Liquids and Solids

Liquids and Solids

AP Chem Unit 10

SectionsIntermolecular ForcesLiquid stateSolid Structures Metal StructuresCarbon and Silicon Networks

SectionsMolecular SolidsIonic SolidsVapor pressure and State ChangePhase Diagrams

States of MatterWhen considering the three states of

matter, properties of gases are strikingly different than solids and liquids. Liquids and solids share many similar characteristics

compressibilitydensityintermolecular forces

States of MatterH2O(s)H2O(l) ΔH°fus = 6.02 kj/molH2O(l)H2O(g) ΔH°vap = 40.7 kj/molWater densities:

25°C and 1atm .99707g/cm3

25°C and 1065 atm 1.046g/cm3

400°C and 1atm 3.26x10-4 g/cm3

400°C and 242 atm .157g/cm3

Intermolecular Forces

10.1

Intermolecular ForcesElectrons shared within the molecule are

called intramolecular bonding.In the condensed states of matter the

attraction between molecules are called intermolecular forces.

Intermolecular ForcesIt is important to realize that when a

molecule changes state, the molecule stays intact. The changes in state are due to the change in forces surrounding the molecule not from changes within the molecule.

40.7kj needed to vaporize water934kj to break the O-H bond

Dipole–Dipole ForcesDipole-dipole forces occur when polar

molecule (molecules with dipole moments) electrostatically attract each other by lining up the positive and negative ends of the dipoles.

Dipole-dipole forces are about 1% as strong as a covalent or ionic bond and rapidly become weaker when distances between the dipoles increases. The distances in a gas make these attractions relatively unimportant

Dipole-Dipole Forces In a condensed

state, molecules line up dipoles to minimize repulsions and maximize attractions.

Dipole-Dipole ForcesSome dipole-dipole forces are unusually

strong. These usually form between H and another very electronegative atom.

These are stronger due to the high polarity of the bond and the closeness of the dipoles between the atoms.These strong attractions have a

strong impact on melting points and boiling points.

Boiling Points of Covalent Hydrides

Hydrogen bondsHydrogen bonds are the strongest in the

smallest and lightest of the covalent molecules. This is primarily due to two factors:

large difference in electronegativitiessmall size of the atoms allows for close

dipole interactions.

Hydrogen bonds

Hydrogen Bonds and OrganicsMethanol (CH3OH) and ethanol

(CH3CH2OH) have much higher boiling points than would be expected from their molar masses because of the O-H bonds that produce hydrogen bonding.

London Dispersion ForcesEven without dipoles, molecules exert

forces on each other.The forces that exist among noble gas

atoms and nonpolar molecules are called London dispersion forces.

London Dispersion ForcesUsually it is assumed that electron

dispersion is uniform throughout the molecule, but this is not always the case.

Since the movements of the electrons around the nucleus are somewhat random, a momentary nonsymmetrical electron distribution can develop that creates a temporary dipolar arrangement of charge.

London Dispersion ForcesThis temporary change in polarity can, in

turn, temporarily change the distribution of the neighboring molecule.

This phenomenon leads to an inter-atomic attraction that is relatively weak and short-lived, but can be significant in larger atoms at lower temperatures.larger atoms have more electrons and

increases the probability of a temporary dipole.

London Dispersion Forces

London Dispersion ForcesPolarizability is the ease at which an

electron cloud can be distorted into a temporary dipole.

large atoms have a larger polarizability than smaller atoms

This also applies to molecules like H2, CH4, CCl4 and CO2; smaller molecules, but nonpolar.

The Liquid State10.2

Liquid Characteristicslack of rigidity low compressibilityhigh densityrounded dropletscapillary actionviscosity

Rounded DropletsOccur due to the intermolecular forces of

the liquid. The liquid molecules are subject to attraction from the side and from below, so liquid tends to form a shape with the minimum surface area – sphere.

The resistance of a liquid to increase surface area is from the energy that it takes to overcome intermolecular forces. This resistance is called surface tension.

Rounded DropletsMolecules that are polar and have

stronger intermolecular forces have stronger surface tensions.

