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CHAPTER 13 –

• LIQUIDS AND SOLIDS

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WHY?WHY?• Why is water usually a liquid

and not a gas?

• Why does liquid water boil at such a high temperature for such a small molecule?

• Why does ice float on water?

• Why do snowflakes have 6 sides?

• Why is I2 a solid whereas Cl2is a gas?

• Why are NaCl crystals little cubes?

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Inter-molecular Forces

InterInter--molecular molecular ForcesForces

Have studied Have studied INTRAmolecularINTRAmolecular forcesforces——the forces holding atoms together to the forces holding atoms together to form molecules.form molecules.

Now turn to forces between molecules Now turn to forces between molecules ——INTERmolecularINTERmolecular forces. forces.

Forces between molecules, between ions, Forces between molecules, between ions, or between molecules and ions.or between molecules and ions.

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Kinetic-Molecular Description of Liquids and Solids•• Solids and liquids are Solids and liquids are condensed states.condensed states.

–– The atoms, ions, or molecules in solids and The atoms, ions, or molecules in solids and liquids are much closer to one another than liquids are much closer to one another than in gases.in gases.

•• Liquids and gases are Liquids and gases are fluids.fluids.

• The intermolecular attractionsintermolecular attractions in in liquids and solids are strong.liquids and solids are strong.

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Liquids and Solids

•• Strengths of interactionsStrengths of interactions among particles

Gases< Liquids < Solids

•• Miscible liquidsMiscible liquids are soluble (dissolve) in each other.

• Water dissolves in alcohol.

••Immiscible liquidsImmiscible liquids are insoluble in each other.

•Water does not dissolve in oil.

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Intermolecular Attractions – 4 Important

1. Ion-ion interactions – Strongest IM Force– The force of attraction between two

oppositely charged ions is governed by Coulomb’s law.

( )( )

ions. ebetween th distance theis d

charges.ion theare q and q

d

qqF

-

2

-

+

+

∝

•Coulomb’s law determines:

1.The melting and boiling points of ionic compounds.

2.The solubility of ionic compounds.

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Ion-Ion Forcesfor comparison of magnitudeIonIon--Ion ForcesIon Forcesfor comparison of magnitudefor comparison of magnitude

NaNa++——ClCl-- in saltin salt

These are the strongest These are the strongest forces. forces.

Lead to solids with high Lead to solids with high melting temperatures.melting temperatures.

NaClNaCl, mp = 800 , mp = 800 ooCC

MgOMgO, mp = 2800 , mp = 2800 ooCC

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Covalent Bonding Forcesfor comparison of magnitudeCovalent Bonding ForcesCovalent Bonding Forcesfor comparison of magnitudefor comparison of magnitude

CC––H, 413 kJ/molH, 413 kJ/mol

C=C, 610 kJ/molC=C, 610 kJ/mol

CC––C, 346 kJ/molC, 346 kJ/mol

CN, 887 kJ/molCN, 887 kJ/mol

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Attraction Between Ions and Permanent DipolesAttraction Between Ions Attraction Between Ions and Permanent Dipolesand Permanent Dipoles

Water is highly polar and Water is highly polar and can interact with positive can interact with positive ions to give ions to give hydrated hydrated ions in water.ions in water.

H

H

water dipole

••

••

O-δδδδ

+δδδδ

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Water is highly polar and Water is highly polar and can interact with positive can interact with positive ions to give ions to give hydrated hydrated ions in water.ions in water.

H

H

water dipole

••

••

O-δδδδ

+δδδδ

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Many metal ions are hydrated. Many metal ions are hydrated. This is the reason metal salts This is the reason metal salts dissolve in water.dissolve in water.

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Attraction between ions and dipole depends on ionAttraction between ions and dipole depends on ioncharge and ioncharge and ion--dipole distance.dipole distance.

Measured by Measured by ∆∆H for H for MMnn++ + H+ H22O O ----> [M(H> [M(H22O)O)xx]]n+n+

--1922 kJ/mol1922 kJ/mol --405 kJ/mol405 kJ/mol --263 kJ/mol263 kJ/mol

Attraction Between Ions and Permanent DipolesAttraction Between Ions and Attraction Between Ions and Permanent DipolesPermanent Dipoles

OH

Hδδδδ+

δδδδ-• • • O

H

Hδδδδ+

δδδδ-• • • O

H

Hδδδδ+

δδδδ-• • •

Na+Mg

2+

Cs+

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Intermolecular Attractions and Phase Changes

3. 3. Dipole-dipole interactions– Consider CO2 vs. NO2

– CO2 – MP –78 ºC

– NO2 – MP 21 ºC

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Dipole-Dipole ForcesDipoleDipole--Dipole ForcesDipole Forces

Such forces bind molecules having Such forces bind molecules having permanent dipoles to one permanent dipoles to one another.another.

