Water and the
Properties of Liquids
Chapter 13
Hein and Arena
Version 1.0
Eugene Passer
Chemistry Department
1 College
Bronx Community
© John Wiley and Sons, Inc.
Chapter Outline
13.1 What is a Liquid
13.8 Occurrence of Water
13.2 Evaporation
13.9 Physical Properties of Water
13.3 Vapor Pressure
13.10 Structure of the Water
Molecule
13.4 Surface Tension
13.5 Boiling Point
13.11 The Hydrogen Bond
13.6 Freezing Point or Melting
Point
13.12 Formation and Chemical
Properties of Water
13.7 Changes of State
13.13 Hydrates
2
What is a Liquid
The properties of liquids are intermediate
between the extremes of gases and liquids.
3
Properties of a Liquid
•
•
•
•
Particles are close together
Incompressible
Definite volume
Takes the shape of its container
4
Evaporation
Evaporation or vaporization is the escape
of molecules from the liquid state to the
gas or vapor state.
evaporation
liquid
vapor
When molecules of higher than average
kinetic energy escape from a liquid, the
remaining liquid is cooler than it was
before they escaped.
5
H2O(l) → H2O(g)
6
Sublimation occurs when a solid changes
directly to a gas bypassing the liquid state.
solid
sublimation
vapor
I2(s) → I2(g)
7
Vapor Pressure
Vapor pressure is the pressure exerted by a
vapor in equilibrium with its liquid.
liquid
evaporation
condensation
vapor
In a closed system after a liquid evaporates some
of the vapor molecules strike the surface of the
container and return to the liquid by condensation.
8
Molecules in an open beaker
evaporate from the liquid and
disperse into the atmosphere.
Evaporation will continue until
all of the liquid is gone.
9
13.1
Molecules leaving the liquid are
confined to a limited space. With
time, the concentration in the
vapor phase will increase until an
equilibrium between liquid and
vapor is established.
10
13.1
Measurement of Vapor Pressure
o
o
at 20 C and 30 C
• Vapor pressure is independent of the
amount of liquid and vapor present.
• It increases with temperature.
11
Vapor Pressure
Measurement
• The system is evacuated.
• The mercury manometer
attached to the flask shows
equal pressure in both legs.
T = 200C
13.2
12
• Water has been added to
the flask.
• The water begins to
evaporate.
• This water exerts a
vapor pressure which
is indicated by the
manometer.
T = 200C
13.2
13
• When equilibrium is
established, the vapor
pressure of the water
inside the flask remains
constant at 17.5 torr.
T = 200C
13.2
14
• The temperature is
changed to 30oC.
• Equilibrium is
reestablished.
• The vapor pressure of
the water has increased
to 31.8 torr.
T = 300C
13.2
15
Vapor pressure at any temperature: ethyl ether > ethyl alcohol > water
Rate of evaporation: ethyl ether > ethyl alcohol > water
16
The rate of evaporation of a liquid is proportional to its vapor pressure.
Volatility
Substances that evaporate readily are said to be volatile.
T = 20oC
vapor pressure ethyl ether = 442.2 torr
volatile
vapor pressure water = 17.5 torr
moderately
volatile
vapor pressure mercury = 0.0012 torr
nonvolatile
17
Surface Tension
Surface tension is the resistance of a liquid
to an increase in its surface area.
18
• Surface tension is the result of the
molecules at the surface of a liquid
being pulled inward by molecules that
lie below the surface layer.
• This pulls the surface of a liquid into a
sphere.
• Substances with large attractions between
molecules have high surface tensions.
19
Capillary Action
Capillary action is the spontaneous
rising of a liquid in a narrow tube.
20
• Capillary action results from the
cohesive forces within the liquid and
the adhesive forces between the liquid
and the walls.
• If the adhesive forces between the
liquid and the walls of its container
exceed the cohesive forces between the
molecules of the liquid, the liquid will
climb the walls of the container.
21
Meniscus
• When a liquid is placed in a glass
cylinder, the surface of the liquid
shows a curve called the meniscus.
22
meniscus
concave
shape
concave shape occurs because
adhesive forces between water
and glass > cohesive forces
between water molecules
23
Boiling Point
• Boiling point is the temperature at which
the vapor pressure of a liquid is equal to
the external pressure above the liquid.
• The normal boiling point is the
temperature at which a liquid boils at
one atmosphere pressure.
24
water boils when its
vapor pressure = 1 atm
or 760 torr
25
The normal boiling points for different substances can be plotted on
a vapor pressure versus temperature curve.
26
1 atmosphere pressure
ethyl ether boiling
ethyl
point
alcohol boiling
water point
boiling point
34.6oC
13.4
78.4oC
100.0oC
Vapor
Eachpressure-temperature
point on the curve represents
curves for
a vapor-liquid
ethyl-ether, ethyl
equilibrium
alcoholatand
a
27
water.
particular temperature and pressure.
The boiling point at any temperature can be found.
28
At 700
500 torr the boiling point of ethyl alcohol = 77
68oC
29
Freezing Point
or Melting Point
• A liquid changing into a solid is said to
be freezing or solidifying.
• A solid that is changing into a liquid is
said to be melting.
30
The temperature at which the solid phase
of a substance is in equilibrium with its
liquid phase is know as the freezing
point or melting point of the substance.
solid
melting
freezing
liquid
31
Pressure = 1 atmosphere
Temperature = 0oC
H2O(s)
melting
freezing
H2O(l)
32
Changes of State
13.5
Heating curve for a pure substance.
33
Heat of Fusion
The energy required to change one gram
of a solid at its melting point into a liquid
is called the heat of fusion.
34
How many joules of energy are needed to change 10.0
g of ice at 0.00oC to water 20.0oC?
The heat of fusion of ice at 0oC is 335 J/g.
Determine the joules necessary to melt 10.0 g of ice.
 335 J 
3
= 3.35 x 10 J
10.0 g  