Surface Tension

Capillary ActionCapillary action is the spontaneous rising

of a liquid in a narrow tube. This action is due to two forcescohesive forces- the intermolecular

forces among the molecules.adhesive forces – the attractive

forces between the liquid and the container.

Adhesive forcesAdhesive forces happen when bonds

within the container have polar bondsFor example: glass has O atoms

that carry a partial negative charge that attracts the partial positive charge of the hydrogen in water. This balance between the strong cohesive forces and the strong adhesive forces produce a meniscus.

Adhesive forcesA nonpolar substance, such as

mercury, has a convex meniscus because the cohesive forces are stronger than the adhesive forces.

Meniscus: Water vs. Mercury

ViscosityViscosity is a fluids resistance to flow. liquids with strong cohesive forces

tend to be highly viscous. Example: glycerol is highly viscous

because of its ability to create hydrogen bonds.

ViscosityMolecular complexity also can affect

viscosity because they can become entangled in each other.Example: Gasoline has carbon chains

from 3-8C long and is nonviscous. Grease is 20-25C long and is very viscous.

Introduction to Structures and Types of

Solids10.3

Types of SolidsCrystalline solidsAmorphous solids

Crystalline SolidsCrystalline solids have a regular

arrangement of components at a microscopic level and produce beautiful, characteristic shapes of crystals:

Crystalline SolidsThe positions of components are usually

represented by a lattice.lattice is a three dimensional system

of units repeating in a pattern. The smallest repeating unit of the lattice is called the unit cell.

Three types of Crystalline Solids

Amorphous SolidsAmorphous solids have considerable

disorder in their structures.Example: Common glass looks like a

solution frozen in place. It has a rigid shape but a great deal of disorder within its structure.

X-ray Analysis of SolidsThe structures of crystalline solids are

commonly determine by X-ray diffraction.

This type of diffraction occurs when beams of light are scattered as they go through spaces between substances. Light scatters when the size of the spaces are similar to the wavelength of light.

X-ray Analysis of Solids

X-ray Analysis of SolidsA single wavelength is directed at the

crystal and a diffraction pattern is obtained. The diffraction pattern is a series of light and dark areas on a photographic plate from constructive and destructive interference from waves of light.

The diffraction pattern can then be used to determine the interatomic spacings.

X-ray Analysis of SolidsA diffractometer is a computer-

controlled instrument used for carrying out the X-ray analysis of crystalsIt rotates the crystal with respect to

the X-ray beam and collects the data produced by the scattering. The techniques have been refined to the point that very complex structures can be determined, such as large biological enzymes.

X-ray Analysis of SolidsThe Bragg equation combines

trigonometry and physics to determine the atomic spaces between crystals:

nλ = 2d sin θd is the distance between atoms and θ

is the angle of incidence and reflection of the light. n is an integer, most commonly 1. (n is usually given)

X-ray Analysis of Solids

Example ProblemX-rays of wavelength 1.54 Â were used to

analyze an aluminum crystal. A reflection was produced at θ = 19.3°. Assuming n=1, calculate the distance d between the planes of atoms producing this reflection

2.33 Á

Types of SolidsIonic solids

ionic solids are made of ionsMolecular solids

Molecular solids have small units of covalently bonded molecules.

Atomic solidsAtomic solids are made of elements

such as carbon (graphite, diamond and the fullerenes), boron, silicon, and all metals.

Fullerenes

Types of Solids

Atomic SolidsAtomic solids are broken down into

subgroups depending on the bond that exists in the solid:

Metallic solidsHas delocalized nondirectional covalent

bonding.Network solids

atoms bond with strong directional covalent bonding that lead to giant molecules and networks

Atomic SolidsGroup 8A solids

noble gases are attracted to each other with London dispersion forces.

Classification of Solids

Structure and Bonding in Metals

10.4

Metal CharacteristicsMost of the properties that we see in

metals is due to the nondirectional covalent bonding found in metal crystals.

High thermal conductivityElectrical conductivityMalleabilityDuctility

Metallic CrystalsMetallic crystals can be pictured as

containing spherical atoms packed together that can be bonded to each other equally in all directions.

