nonpolar moleculesOnly force present in nonpolar molecules Increases with size of atoms/moleculesIncreases with size of atoms/molecules

Dipole-dipole forceDipole-dipole force Usually stronger than dispersion forcesUsually stronger than dispersion forces Present in polar moleculesPresent in polar molecules Polar substances higher boiling point than Polar substances higher boiling point than

nonpolar substancesnonpolar substances

MoleculeMolecule Molar mass Molar mass (g/mol)(g/mol)

Boiling point Boiling point ((ºC)ºC)

EthaneEthane (C(C22HH66))

Non-polarNon-polar30.030.0 -88.0-88.0

Formaldehyde Formaldehyde (CH(CH22O)O)

PolarPolar30.030.0 -19.5-19.5

Polar or nonpolar: that is the Polar or nonpolar: that is the question?question?

Polar molecules must contain polar bondsPolar molecules must contain polar bonds Determine bond polarity from electronegativityDetermine bond polarity from electronegativity

The polar bonds must not cancel outThe polar bonds must not cancel out Determine molecular shapeDetermine molecular shape

Examples:Examples:• OO22 nonpolar (no polar bond) nonpolar (no polar bond)• HCl polar (one polar bond)HCl polar (one polar bond)• CHClCHCl33 polar (three polar bonds) polar (three polar bonds)• CClCCl44 nonpolar (four polar bonds but they all cancel) nonpolar (four polar bonds but they all cancel)

Hydrogen bondingHydrogen bonding

The ultimate expression The ultimate expression of polarityof polarity

Small positive H atom Small positive H atom exerts strong attraction exerts strong attraction on O atomon O atom

Other H-bonding Other H-bonding molecules: HF, NHmolecules: HF, NH33

HH22O is the supreme O is the supreme

example: two H atoms example: two H atoms and two lone pairs per and two lone pairs per moleculemolecule

Terms in solutionTerms in solution

Solute: Solute: the dissolved substancethe dissolved substance Solvent:Solvent: the dissolving substancethe dissolving substance

MolarityMolarity

Concentration is usually expressed in Concentration is usually expressed in terms of molarity:terms of molarity:

Moles of solute/liters of Moles of solute/liters of solutionsolution (M) (M)

Moles of solute = molarity x volume of solutionMoles of solute = molarity x volume of solution

ExampleExample

What is molarity of 50 ml solution What is molarity of 50 ml solution containing 2.355 g Hcontaining 2.355 g H22SOSO44?? Molar mass HMolar mass H22SOSO44 = 98.1 g/mol = 98.1 g/mol

Moles HMoles H22SOSO44 = .0240 mol = .0240 mol Volume of solution = 50/1000 = .050 LVolume of solution = 50/1000 = .050 L Concentration = moles/volumeConcentration = moles/volume

= .0240/.050 = 0.480 M= .0240/.050 = 0.480 M

DilutionDilution

More dilute solutions are prepared from More dilute solutions are prepared from concentrated ones by addition of solventconcentrated ones by addition of solvent

MM11VV11 = M = M22VV22

Molarity of new solution MMolarity of new solution M22 = M = M11VV11/V/V22

To dilute by factor of ten, increase volume by factor To dilute by factor of ten, increase volume by factor of tenof ten

Trace quantitiesTrace quantities

Percent means one in a hundredPercent means one in a hundred PPM measures trace amounts – 1 in a PPM measures trace amounts – 1 in a

millionmillion Iodized salt contains tiny amounts of KI – 7.6 Iodized salt contains tiny amounts of KI – 7.6

x 10x 10-5-5 g in 1 g of salt g in 1 g of salt 7.6 x 107.6 x 10-5-5 g = 7.6 x 10 g = 7.6 x 10-2-2 mg = 76 mg = 76 μμgg