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Dipole-Dipole ForcesDipoleDipole--Dipole ForcesDipole Forces

Influence of dipoleInfluence of dipole--dipole forces is seen in the dipole forces is seen in the boiling points of simple molecules.boiling points of simple molecules.

CompdCompd Mol. Wt.Mol. Wt. Boil PointBoil Point

NN22 2828 --196 196 ooCC

COCO 2828 --192 192 ooCC

BrBr22 160160 59 59 ooCC

IClICl 162162 97 97 ooCC

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Intermolecular Attractions

2. 4. Hydrogen bonding – IM Force between dipole induced by H and N, O, F only

– H2O - very polar molecule.

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Hydrogen BondingHydrogen BondingHydrogen Bonding

A special form of dipoleA special form of dipole--dipole attraction, dipole attraction, which enhances dipolewhich enhances dipole--dipole attractions.dipole attractions.

HH--bonding is strongest when X and Y are bonding is strongest when X and Y are N, O, or FN, O, or F

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H-Bonding Between Methanol and WaterHH--Bonding Between Methanol Bonding Between Methanol and Waterand Water

H-bondHH--bondbond--δδδδδδδδ

++δδδδδδδδ

--δδδδδδδδ

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H-Bonding Between Two Methanol MoleculesHH--Bonding Between Two Bonding Between Two Methanol MoleculesMethanol Molecules

H-bondHH--bondbond

--δδδδδδδδ

++δδδδδδδδ

--δδδδδδδδ

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H-Bonding Between Ammonia and WaterHH--Bonding Between Ammonia Bonding Between Ammonia and Waterand Water

H-bondHH--bondbond

--δδδδδδδδ

++δδδδδδδδ --δδδδδδδδ

This HThis H--bond leads to the formation of bond leads to the formation of

NHNH44++ and OHand OH--

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Hydrogen Bonding in H2OHydrogen Bonding in HHydrogen Bonding in H22OO

HH--bonding is especially bonding is especially strong in water strong in water becausebecause

•• the Othe O——H bond is very H bond is very polarpolar

•• there are 2 lone pairs there are 2 lone pairs on the O atomon the O atom

Accounts for many of Accounts for many of waterwater’’s unique s unique properties.properties.

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Ice has open Ice has open latticelattice--like like structure.structure.

Ice density is Ice density is < liquid.< liquid.

And so solid And so solid floats on floats on water.water.

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Ice has open latticeIce has open lattice--like structure.like structure.

Ice density is < liquid and so solid floats on water.Ice density is < liquid and so solid floats on water.

One of the VERY few One of the VERY few

substances where substances where

solid is LESS DENSE solid is LESS DENSE

than the liquid.than the liquid.

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Boiling Points of Simple Boiling Points of Simple HydrogenHydrogen--Containing Containing CompoundsCompounds

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Intermolecular Attractions

4. 5. London Forces are very weak.– Only attractive force in nonpolar molecules.

– Induced dipole-Induced dipole interactions

• Consider I2 as an isolated molecule.

• I2 – MP 110 ºC

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FORCES INVOLVING INDUCED DIPOLESFORCES INVOLVING FORCES INVOLVING INDUCED DIPOLESINDUCED DIPOLES

How can nonHow can non--polar molecules such as Opolar molecules such as O2 2 and and II22 dissolve in water?dissolve in water?

The water dipole The water dipole INDUCESINDUCES a dipole in a dipole in

the Othe O22 electric cloud. electric cloud.

Dipole-induced

dipole

DipoleDipole--induced induced

dipoledipole

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Solubility increases with mass the gasSolubility increases with mass the gas

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•• Process of inducing a dipole is Process of inducing a dipole is polarizationpolarization

•• Degree to which electron cloud Degree to which electron cloud of an atom or molecule can be of an atom or molecule can be distorted in its distorted in its polarizabilitypolarizability..