This arrangement is called closest packing.

Closest PackingThe spheres pack in layers. Each

sphere is surrounded by six others. These layers do not lie directly over those in the first layer, instead they fill the indentations of the layer below. The third layer is in the same position as the first. This is called aba arrangement.

Closest PackingThe aba arrangement has the

hexagonal unit cell and the resulting structure is called the hexagonal closest packed (hcp) structure.

The abc arrangement has a face-centered cubic unit cell and the resulting structure is called the cubic closest packed (ccp) structure. This has a repeating vertical placement every fourth layer.

Closest Packing

Closest Packing: Hexagonal

Closest Packed: Cubic

Closest PackingKnowing the net number of atoms in a

particular unit cell is important for many applications involving solids.

Closest PackingExample: A face centered cube (unit cell)

is defined by the centers of the spheres on the cube’s corners. Therefore 8 cubes share a given corner sphere, so 1/8 of this sphere lies inside the unit cell. (8 corners x 1/8 sphere = 1sphere). The sphere at the center of each face is shared by two cubes. (6 faces x ½ sphere = 3 spheres). The total number of spheres for a face centered cube is 4.

Closest PackingFace – Centered Cubic Unit Cell

Cubic SubstancesMetals that form cubic closest packed

solids are:aluminumironcoppercobaltnickel

Hexagonal SubstancesMetals that form hexagonal closest

packed solids are:magnesiumzinc

Other Metal SolidsCalcium and certain other metals can

crystallize in either cubic or hexagonal solids.

Some metals, including many alkali metals, have structures that are characterized by a body-centered cubic (bcc) unit cell. In this structure, each sphere has 8 neighbors.

Example ProblemSilver crystallizes in a cubic closest

packed structure. The radius of a silver atom is 144pm. Calculate the density of solid silver?

10.6 g/cm3

Bonding Models for MetalsIn order to determine bonding for metals,

one must account for the typical properties: durable, high melting point, malleable, ductile, and efficient in uniform conduction of heat and electricity in all directions.

These characteristics indicate that the bonds are strong and nondirectional. In other words, it is not easy to separate metal atoms but easy to move them.

Electron Sea ModelMetal cations ‘swim’ in a sea of valence

electrons that are mobile and shared.This accounts for conduction and

malleability and ductility.

Electron Sea Model

Band Model (MO Model)In this model, the electrons are assumed

to travel around the metal crystal in molecular orbitals formed from the valence atomic orbitals of the metal atoms.

When metals atoms interact, the large number of resulting molecular orbitals become more closely spaced and finally form a virtual continuum of levels, called bands.

Band Model (MO Model)

Band ModelThe electrons in partially filled MO’s

are mobile. These conduction electrons are free to travel throughout the metal crystal. The MO occupied by these conducting electrons are called conduction bands.

Band Model

Metal AlloysAn alloy is best defined as a substance that

contains a mixture of elements and has metallic properties. There are two types of alloys:

Substitutional alloy– some of the host metal atoms are replaced by other metal atoms of similar size.

Interstitial alloy – is formed when some of the interstices (holes) in the closest packed lattice are occupied by smaller atoms.

Metal Alloys

Substitutional AlloysExample: brass: 1/3 of copper metal

atoms are replaced by zinc atomsSterling silver- 93% silver and 7% copper.Pewter- 85% tin, 7% copper, 6% bismuth

and 2% antimony.Plumbers solder – 95% tin and 5%

antimony

Interstitial AlloyExample: Steel contains carbon atoms in

the holes of an iron crystal. The presence of the interstitial atoms changes the properties of the host metal. Iron is relatively soft, ductile and malleable, but when carbon (which forms directional bonds), is introduced into the crystal, it makes the iron bonds stronger and less ductile.

Interstitial AlloyThe amount of carbon directly affects the

properties of steel:Mild steels- contains less than .2%

carbon: nails chains and cables.Medium steels- contain .2-.6% carbon:

rails and structural steelHigh-carbon steel – .6-1.5% carbon:

springs, tools and cutlery.