5 6

6 6

7.6 10 76 76 1076

1 110 10

x g KI g KI g KI ppmx ppm KI

g salt gg salt g salt

grams soluteppm =

grams solution6x10

Milligrams per literMilligrams per liter

Units for impurities in drinking waterUnits for impurities in drinking water

0.38 mg lead in 250 mL water0.38 mg lead in 250 mL water Concentration in mg/LConcentration in mg/L

milligrams solutemg/L =

liters solution

0.38 1000 0.38mg/L = 1.5 /

250 1 0.250

mg mL mgx mg L

mL L L

Something about waterSomething about water

High boiling point High boiling point compared with similar compared with similar compoundscompounds

Liquid at earth temperatureLiquid at earth temperature

Solid less dense than Solid less dense than liquidliquid

Essential for life on earthEssential for life on earth

High heat capacityHigh heat capacity Modifying influence on Modifying influence on

climateclimate

Universal solventUniversal solvent

HH22O has optimum combination of O has optimum combination of

lone pairs and H atomslone pairs and H atoms

CompoundCompound Number of lone Number of lone pairspairs

Number of H Number of H atomsatoms

HFHF 33 11

HH22OO 22 22

NHNH33 11 33

H bonding generates three-H bonding generates three-dimensional networkdimensional network

Ice floats!Ice floats!

Something so familiar we might believe all Something so familiar we might believe all solids float on their liquids. Not so. Water solids float on their liquids. Not so. Water is the exception.is the exception.

Hydrogen bonding and lifeHydrogen bonding and life

hold the two strands of the DNA double helix hold the two strands of the DNA double helix together together

hold polypeptides together in such secondary hold polypeptides together in such secondary structures as the alpha helix and the beta structures as the alpha helix and the beta conformationconformation

help enzymes bind to their substratehelp enzymes bind to their substrate help antibodies bind to their antigen help antibodies bind to their antigen help transcription factors bind to each otherhelp transcription factors bind to each other help transcription factors bind to DNA help transcription factors bind to DNA

Implications for life on earth Implications for life on earth

Without H-bonds molecules like DNA Without H-bonds molecules like DNA would not existwould not exist

H-bonds hold the two strands togetherH-bonds hold the two strands together Comparative weakness of bonding allows Comparative weakness of bonding allows

for DNA replicationfor DNA replication

Unusual variation of the density of Unusual variation of the density of water with T: maximum density at 4°Cwater with T: maximum density at 4°C

Effects of density on lakesEffects of density on lakesseasonal cycling of waterseasonal cycling of waterreplenishment of oxygenreplenishment of oxygen

Warm water on top

Cold water on top

Water contaminationWater contamination BiologicalBiological

Human and animal waste – bacteria leading to hepatitis, cholera, Human and animal waste – bacteria leading to hepatitis, cholera, typhoid, dysenterytyphoid, dysentery

ChemicalChemical OrganicOrganic

• BenzeneBenzene• ChlorohydrocarbonsChlorohydrocarbons

InorganicInorganic• AsbestosAsbestos• NitratesNitrates• LeadLead• Mercury Mercury

RadioactivityRadioactivity• UraniumUranium• Tritium spillsTritium spills

Legislating cleanliness: The Safe Legislating cleanliness: The Safe Drinking Water Act 1974Drinking Water Act 1974

Establish maximum contaminant levels Establish maximum contaminant levels (MCLs) for 84 contaminants(MCLs) for 84 contaminants

All water supplies must passAll water supplies must pass Periodic sampling requiredPeriodic sampling required Too much or too little? Water treatment Too much or too little? Water treatment

costs moneycosts money

Don’t trust the EPA? Treat at homeDon’t trust the EPA? Treat at home

Active carbon filtersActive carbon filters Effective on organic contaminantsEffective on organic contaminants Need regular replacementNeed regular replacement

Water softenersWater softeners Specific to hard waterSpecific to hard water Ion exchange using zeolitesIon exchange using zeolites

Reverse osmosisReverse osmosis Apply pressure to push water from saline to Apply pressure to push water from saline to

pure side of membranepure side of membrane