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SECTION 10.5
Water
Teacher Notes and Answers
SECTION 5 Water
1.water vapor, ice, liquid water
2.Because liquid water molecules are polar and
form hydrogen bonds, a relatively large amount
of energy is necessary to break the bonds and
vaporize the water.
Practice
A.2.19 × 1​05​ ​g
Review
1.Because of the bent (angular) shape of the
5.The regular hydrogen bonding pattern in ice
leads to an open structure with significant empty
space between molecules. The hydrogen bonds
in liquid water are fewer and more disordered,
allowing the molecules in the liquid to crowd
closer together.
6.Boiling requires more energy. We know this
because the molar enthalpy of vaporization
of water is greater than its molar enthalpy of
fusion.
7.The high enthalpy of vaporization of water
means that a great quantity of energy is released
as heat when the steam condenses on contact
with the skin. Steam burns can be severe.
molecule, the large electronegativity difference
between hydrogen and oxygen atoms results in
the overall polarity of the molecule.
2.The structure allows for hydrogen bonding,
resulting in the formation of molecular groups
that produce a high enthalpy of fusion, boiling
point, and enthalpy of vaporization.
3.There are intermolecular hydrogen bonds in
both. The liquid usually has 4 to 8 molecules
per group. In the solid, all the molecules are
arranged in a hexagonal structure.
4.Ice floats because it is less dense than liquid
water. The low density of ice allows it to form
only on the surface of most lakes. The solid
top layer insulates the liquid water beneath
it, preventing a total freeze. This enables lake
organisms to survive the winter.
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SECTION 10.5
Water
Water commonly exists in all three physical states on Earth,
where it is by far the most abundant liquid. It covers nearly
three-quarters of Earth’s surface. Water is an essential
component of life; from 70% to 90% of the mass of living
things is water. Chemical reactions that are necessary for
life take place in water, and often involve water as a
reactant or a product. The structure of a water molecule
gives water the unique properties that make it such an
important molecule.
Hydrogen
bond
The properties of water in all phases are determined
by its structure.
Water molecules consist of two atoms of hydrogen linked
to one atom of oxygen by polar-covalent bonds. A molecule
of water is bent, with its two bonds forming an angle of
about 105°.
O
H
H
105˚
Different molecules of water are linked by hydrogen
bonding. The number of linked molecules decreases with
increasing temperature because hydrogen bonds have
difficulty forming between molecules with greater kinetic
energies. Usually, four to eight molecules of water are
linked in a group, as shown at the right.
This ability of water molecules to form groups prevents
molecules from escaping to become gas particles. Water
would be a gas at room temperature without this ability.
The diagram on the next page shows water molecules in
the solid state. These molecules form an orderly, hexagonal
arrangement in ice crystals. The large empty spaces between
molecules in this pattern explain why solid water has the
unusual property of being less dense than its liquid form.
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Liquid water
In a group of liquid water molecules,
hydrogen and oxygen are bonded
within each molecule, and different
molecules are held together by
hydrogen bonds.
READING CHECK
1. Order the three forms of water
from least dense to densest.
Density of Water The diagrams of water molecules to the
right and on the preceding page represent water in the liquid
state and the solid state at 0°C. Liquid water has fewer and
more disorderly hydrogen bonds than ice. When energy is
added to ice, and the crystal structure breaks down, the water
molecules can actually crowd closer together in the liquid
state. This is why water is more dense than ice.
As liquid water is warmed from 0°C, its particles pack
closer together until the temperature of 3.98°C is reached.
At temperatures above 3.98°C, the kinetic energy of the water
molecules moving around in the liquid keeps the molecules
from being packed so closely close together.
This unusual property of water helps protect organisms
that live in water. Most liquids freeze from the bottom up.
Water freezes from the top down, because the surface water is
cooler than the deeper water. In addition, the ice stays on the
surface of a lake or pond because it is less dense than water
and acts as an insulator. This effect makes it difficult for a
large body of water to freeze solid.
Hydrogen
bond
Ice
Ice contains the same types of bonding
as liquid water. However, the structure
of the hydrogen bonding is more rigid
than in liquid water.
The molar enthalpy of water determines many of its
physical characteristics.
At room temperature, liquid water is transparent, odorless,
tasteless, and nearly colorless. Any observed odor or taste is a
result of dissolved substances in the water. The density of
water is 0.999 84 g/​cm​3​, while the density of ice is 0.917 g/​cm​3​.
Water has a relatively high boiling point. A large amount of
kinetic energy is necessary for the water molecules to completely overcome the hydrogen bonding.
At atmospheric pressure, ice’s molar enthalpy of fusion is
6.009 kJ/mol. That value is relatively large compared to other
solids. Water also has a relatively high molar enthalpy of
vaporization, 40.79 kJ/mol. These high values both result from
the strong attractive forces in hydrogen bonds. The high molar
enthalpy of vaporization makes steam (vaporized water) ideal
for household heating systems. Steam can store a great deal of
energy as heat. When the steam condenses in a radiator, it
releases this energy.
READING CHECK
2. Why is a relatively large amount
of energy required to turn liquid
water into water vapor?
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SAMPLE PROBLEM
How much energy is absorbed when 47.0 g of ice melt at a
temperature of 0°C and a pressure of 1 atm?
SOLUTION
1 ANALYZE
Determine what information is given and unknown.
Given: mass of H
​ 2​ ​O(s) = 47.0 g
molar enthalpy of fusion = 6.009 kJ/mol
Unknown: energy absorbed when ice melts
2 PLAN
Determine how to find the value of the unknown.
First, convert the mass of water to moles. Then use the molar
enthalpy of fusion as a conversion factor.
3 SOLVE
Find the value of the unknown using the given information.
1 mol ​H​2​O
47.0 g H
​ 2​ ​O = 47.0 g ​H2​ ​O × ___________
 ​
​ 
   