Mixed AlloysSome steels contain elements in addition

to iron and carbon. These are called alloy steels and are viewed as being mixed interstitial and substitutional alloys.

Bicycle frames are usually constructed from a wide variety of alloy steels.

Carbon and Silicon Network Atomic Solids

10.5

Network SolidsMany atomic solids contain strong

directional covalent bonds to form a solid that might be viewed as a “giant molecule.” These materials are typically brittle and do not efficiently conduct heat and electricity. Two examples of these network solids are carbon and silicon.

CarbonTwo most common forms of carbon are

diamond and graphite. They are typical network solids.

Diamond is the hardest naturally occurring substance.

Graphite is slippery, black and a conductor.

DiamondEach carbon is surrounded by a tetrahedral

arrangement of other carbon atoms to form a large molecule. Diamond is an insulator not a conductor. Each carbon is sp3 hybridized with localized bonding and therefore does not conduct.

Diamonds are often used for industrial cutting implements.

The application of 150,000 atm at 2800°C can break graphite bonds and rearrangement into a diamond structure.

GraphiteThe structure of graphite is based on layers of

carbon atoms arranged in fused 6 C rings. The unhybridized p orbitals allow for delocalized electrons and therefore conductivity.

Graphite is used as a industrial lubricant. Because graphite has strong bonds within the layers and weak bonding between the layer, the layers slide past one another readily.

Carbon

Carbon: Graphite layers

SiliconSilicon is an important constituent of the

compounds that make up the earth’s crust. Silicon is to geology what carbon is to biology and is fundamental to most rocks, sands and soils found in the earth’s crust.

Carbon compounds typically have long strings of C-C bonds

Silicon compounds typically involve chains of Si-O bonds.

SilicaThe fundamental

silicon-oxygen compound is silica, which has the empirical formula SiO2. The structure that is formed is based on a network of SiO4 tetrahedra with shared oxygen atoms rather than smaller SiO2 units.

SilicaWhen silica is heated

above its melting point (1600°c) and cooled rapidly, an amorphous solid called glass results. Glass has a lot of disorder as opposed to the crystalline nature of quartz. Glass, also homogeneous, more closely resembles a very viscous solution than it does a crystalline solid.

GlassThe properties of glass can vary greatly

depending on the additives. Common glass results when substances

like Na2CO3 are added to the silica melt. B2O3 produce borosilicate glass which

does not expand and contract during large temperature changes. (Pyrex)

K2O produces especially hard glass that can be ground into shapes for lenses and contacts.

Glass

SilicatesCompounds closely related to silica and

found in most rocks, soils and clays are the silicates. Like silica, the silicates are based on interconnected SiO4 tetrahedra, but instead of a O/Si ratio of 2:1, the ratio is typically higher. This higher ratio tends to make silicon-oxygen anions.

Silicates

SilicatesCeramics are typically made from clays

(which contain silicates) and hardened by firing at high temperatures. They tend to be strong, brittle and heat and chemical resistant.

Ceramic is heterogeneous and contain two phases: minute crystals of silicates that are suspended in a glassy cement.

ClaysClay comes from the

weathering of feldspar, an Aluminosilicate (Na2O/K2OAl2O36SiO2). This weathering produces kaolinite, that consists of tiny thin platelets of Al2Si2O5(OH)4. When dry these platelets cling together and lock into place; when wet they can slide over one another. During firing, these platelets bind and form a glass.

CeramicsCeramics constitute one of the most

important classes of ‘high-tech” materials. Their stability at high temperatures and resistance to corrosion, make them an obvious choice for constructing jet and car engines.

Organoceramics are taking form by the addition of organic polymers to ceramics. This reduces some of the brittle nature of ceramics and allows them to be used for things such as flexible superconducting wire, microelectronic devices, prosthetic devices and artificial bones.

SemiconductorsElemental silicon has the same structure

as diamond. The structure is different in that the energy gap between filled and empty MO’s is not as large and electrons can delocalize and make silicon a semi-conductor. At higher temperatures, more electrons get excited in the conduction bands and the conductivity of silicon increases.