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IM FORCES — INDUCED DIPOLESIM FORCES IM FORCES —— INDUCED DIPOLESINDUCED DIPOLES

Consider IConsider I22

dissolving dissolving in ethanol, in ethanol, CHCH33CHCH22OH.OH.

OH

-δδδδ

+δδδδ

I-I

R

-δδδδ

+δδδδ

OH+δδδδ

-δδδδ

I-I

R

The alcohol The alcohol

temporarily temporarily

creates or creates or

INDUCESINDUCES a a

dipole in Idipole in I22..

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Formation of a dipole in two Formation of a dipole in two nonpolarnonpolar II22

molecules.molecules.

Induced dipole-

induced dipole

Induced dipoleInduced dipole--

induced dipoleinduced dipole

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The induced forces between IThe induced forces between I22

molecules are very weak, so solid Imolecules are very weak, so solid I22

sublimessublimes (goes from a solid to (goes from a solid to

gaseous molecules).gaseous molecules).

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The magnitude of the induced dipole depends on The magnitude of the induced dipole depends on the tendency to be distorted. the tendency to be distorted.

Higher Higher molecmolec. weight . weight ------> larger induced > larger induced dipoles.dipoles.

MoleculeMolecule Boiling Point Boiling Point ((ooCC))

CHCH44 (methane) (methane) -- 161.5161.5

CC22HH66 (ethane)(ethane) -- 88.6 88.6

CC33HH88 (propane) (propane) -- 42.142.1

CC44HH1010 (butane) (butane) -- 0.50.5

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Boiling Points of HydrocarbonsBoiling Points of Hydrocarbons

Note linear relation between bp and molar mass.

CHCH44

CC22HH66

CC33HH88

CC44HH1010

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Intermolecular Forces SummaryIntermolecular Forces Summary

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LiquidsLiquidsLiquidsIn a liquidIn a liquid

•• molecules are in constant molecules are in constant motionmotion

•• there are appreciable there are appreciable intermolecintermolec. forces. forces

•• molecules close togethermolecules close together

•• Liquids are almost Liquids are almost incompressibleincompressible

•• Liquids do not fill the Liquids do not fill the containercontainer

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LiquidsLiquids

The two key properties we need to describe are EVAPORATION and its opposite—CONDENSATION

The two key properties we need to The two key properties we need to describe are EVAPORATION and its describe are EVAPORATION and its oppositeopposite——CONDENSATIONCONDENSATION

break IM bonds

make IM bonds

Add energy

Remove energy

LIQUID VAPOR

<<------condensationcondensation

evaporationevaporation------>>

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LiquidsLiquids——EvaporationEvaporation

To evaporate, To evaporate, molecules must molecules must have sufficient have sufficient energy to break energy to break IM forces.IM forces.

Breaking IM forces Breaking IM forces

requires energy. The requires energy. The

process of process of

evaporation is evaporation is

endothermic.endothermic.

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LiquidsLiquids——Distribution of EnergiesDistribution of Energies

Distribution of molecular energies in a liquid.

KE is propor-tional to T.

Distribution of Distribution of molecular molecular energies in energies in a liquid.a liquid.

KE is KE is proporpropor--tionaltional to T.to T.0

Number of molecules

Molecular energy

higher Tlower T

See Figure 13.14See Figure 13.14

Minimum energy req’d to break IM

forces and evaporate

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Distribution of Energy in a LiquidDistribution of Energy in a Liquid

•Molecules escape from the surface of a liquid and

•become a gas - temperature dependent.

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The Liquid State - Vapor Pressure

• Pressure exerted by a liquid’s vapor on its surface at equilibrium.

• Vapor Pressure (torr) and boiling point for three liquids at different temperatures.

0 oC 20 oC 30 oC normal boiling point

diethyl ether 185 442 647 36oCethanol 12 44 74 78oC

water 5 18 32 100oC

Boiling Point – Temperature at which VP = Atm. Pressure

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LiquidsLiquidsWhen molecules of liquid are in When molecules of liquid are in

the vapor state, they exert a the vapor state, they exert a VAPOR PRESSUREVAPOR PRESSURE

EQUILIBRIUM VAPOR EQUILIBRIUM VAPOR PRESSUREPRESSURE is the pressure is the pressure exerted by a vapor over a exerted by a vapor over a liquid in a closed container liquid in a closed container when the when the rate of evaporation rate of evaporation = the rate of condensation.= the rate of condensation.