18.02 g ​H2​ ​O
= 2.61 mol ​H​2​O
6.009 kJ H
​ 2​ ​O
Energy absorbed =
2.61 mol ​H​2​O × ____________
​ 
  
  
 ​
1 mol ​H2​ ​O
= 15.7 kJ
4 CHECK
YOUR
WORK
Check to see if the answers make sense.
A mass of 47 g of water is about 3 mol and 3 × 6 = 18. The
answer has the right units and is close to the estimate.
PRACTICE
mass of steam is required to release 4.97 × 1​ 0​5​kJ
of energy on condensation?
A.What
1 mol ​H​2​O
​ 
  
  
 ​
Moles of steam required =
4.97 × 1​ 0​5​kJ × _______________
40.79 kJ ​H2​ ​O
=
Mass of steam required =
=
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18.02 g ​H2​ ​O
mol ​H2​ ​O × ___________
​ 
   ​ 
1 mol ​H2​ ​O
SECTION 10.5 REVIEW
REVIEW
1. Why is the water molecule polar?
2. How is the structure of water responsible for its unique characteristics?
3. Describe the arrangement of molecules in liquid water and ice.
4. Why does ice float? Why is this phenomenon important?
5. Why is ice less dense than liquid water?
6. Is more energy required to melt one gram of ice at 0°C or to boil one gram
of water at 100°C? How do you know?
Critical Thinking
7. RELATING IDEAS Why is exposure to steam dangerous?
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Math Tutor Calculating Using Enthalpies
of Fusion
When one mole of a liquid freezes to a solid, a certain amount of energy is released. The
attractive forces between particles pull the disorderly particles of the liquid into a more
orderly crystalline solid. When the solid melts to a liquid, the solid must absorb the same
amount of energy in order to separate the particles of the crystal and overcome the
attractive forces opposing separation. The energy used to melt or freeze one mole of a
substance at its melting point is called its molar enthalpy of fusion, ∆​Hf​ ​.
Problem-Solving TIPS
• The enthalpy of fusion of a substance can be given as either joules per gram or
kilojoules per mole.
• Molar enthalpy of fusion (kilojoules per mole) is the value that is most commonly
used in calculations.
• The enthalpy of fusion is the energy absorbed or given off as heat when a substance
melts or freezes at its melting point.
• No net change in temperature occurs as the change in state occurs.
SAMPLE
Determine the quantity of energy that will be needed to melt
2.50 × 1​05​ ​kg of iron at its melting point, 1536°C. The Δ​H​f​
of iron is 13.807 kJ/mol.
The number of moles of a substance that is equal to a given mass of a
substance can be determined from the following equation.
moles of a substance = mass of substance/molar mass of substance
The energy as heat absorbed by a substance as it is going through a
phase change from a solid to a liquid is
energy absorbed = ∆​H​f​× moles of a substance
The first equation can be substituted into the second equation to give
the energy absorbed in terms of the given information. The given
information can then be used to solve the problem.
substance
​  mass of    
    ​
energy absorbed = ∆​H​f​× ______________________
molar mass of substance
2.50 × 1​08​ ​g Fe
energy absorbed =_________
​ 13.807 kJ
 ​ 
× ______________
​ 
 
  
   ​
1 mol
55.847 g Fe/mol
= 6.18 × 1​0​7​kJ
Practice Problems: Chapter Review practice problems 16 –18
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