N-type SemiconductorWhen small fraction of silicon atoms

are replaced by arsenic atoms (one more valence electron), extra electrons become available for conduction and produce an n-type semi-conductor. These can conduct an electric current.

P-type SemiconductorWhen small fraction of silicon atoms

are replaced by boron atoms (one less valence electron), an electron ‘vacancy’ is made. As electrons move, the fill the ‘hole’ and make a new one. This movement of electrons can therefore carry a current. This type of conductor (less electrons) is called a p-type semiconductor.

Energy Level Diagrams for N-type and P-type Semiconductors.

P-N JunctionMost important

applications of semiconductors involve connection of a p-type and an n-type to form a p-n junction.

The red dots represent excess electrons in the n-type semiconductor and the white circles represent holes (electron vacancies.

P-N JunctionAt the junction a small

number of electrons migrate from the n-type region into the p-type region. The effect of these migrations is to place a negative charge on the p-type region and a positive charge on the n-type region.

P-N JunctionThis charge buildup,

called the contact potential or junction potential, prevents further migration of electrons. This transfer of electrons is therefore a ‘one-way’ transfer and under an external battery source will allow flow of electrons from the n to the p type regions.

P-N JunctionWhen current is

opposed it is said to be under reverse bias. When current flows easily, the junction is said to be under forward bias.

A p-n junction is a good rectifier, a device that produces a pulsating direct current from an alternating current.

P-N JunctionWhen placed in a circuit

where the current is constantly reversing, a p-n junction only transmits current under forward bias. Radios, computers and other electronic devices all use this rectifiers. This p-n junction revolutionized electronics.

Molecular Solids10.6

Molecular SolidsSometimes Molecular solids can have

large discrete molecular units in a lattice-type position. These molecules have strong bonds within the molecules but relatively weak between the molecules.

Common examples: Ice, dry ice (solid carbon dioxide), Sulfur (S8), Phosphorus (P4)

Molecular

Molecular SolidsWhen molecules do have dipole

moments, their intermolecular forces are significantly greater, especially when hydrogen bonding is possible.

Water not only has polar bonds, a dipole moment, has hydrogen bonds, but it also can have a total of four hydrogens associated with every oxygen atom.

Ionic Solids10.7

Ionic SolidsIonic solids are stable, high melting

substances held together by the strong electrostatic forces that exist between oppositely charged ions.

Ionic SolidsMost binary ionic solids can be

explained by the closest packing of spheres. Typically the larger ions, usually anions, are packed in one of the closest packed arrangements (hcp and ccp).

The smaller cations fit into the holes among the closest packed anions. This packing maximizes the electrostatic attractions among oppositely charged ions and minimizes the repulsion of like charges.

Ionic SolidsThere are three

types of holes in closest packed structures:

1.Trigonal holes are formed by three spheres in the same layer

Ionic SolidsThere are three

types of holes in closest packed structures:

2.Tetrahedral holes are formed when a sphere sits in the dimple of three spheres in an adjacent layer.

Ionic SolidsThere are three

types of holes in closest packed structures:

3.Octahedral holes are formed between two sets of three spheres in adjoining layers of the closest packed structures.

Ionic SolidsThe holes increase in size in the order:

trigonal < tetrahedral < octahedralThe trigonal holes are so small that

they are never occupied in binary ionic compounds. Tetrahedral and octahedral holes are occupied if the relative sizes of the ions allow.

Ionic SolidsExample: Zinc Sulfide (ZnS) creates a ccp

structure. The Zn2+ has a radius of 70pm and the S2- ion has an ionic radius of 180pm. There are 4 spheres (atoms/anions) in a face-centered cubic unit cell and 8 tetrahedral holes. So only half of the holes in the ccp unit are filled with cations.

Ionic SolidsExample: Sodium chloride can be described in

terms of a ccp structure. Na+ resides in octahedral holes. The locations of the octahedral holes in the face-centered cubic unit is marked by X. The number of spheres (anions) in the structure is the same number of octahedral holes. Since NaCl is a 1:1 binary compound. All octahedral holes are used.

Example ProblemDetermine the net number of Na+ and Cl-

ions in the sodium chloride unit cell.