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Measuring Equilibrium Measuring Equilibrium Vapor PressureVapor Pressure

Liquid in flask evaporates and exerts pressure on manometer.

43

LiquidsLiquids

HEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat reqreq’’dd (at (at constant P) to vaporize the liquid.constant P) to vaporize the liquid.

LIQ + heat LIQ + heat ------> VAP> VAP

Compd.Compd. ∆∆HHvapvap (kJ/mol) (kJ/mol) IM ForceIM Force

HH22OO 40.7 (100 40.7 (100 ooCC)) HH--bondsbonds

SOSO22 26.8 (26.8 (--47 47 ooCC)) dipoledipole

XeXe 12.6 (12.6 (--107 107 ooCC)) induced dipole induced dipole

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The Liquid State – Energy and Phase Change

• Heat of Vaporization - amount of heat required to change 1.00 g of a liquid substance to a gas at constant temperature (units of J/g).

• Heat of Condensation is the reverse of heat of vaporization, phase change from gas to liquid.

← →+

J 2260-

o

(g)2

J 2260o

)(2 C100.0at OH g 00.1C100.0at OH g 1.00l

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The Liquid State

Molar heat of vaporization or ∆∆∆∆Hvap

• Heat required to change 1.00 mole of a liquid to a gas at constant temperature.

∆Hvap has units of J/mol.

Molar heat of condensation• The reverse of molar heat of vaporization is the heat of condensation.

← →+

kJ 40.7-

o

(g)2

kJ 40.7o

)(2 C100.0at OH mol 00.1C100.0at OH mol 1.00l

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Equilibrium Vapor Pressure & the Equilibrium Vapor Pressure & the ClausiusClausius--ClapeyronClapeyron EquationEquation

•• ClausiusClausius--ClapeyronClapeyron equation equation

—— used to find used to find ∆∆HH˚̊vapvap..

•• The logarithm of the vapor The logarithm of the vapor

pressure P is proportional to pressure P is proportional to

∆∆HHvaporiationvaporiation and to 1/T.and to 1/T.

•• lnln P = P = ––((∆∆HH˚̊vapvap/RT) + C/RT) + C

lnP2P1

= ∆HvapR

1

T1 -

1

T2

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The Liquid State

48

The Liquid State - Heat Transfer

q = m C T∆

•Example: How much heat is released by 225 g of H2O as it cools from 85.0

oC to

40.0 oC? The specific heat of water is

4.184 J/goC.

49

The Liquid State• Example: How many joules of energy must

be absorbed by 5.00 x 102 g of H2O at 50.0oC to convert it to steam at 120oC? The molar heat of vaporization of water is 40.7 kJ/mol and the molar heat capacities of liquid water and steam are 75.3 J/mol oCand 36.4 J/mol oC, respectively.

50

Boiling LiquidsBoiling Liquids

Liquid boils when its vapor pressure equals atmospheric pressure.

Liquid boils when its vapor pressure equals atmospheric pressure.

51

The Liquid State

• Boiling Points of Various Kinds of Liquids

Gas MW BP(oC)He 4 -269

Ne 20 -246

Ar 40 -186

Kr 84 -153

Xe 131 -107

Rn 222 -62

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The Liquid State

Compound MW(amu) B.P.(oC)

CH4 16 -161

C2H6 30 -88

C3H8 44 -42

n-C4H10 58 -0.6

n-C5H12 72 +36

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The Liquid State - ViscosityViscosity

• Viscosity is the resistance to flow.– For example, compare how water pours

out of a glass compared to molasses, syrup or honey.

• Oil for your car is bought based on this property.– 10W30 or 5W30 describes the viscosity of

the oil at high and low temperatures.

– Greater IM Force � Greater viscosity

54

The Liquid State - Surface TensionSurface Tension

• Molecules at the surface are attracted unevenly.

• Measure of inward forces that must be overcome to expand the molecules at the surface.

• Consider water – surface vs submerged

55

LiquidsLiquids

Molecules at surface behave differently than those Molecules at surface behave differently than those in the interior.in the interior.

Molecules at surface experience net INWARD Molecules at surface experience net INWARD

force of attraction. force of attraction.

This leads to SURFACE TENSION This leads to SURFACE TENSION —— the energy the energy

reqreq’’dd to break the surface.to break the surface.