 1g 
35
How many joules of energy are needed to change 10.0
g of ice at 0.00oC to water 20.0oC?
Determine the joules necessary to heat 10.0 g of
water from 0.00oC to 20.0oC.
The specific heat of water is 4.184 J/goC.
 4.184 J 
o
10.0 g   o   20.0 C  = 837 J
 1g C 
The total heat absorbed by the system is the heat
required to melt the ice plus the heat required to raise
the water temperature from 0.00oC to 20oC.
3350 J + 837 J = 4.19 x 103 J
36
The energy required to change one gram of a
liquid at its boiling point into a vapor is called
the heat of vaporization.
37
How many kilojoules of energy are needed to change
20.0 g of water at 20.0oC to steam at 100.0oC?
The specific heat of water is 4.184 J/goC.
(mass) (sp.ht) (Δt) = energy
Determine the kilojoules necessary to heat 20.0 g
from 20oC to 100oC.
 4.184 J   1 kJ 
o
o
 20.0 g   o  

100. C - 20.0 C   6.71 kJ

g
C
1000
J



38
How many kilojoules of energy are needed to change
20.0 g of water at 20.0oC to steam at 100.0oC?
The heat of vaporization of water at 100.oC is 2.26 kJ/g.
Determine the kilojoules necessary to change 20.0 g
of water at 100.oC to steam at 100.oC.
 2.26 kJ 
 45.2 kJ
 20.0 g  

