15
Water and Aqueous Systems
Planning G
Guide
Introducing the
BIGIDEA:
BONDING AND INTERACTIONS
Chemical and physical properties of materials can
ca be explained by the structure and arrangement
arrangemen of atoms, ions, and molecules and the
NSES
Lessons and Objectives
Print Resources
For the Student
For the Teacher
A-1, B-2
15.1
Water and Its Properties p 488–493
15.1.1 Identify the factor that causes the high
surface tension, low vapor pressure, and
high boiling point of water.
15.1.2 Describe the structure of ice.
Reading and Study
Workbook Lesson 15.1
Lesson Assessment 15.1
p 493
Quick Lab: Surface Tension,
p 491
Teaching Resources,
Lesson 15.1 Review
A-1, B-2, G-1
15.2
Homogeneous Aqueous Systems
p 494–501
15.2.1 Identify the types of substances that
dissolve most readily in water.
15.2.2 Explain why all ionic compounds are
electrolytes.
15.2.3 Explain why hydrates easily lose and
regain water.
Reading and Study
Workbook Lesson 15.2
Lesson Assessment 15.2
p 501
Teaching Resources,
Lesson 15.2 Review
Class Activity, p 497:
Electrolytes
Teacher Demo, p 498:
Magic Writing
E-2
15.3
Reading and Study
Workbook Lesson 15.3
Lesson Assessment 15.3
p 507
Small-Scale Lab: Electrolytes,
p 508
Teaching Resources, Lesson
15.3 Review
Teacher Demo, p 506:
Tyndall Effect
Heterogeneous Aqueous Systems
p 504–507
15.3.1 Distinguish between a suspension and
a solution.
15.3.2 Identify how to distinguish a colloid from
a suspension and a solution.
Assessing the
BIGIDEA:
BONDING AND INTERACTIONS
Essential Questions
1
1. How do the interactions between water
molecules account for the unique properties
of water?
2. How do aqueous solutions form?
486A Chapter 15
Study Guide p 509
STP p 515
Reading and Study
Workbook Self-Check
and Vocabulary Review
Chapter 15
Materials List
FFor the
h S
Student
d
forces between them.
Digital Resources
Editable Worksheets
PearsonChem.com
L
ESSON
W
OV
ERVIE
ET
KIN IC
ART
L
ET
KIN IC
ART
CHEM
TU
TOR
Hydrogen Bonding
15.2 Lesson Overview
Solvation of an Ionic
Solid
Finding the Percent by
Mass of Water in a
Hydrate
Small-Scale Lab, p 508
• paper
• pencil
• ruler
• reaction surface
• water
• micropipet or dropper
• NaCl(s); Na2CO3(s); NaHCO3(s); KCl(s); MgSO4(s)
• table sugar
• cornstarch
• KI(s)
• aqueous solutions of HCl, H2SO4, HNO3, CH3COOH,
NH3, and NaOH
• rubbing alcohol
• distilled water
• various liquid foods
For the Teacher
O
NLIN
E
O
ACTI
PR
S
IN
N
CO
NCEP
M
OBLE
15.3 Lesson Overview
Why Oil and Water
Don’t Mix
Class Activity, p 497
• light bulb in a porcelain
socket
• 9 V or lantern battery
• 2 copper metal strips
• lamp cord
• alligator clips
• 0.1M solutions of
glucose, alanine, glycine,
ascorbic acid, malonic
acid, citric acid, acetic
acid, hydrochloric acid
• 8 beakers
Chapter 15 Problem Set
Teacher Demo, p 498
• CoCl2 6H2O solution
• cotton swab
• white paper
• hot plate
• misting bottle
• water
Teacher Demo, p 506
• whole milk
• water
• beaker
• stirring rod
• projector or laser pointer
Additional Digital Resources
Online Student Edition
Online Teacher’s Edition
15.2 Virtual Chem Lab 21: Electrolytes
L
U
IRT A
V
W
L
ESSON
ERVIE
TS
Exam View Assessment Suite
Classroom Resources Disc
(includes editable worksheets)
• Lesson Reviews
• Practice Problems
• Interpret Graphs
• Vocabulary Review
• Chapter Quizzes and Tests
• Lab Record Sheets
W
ERVIE
OV
Small-Scale Lab Manual Lab 25:
Hard and Soft Water
ESSON
OV
Lab 26: Distillation
Lab 27: The Solvent Properties
of Water