4 Na+ and 4 Cl-

Example ProblemClassify each of the following substances

according to the type of solid it forms:goldcarbon dioxidelithium fluoridekrypton

metallicmolecularionic8a

Types and Properties of Solids

Vapor Pressure and Changes of

State10.8

VaporizationVaporization, or evaporation, is the

process of liquid molecules escaping the liquid’s surface and forming a gas.

Vaporization is endothermic because energy is required to overcome the relatively strong intermolecular forces in the liquid. Water has strong intermolecular forces

and this increases the energy required to vaporize. Also making it a great coolant.

VaporizationThe energy required to vaporize 1

mole of liquid at 1 atm is called the heat of vaporization or the enthalpy of vaporization.The symbol for this is ΔHvap.

Vapor PressureCondensation is the process by which

vapor molecules re-form a liquid. The evaporation process occurs at a

constant rate at a given temperature, and once an equilibrium has been reached, the rate of condensation will equal the rate of evaporation.

Vapor PressureMolecules in a

given system are constantly escaping from and entering the liquid at high rate. However, there is not net change because the two opposite processes just balance each other.

Rates of Condensation and Evaporation.

Vapor PressureThe pressure of the vapor present at

equilibrium is called the equilibrium vapor pressure, or more commonly, the vapor pressure of the liquid.

A simple barometer can measure the vapor pressure of a liquid.

Vapor PressureLiquid is injected at the bottom of the tube of

mercury and floats to the surface. A portion of the liquid evaporates at the top of the column, producing a vapor whose pressure pushes some mercury out of the tube.

Vapor PressureWhen the system reaches equilibrium,

the vapor pressure can be determined from the change in the height of the mercury column

Vapor PressurePatmosphere = Pvapor + PHg column

Pvapor = Patmosphere - Phg column

Vapor PressureThe vapor pressure of liquids vary

widely. Liquids with high vapor pressures are said to be volatile. They evaporate rapidly in an open dish.

Vapor PressureThe vapor pressure of a liquid is

principally determined by the size of the intermolecular forces in the liquid.Liquids with strong molecular forces

have relatively low vapor pressures because it takes so much energy for the molecules to escape.

In general, substances with large molar masses have relatively low vapor pressures due of large dispersion forces.

Vapor PressureVapor pressure increases significantly

with temperature. In order to break intermolecular forces,

a sufficient amount of kinetic energy is needed.

As temperature of the liquid increases, so does kinetic energy of the liquid.

Vapor Pressure of Water

Vapor PressureThe nature of the temperature, vapor

pressure relationship is quadratic. Pvap vs. 1/T (Kelvin) gives a direct relationship.

Vapor PressureVapor pressure equation:

R is the universal gas constant (8.3145 J/K). In means natural logarithm.

Example ProblemUsing the graph, determine whether water or

diethyl ether has the larger enthalpy of vaporization.

Ether has the smaller slope therefore smaller ΔHvap

Example ProblemThe vapor pressure of water at 25°C is

23.8 torr, and the heat of vaporization of water at 25°C is 43.9 kj/mol. Calculate the vapor pressure of water at 50°C.

93.7 torr

SublimationLike liquids, solids have vapor pressures.

When a solid sublimes, it goes directly from the solid to the gaseous state without passing through the liquid state.

Example: Dry Ice.

Changes of StateTypically when a solid is heated, it will

form a liquid and then boil to form a vapor. This process can be represented by a heating curve.

Temperature vs. time when energy is added a constant rate.

Heating Curve

Heating CurveThe plateaus in the heating curve

represent the positions of phase change.

At the melting point, the temperature remains constant until the solid has completely changed to liquid.

At the boiling point the temperature remains constant as the added energy is used to vaporize the liquid

Heating CurveThe energy change that occurs at the

melting point when a solid melts is called heat of fusion or enthalpy of fusion.Note that changes of state are physical

changes, although intermolecular forces have been broken, no chemical bonds have been broken.

Enthalpy of Fusions and Melting Points

Melting and BoilingThe melting and boiling points for a

substance are determined by the vapor pressure of the solid and liquid states.