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The Liquid State

• Cohesive forces are the forces that hold liquids together.

• Adhesive forces are the forces between a liquid and another surface.– Capillary rise implies that the:

• Adhesive forces > cohesive forces

– Capillary fall implies that the:• Cohesive forces > adhesive forces

• affects the meniscus of liquidsWater

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LiquidsLiquidsIntermolecIntermolec. forces also lead to CAPILLARY action . forces also lead to CAPILLARY action

and to the existence of a concave meniscus for a and to the existence of a concave meniscus for a water column.water column.

concave

meniscus

H2O in

glass

tube

ADHESIVE FORCESbetween waterand glass

COHESIVE FORCESbetween watermolecules

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Capillary ActionCapillary Action

Movement of water up a piece of paper Movement of water up a piece of paper depends on Hdepends on H--bonds between Hbonds between H22O and O and the OH groups of the cellulose in the the OH groups of the cellulose in the paper.paper.

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The Solid State - Normal Melting Point

• Temperature at which the solid melts (liquid and solid in equilibrium) at exactly 1.00 atm of pressure.

• MP increases as IM Forces increase.

60

The Solid State

• Which requires more energy?

( ) ( )

( ) ( )←→

←→

l

l

OHOH

or

NaClNaCl

2s2

s

� What experimental proof do you have?

61

Heat Transfer Involving Solids

Heat of fusionHeat of fusion - heat required to melt one gram of a solid at its melting point

at constant temperature.

•• Heat of crystallization Heat of crystallization is the reverse of

the heat of fusion.

← →+

J 334-

o

)(2

J 334 o

(s)2 C0at OH g 1.00 C0at OH g 1.00l

62


Molar heat of fusion or ∆∆∆∆Hfusion• The molar heat of fusion per mole of a

substance at its melting point.

• The molar heat of crystallization is thereverse of molar heat of fusion

← →+

J 6012-

o

)(2

J 6012o

(s)2 C0at OH mole 1.00 C0at OH mole 1.00l

63


• Example: Calculate the amount of heat required to convert 150.0 g of ice at

• -10.0oC to water at 40.0oC.specific heat of ice is 2.09 J/goC

Heat of fusion – 334 J/g

Specific heat of water 4.18 J/goC

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Sublimation

• Solid transforms directly to the vapor phase

• Solid CO2 or “dry” ice does this well.

← →oncondensati

nsublimatio

gas solid

65

Phase DiagramsPhase Diagrams

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TRANSITIONS TRANSITIONS BETWEEN PHASESBETWEEN PHASESSection 13.10Section 13.10

Lines connect all conditions of T and P where Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on EQUILIBRIUM exists between the phases on either side of the line.either side of the line.

(At equilibrium particles move from liquid to (At equilibrium particles move from liquid to gas as fast as they move from gas to liquid, gas as fast as they move from gas to liquid, for example.)for example.)

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Critical T and PCritical T and P

.

LIQUID

GAS

Pcritical

High Pressure

High Temperature

Tcritical

Note that linegoes straight up

Above critical T

no liquid exists

no matter how

high the

pressure.

Above critical T Above critical T

no liquid exists no liquid exists

no matter how no matter how

high the high the

pressure.pressure.

As P and T increase, you finally As P and T increase, you finally

reach the reach the CRITICAL TCRITICAL T and and PP

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Phase Diagrams (P versus T)• Display the different phase transitions of a

substance - water.

69

Triple Point Triple Point ——WaterWater

At the At the TRIPLE POINT TRIPLE POINT all all

three phases are in three phases are in

equilibrium.equilibrium.

70

Phases Phases DiagramsDiagrams——Important Points for Important Points for WaterWater

T(T(˚̊C)C)P(mmHg)P(mmHg)

Normal boil point Normal boil point 100100 760760

Normal freeze pointNormal freeze point 00 760760

Triple point Triple point 0.00980.0098 4.58 4.58

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COCO22 Phase DiagramPhase Diagram

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Critical T and PCritical T and P

COMPDCOMPD TTcc((ooCC)) PPcc(atm(atm))

HH22OO 374374 218218

COCO22 3131 7373

CHCH44 --8282 4646

FreonFreon--1212 112112 4141(CCl(CCl22FF22))

Notice that Notice that TTcc and Pand Pcc depend on depend on intermolecular forces.intermolecular forces.

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