Lab 28: Water of Hydration
Lab 29: Electrolytes and
Nonelectrolytes
Small-Scale Lab Manual Lab 22:
Reactions of Aqueous Ionic
Compounds
Small-Scale Lab Manual Lab 23:
Identification of Eight
Unknown Solutions
Small-Scale Lab Manual Lab 24:
Electrolytes
Lab Practical 15-1: Electrolytes
and Nonelectrolytes
Probeware Lab: Electrolytes
15.1 Lesson Overview
Quick Lab, p 491
• shallow dish or Petri dish
• water
• paper clip
• rubber band (approximately 5 cm in diameter)
• 2 micropipets or droppers
• vegetable oil
• liquid dish detergent
LAB
Conductivity of Electrolyte Solutions
Water and Aqueous Systems 486B
CHEM
TOR
NLIN
S
E
O
TU
PR
M
O BL E
MATH
T
UTOR
L
V
IRTUA
LAB
CHEM TUTOR Students access guided, stepby-step tutorials for calculating the percent
by mass of water in a hydrate.
PROBLEM SETS Students can practice key
problem-solving skills in an online
problem set.
MATH HELP Identify the students that
struggle with math by assigning an online
math skills diagnostic test. These students
can then improve and practice math skills
by using the MathXL tutorial system.
VIRTUAL LAB Students go on an animated
virtual lab tour in which various electrolytes
are studied in a simulated laboratory
environment.
15
Water has many unique
properties. In this chapter, you
will learn about the interactions
between water molecules.
Water and
Aqueous
Systems
INSIDE:
t15.1 8BUFSBOE*UT1SPQFSUJFT
t15.2 )
PNPHFOFPVT
"RVFPVT4ZTUFNT
N
O
N
V
LAB
MATH
NLIN
CHEM
O
ART
CONCEPTS IN ACTION Students watch an
S
TOR
O
O BL E
M
TU
E
IN
ACTI
ACTI
PR
TS
NCEP
IRTUA
KINETIC ART Students watch animations of
selected figures from the chapter followed
by questions to check for understanding.
AR T
ET
KIN IC
L
IN
NET
KI IC
NCEP
TS
CO
t15.3 )
FUFSPHFOFPVT
"RVFPVT4ZTUFNT
CO
CHAPTER 15
What’s Online
TU
TOR
overview of a key chapter concept using
real-world contexts and concrete examples
and analogies. Each activity includes an
interactive animation followed by analysis
questions.
National Science Education Standards
486
A-1, B-2, E-2, F-3, F-4, F-6
Focus on ELL
1 CONTENT AND LANGUAGE Write the words homogeneous, heterogeneous,
and aqueous on the board. Assign students to three groups. Assign each group of
students one of the words, and ask them to brainstorm the meaning of the prefix of
their word. Then have each group complete a Frayer model diagram for the word,
with a definition, characteristics, an example, and a nonexample. Have each group
teach the word to the class.
BEGINNING
LOW Draw a symbol to represent the meaning of the prefix.
HIGH Use a bilingual dictionary or glossary to find any cognates for the prefix.
INTERMEDIATE: LOW/HIGH List other words that use the same prefix.
ADVANCED: LOW/HIGH After the class presentations, have students predict the
486
Chapter 15
meaning of the lesson titles Homogeneous Aqueous Systems and Heterogeneous
Aqueous Systems.
BONDING
AND INTERACTIONS
Essential Questions:
Understanding by Design
1. How do the interactions between
water molecules account for the
unique properties of water?
Students are building toward the ability to predict
how particles bond and interact by understanding
how the properties of water and aqueous solutions
are affected by interactions among water molecules.
2. How do aqueous solutions form?
PERFORMANCE GOALS At the end of Chapter
CHEMYSTERY
Coming Clean
On a beautiful Saturday
afternoon, Wes decided
to take his bicycle out
for a ride. He set off on
a long ride through the
trails at a nearby park.