 1g 
The total heat absorbed by the system is the heat
required to raise the water temperature from 20oC to
100oC plus the heat required to raise to change the
water to steam.
39
3350 J + 837 J = 4.19 x 103 J
Occurrence of Water
• Water covers about 75% of the earth’s
surface.
• About 97% of the earth’s water is in
the oceans.
• About 3% of the earth’s water is in the
fresh water and two-thirds of this is
locked up in polar ice caps and
glaciers.
40
• 70 elements have been detected in
seawater.
– Chlorine, sodium, magnesium and
bromine are commercially extracted from
sea water.
• Water constitutes about 70% of human
body mass.
– 92% of blood plasma is water.
– 80% of muscle tissue is water.
– 60% of a red blood cell is water.
41
Physical Properties of Water
Ice and water exist together
in equilibrium at 0oC.
42
Pressure = 1 atmosphere
Temperature = 0oC
H2O(s)
melting
freezing
H2O(l)
43
44
Density of Water
• The density of water reaches a maximum at
4oC.
• As it cools to 4oC it contracts in volume and
rises in density.
• When it is cooled from 4oC to 0oC it
expands in volume and decreases in density.
• When water changes to ice its volume
expands by 9%. The density of ice at 0oC is
less than the density of water at 0oC. This is
45
why ice floats in water.
Structure of the
Water Molecule
• O-H bond length = 0.096 nm
• H-O-H bond angle = 105o
• Shape: nonlinear, bent
46
• Each OH bond is polar.
– Electronegativity hydrogen = 2.1
– Electronegativity oxygen = 3.5
– Hydrogen has a partial positive charge
and oxygen has a partial negative charge.
• The nonlinear structure of water
accounts for the polarity of the
molecule.
47
unpaired
electrons
EN = 3.5
EN = 2.1
(a) Lewis structure of water showing electron distribution.
(b) Bond angle and bond length.
(c) Molecular orbital structure.
(c) Dipole representation.
48
13.7
The Hydrogen Bond
water
waterhas
has
water
water
thehashas
water
the
has
thehighest
lowest
theheat
highest
highest
the
heat
highest
of
molarofmass
fusion
melting
vaporization
point
boiling point
The melting point, boiling point, heat of fusion and heat of
vaporization of water are extremely high and do not fit the trend
49
of properties relative to molar mass within Group VIA.
Water exhibits these unusual properties
because of hydrogen bonding between
water molecules.
50
• A hydrogen bond is an intermolecular
bond.
• A hydrogen bond is formed between
polar molecules that contain hydrogen
covalently bonded to a small, highly
electronegative atom: F, O, N.
F—H
O—H
N—H
51
• When hydrogen is attached to one of
the small electronegative atoms: F, O,
or N, it will be attracted to another F,
O, or N, on another molecule.
• A dipole-dipole bond bond will be
formed between the two molecules
which is called a hydrogen bond.
hydrogen
bond
covalent
bond
covalent
bond
52
Water in the liquid and solid states exists as aggregates
in which the water molecules are linked together by
hydrogen bonds.
53
13.8
Formation and Chemical
Properties of Water
Decomposition
• Up to 2000oC water decomposes no
more than 1%.
• If small amounts of H2SO4 or NaOH
are added to water it will be
decomposed by an electric current.
2H2O(l)
electrical energy
H2SO4 or NaOH
2H2(g) + O2(g)
54
Formation
1. Combination
2H2 + O2 → 2H2O + 484 kJ
2. Neutralization
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
3. Combustion of hydrogen containing substances.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 803kJ
4. Metabolic oxidation in living cells.
enzymes
C6H12O6(aq) + 6O2(g)
6CO2(g) + 6H2O(l) + 251955 kJ
Reactions of Water with
Metals and Nonmetals
56
1. Metals that react with cold water to produce
hydrogen and a metal hydroxide
2K(s) + 2H2O(l) → H2(g) + 2KOH(aq)
2Na(s) + 2H2O(l) → H2(g) + 2NaOH(aq)
reactivity
Ca(s) + 2H2O(l) → H2(g) + Ca(OH)2(aq)
2. Metals that react with high temperature steam to
produce hydrogen and a metal oxide.
Zn(s) + H2O(g) → H2(g) + ZnO(aq)
2Al(s) + 3H2O(g) → 3H2(g) + Al2O3(aq)
3Fe(s) + 4H2O(g) → H2(g) + 3FeO(aq)
57
3. Nonmetals that react with water.
2F2(g) + 2H2O(l) → 4HF(aq) + O2(g)
violent
reaction
Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq)
Br2(l) + H2O(l) → HBr(aq) + HOBr(aq)
C(s) + H2O(g)
1000oC
CO(g) + H2(g)
mild
reaction
mild
reaction
water gas
forms
58
Reactions of Water with Metal
and Nonmetal Oxides
1. Basic anhydrides are metal oxides that react
with water to form hydroxides
CaO(s) + H2O(l) → Ca(OH)2(aq)
Na2O(s) + H2O(l) → 2NaOH(aq)
2. Insoluble metallic oxides do not react with water
to form hydroxides.
CuO(s) + H2O(l) → no reaction
Al2O3(s) + H2O(l) → no reaction
59
2. Acidic anhydrides are nonmetal oxides that react
with water to form hydroxides
CO2(g) + H2O(l) → H2CO3(aq)
SO2(g) + H2O(l) → H2SO3(aq)
N2O5(g) + H2O(l) → 2HNO3(aq)
61
Hydrates
Solids that contain water as part of
their crystalline structure are known as
hydrates.
62
Solids that contain water as part of
their crystalline structure are known as
hydrates.
Water in a hydrate is known as water of
hydration or water of crystallization.
63
Formulas of hydrates are written by first writing
the formula for the anhydrous compound and
then adding a dot followed by the number of
water molecules present.
CoCl2 6H2O
64
• Water molecules in hydrates are
bonded by electrostatic forces between
polar water molecules and the positive
or negative ions of the compound.
• These forces are weaker than covalent
or ionic bonds.
• As a result water of crystallization can
be removed by moderate heating of the
compound.
BaCl2
2H2O(s)
100oC
BaCl2(s) + 2H2O(g)
65
66
13.1 What is a Liquid –
Common properties
13.2 Evaporation – Kinetic
Energy
13.3 Vapor Pressure – relative
rates of evaporation and
boiling points
13.8 Occurrence of Water
13.9 Physical Properties of Water
13.4 Surface Tension – Forces
of; Meniscus
13.10 Structure of the Water
Molecule
13.5 Boiling Point – Heating
curves
13.11 The Hydrogen Bond
13.6 Freezing Point or Melting
Point
13.12 Formation and Chemical
Properties of Water
13.7 Changes of State
13.13 Hydrates
67