Melting and BoilingAt 0°C the vapor

pressure of ice is less than that of liquid. Vapor pressure of ice increases more rapidly than water.

Melting and BoilingA point is

reached when the liquid and solids have identical vapor pressures. This is the melting point.

Freezing PointAt a temperature at

which the vapor pressure of the solid is greater than that of the liquid, the solid would sublime and the vapor would be added to the water.

Freezing PointAt a temperature at

which the vapor pressure of the solid is less than that of the liquid, the liquid would evaporate and the vapor would be added to the ice.

Freezing PointAt a temperature at

which the vapor pressures of the solid and liquid are identical, the vapor is in equilibrium. This is the freezing point of water.

Melting Point and Boiling PointThe normal melting point of a liquid is

the temperature at which the solid and liquid states have the same vapor pressure under conditions where the total pressure is 1 atmosphere.

The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid is exactly 1 atmosphere.

Supercooled and SuperheatedChanges of state do not always occur

exactly at the boiling point or melting point.

Water can be supercooled below 0°C at 1 atm and remain in the liquid state. At some point the correct ordering of molecules occurs and ice forms, releasing energy in the exothermic process and bringing the temperature back up to the melting point.

Supercooled and Superheated

Supercooled and SuperheatedChanges of state do not always occur

exactly at the boiling point or melting point.

A liquid can also be superheated, or raised to temperatures above its boiling point, especially if it is heated rapidly. Boiling requires high-energy molecules to gather in the same vicinity for bubble formation. This may not happen at the boiling point.

Supercooled and SuperheatedOnce a bubble does form, when a liquid

is superheated, its internal pressure is greater than the atmospheric pressure. This bubble can burst before rising to the surface, blowing the surrounding liquid out of the container. This is called bumping and is a common experimental problem.

Supercooled and SuperheatedBoiling chips are often added to

prevent bumping. These are bits of porous ceramic material containing trapped air that escapes on heating, forming tiny bubbles that act as ‘starters’ for the vapor bubble formation. This allows for smooth onset of boiling.

Phase Diagrams10.9

Phase DiagramA phase diagram is a convenient way of

representing the phases of a substance as a function of temperature and pressure. It shows which state exists at a given temperature and pressure.

Conditions for these phase diagrams are assumed to be a closed system and is plotted as temperature vs pressure.

Phase Diagram

Phase DiagramThe solid/liquid boundary has a

negative slope. Melting point of ice decreases as

external pressure increases. This is different for most substances

other than water because the density of ice is less than that of liquid water at the melting point.

Boiling Points of Water vs. Pressure

Example 1Pressure is 1 atm.

Water moves through the changes of state according to the vapor pressure at the corresponding temperatures.

Example 2Pressure is 2 torr.

Water will sublime at -10°C. This is when the vapor pressure of the ice is equal to the external pressure of 2 torr. Vapor pressure of liquid water is always greater than 2 torr and therefore will not form.

Example 3Pressure is 4.58 torr.

When temperature reaches .01°C (273.16K), water reaches the triple point. Solid and liquid water have identical vapor pressures and all three states of water exist. This is the only condition in a closed system that allows this.

Example 4Pressure is 225 atm.

Liquid water can be present at this temperature because of the high external pressure. As temperature increases, liquid gradually turns to vapor, but goes through a ‘fluid’ region.

Example 4The fluid region is

neither true liquid or vapor. This unusual behavior occurs because the conditions are beyond the critical point for water.

Critical PointThe critical temperature can be

defined as the temperature above which the vapor cannot be liquefied no matter what pressure is applied.

The critical pressure is the pressure required to produce liquefaction at the critical temperature

Together, the critical temperature and critical pressure define at the critical point.

Critical PointThe critical point for water is 374°C

and 218 atm. Anything beyond this point, involves the intermediate “fluid” region.

Phase DiagramsThe phase diagram

for CO2 shows the liquid state does not exist at a pressure of 1 atm. The solid/liquid line has a positive slope, since the density of solid CO2 is greater than that of liquid CO2

THE END!!