When Wes returned from
his bike ride, he found that his socks had quite
a few stains on them. There was dirt from the
trail as well as grease from the bicycle chain.
Wes figured he could just clean the socks in
the sink. He tried soaking the socks in water,
but neither the dirt, nor the grease would rinse
off. Isn’t water supposed to clean everything?
Apparently not. If Wes had known more about
the chemistry of water, he might have tried
something different to clean those dirty socks.
Connect to the BIGIDEA As you read
about water and aqueous systems, think about
how Wes could remove the dirt and grease
from his socks.
NATIONAL SCIENCE EDUCATION STANDARDS
A1, B-2, E-2, F-3, F-4, F-6
15, students will be able to answer the essential
questions by applying their knowledge of water
and aqueous solutions. Students will also be able to
calculate the percent by mass of water in a hydrate.
ESSENTIAL QUESTIONS Read the essential
questions aloud. Ask What is the structure of a
water molecule? (two hydrogen atoms covalently
bonded to an oxygen atom) Ask What are some
properties of water? (Sample answers: It forms
spherical drops. It can dissolve many substances. It
has three states of matter.) Ask How do you think a
water molecule’s structure relates to water’s ability
to form spherical drops? How do you think its
structure relates to water’s ability to dissolve many
substances? (Engage students in a discussion of
reasonable answers.)
Use the photo of water drops to
help students connect to the
concepts they will learn in this chapter. Activate prior
knowledge by reviewing the covalent bonding in a
water molecule. Ask Why do you think people use
water to wash hands, dishes, and laundry? (Sample
answers: Water is abundant on Earth. Water often
cleans things well.) Point out that the covalent
bonding in a water molecule affects the way it
interacts with other substances.
BIGIDEA
Have students read over the
CHEMystery. Connect the
CHEMystery to the Big Idea of bonding and
interactions by asking students to think about how
the bonding in a water molecule might affect the
ability of water to remove the dirt and grease from
Wes’s socks. Challenge students to predict what Wes
could use to clean his dirty socks. Have students
write their predictions on notecards. As a hint,
suggest that students think about what they use
at home when they wash their hands, dishes,
or laundry.
CHEMYSTERY
Introduce the Chapter
IDENTIFYING MISCONCEPTIONS Students may think a solution is always tiny bits
of a solute floating in a solvent that is continuous; that is, the solvent is not made
of individual particles. As a result, they may think any solute will dissolve in water.
Use the following activity to help them realize that water will not dissolve all
substances.
Activity You will need glass beakers, stirring rods, water, and various substances that
are or are not soluble in water. Ask Do you think all these substances will dissolve
in water? (Answers will vary.) Have students predict whether each substance will
dissolve in water at room temperature. Then have student volunteers try to dissolve
the substances in water. Conclude by stating that water’s properties cause it to
dissolve some substances but not others. Explain that students will learn the
difference as they study the chapter.
Water and Aqueous Systems
487
CHAPTER 15
BIGIDEA
LESSON 15.1
Key Objectives
15.1.1 IDENTIFY the factor that causes the
high surface tension, low vapor pressure, and high
boiling point of water.
15.1.2 DESCRIBE the structure of ice.
15.1 Water and Its Properties
CHEMISTRY
Additional Resources
Q: What properties of water make it essential to life on Earth? When the
Apollo 8 astronauts first saw their home planet from a distance of thousands
of kilometers, they called it the big blue marble. Water covers about three
quarters of Earth’s surface. In addition to making up Earth’s oceans, water
forms the polar ice caps and cycles through the atmosphere. All known life
forms, including the penguin in Figure 15.1, are made mostly of water.
• Reading and Study Workbook, Lesson 15.1
• Probeware Laboratory Manual, Core Teaching
Resources, Lesson 15.1
Key Questions
Engage
&
CHEMISTRY
Y
YO
YOU
U Have students study the
photograph and read the text. Ask What are the
white swirls in the photo? (clouds) Ask How are the
white swirls relevant to the subject matter of this
chapter? (Clouds form when water vapor in the
atmosphere condenses into tiny liquid droplets.)
Activate Prior Knowledge
Before students begin to read this chapter, review
the concepts of polarity and hydrogen bonding.
Remind students that water is a liquid rather than
a gas at room temperature because of the strong
hydrogen bonding between water molecules.
National Science Education Standards
Y U
YO
&YOU
What factor causes the high
surface tension, low vapor
pressure, and high boiling point
of water?
How can you describe the
structure of ice?
Vocabulary
tTVSGBDFUFOTJPO
tTVSGBDUBOU
Figure 15.1 Water Is Vital to Life
The oceans supply penguins with an
abundant supply of food.
Water in the Liquid State
What factor causes the high surface tension, low vapor
pressure, and high boiling point of water?
You couldn’t live without water, nor could all the plants and animals that
share space on the “big blue marble.” Besides the water visible on Earth’s surface, immense reserves of water exist deep underground. Water in the form
of ice and snow dominates the polar regions of Earth. Water vapor from the
evaporation of surface water and from steam spouted from geysers and volcanoes is always present in Earth’s atmosphere.
Recall that water, H2O, is a simple molecule consisting of three atoms.
The oxygen atom forms a covalent bond with each of the hydrogen atoms.
Oxygen has a greater electronegativity than hydrogen, so the oxygen atom
attracts the electron pair of the covalent O ĿH bond to a greater extent than
the hydrogen atom. Thus, the OĿ H bond is highly polar. As a result, the
oxygen atom acquires a partial negative charge (ĠŹ). The less electronegative
hydrogen atoms acquire partial positive charges (Ġà).
488 $IBQUFSt-FTTPO
A-1, B-2
Focus on ELL
1 CONTENT AND LANGUAGE Create a KWL chart with the students, beginning with
the things they know about water in all of its forms. Ask students to describe personal
experiences they have had with liquid water or ice such as swimming, ice-skating, and
seeing fog.
2 FRONTLOAD THE LESSON Display pictures of the planets of our solar system, and
encourage students to share what they know about each. Point out that only Earth
has the characteristic blue hue that indicates the presence of surface water. Explain
that the presence of water on Earth is one of the primary reasons that Earth can
support life. Tell students to make note of each of water’s life-sustaining properties
that they will read about in the lesson.
3 COMPREHENSIBLE INPUT Have students view the Kinetic Art animation on
488
Chapter 15 • Lesson 1
hydrogen bonding. Use a role-playing activity to model the effect of adding a
surfactant to water to demonstrate how surfactant molecules limit the ability of
water molecules to hydrogen bond.
ĠĽ
O
H
H
Ġà
Polar bonds
Ġà
Figure 15.2 Polarity of H2O
In a water molecule, the bond polarities
are equal, but the two poles do not
cancel each other because a water
molecule is bent. The molecule as a
whole is polar.
Apply Concepts Which element in
water has the higher electronegativity?
Molecule has net polarity
ĠĽ
ĠĽ
BUILD VOCABULARY The word tension comes
from the Latin tendere, which means “to stretch.”
Thus, surface tension causes the surface of a liquid
to appear stretched and taut. Have students use
the derivation of tension to explain the expression
nervous tension. (a condition of stress or tautness in
an individual, brought about by some external cause)
READING STRATEGY Have students preview the
O
H
Foundations for Reading
H
Ġà
Ġà
How do the polarities of the two OĿ H bonds affect the polarity of
the molecule? The shape of the molecule is the determining factor. The
bond angle of the water molecule is approximately 105°, which gives the
molecule a bent shape. The two OĿ H bond polarities do not cancel, so
the water molecule as a whole is polar. The net polarity of the water molecule is illustrated in Figure 15.2.
In general, polar molecules are attracted to one another by dipole
interactions. The negative end of one molecule attracts the positive end
of another molecule. However, in water, this attraction results in hydrogen bonding, as illustrated in Figure 15.3. Recall that hydrogen bonds are
attractive forces that arise when a hydrogen atom is covalently bonded
to a very electronegative atom and also weakly bonded to an unshared
electron pair of another electronegative atom. Hydrogen bonds are not
as strong as covalent bonds, but they are stronger than other intermoMany unique and important properties of water—
lecular forces.
including its high surface tension, low vapor pressure, and high
boiling point—result from hydrogen bonding.
b
a
figures and read the related captions. Encourage
students to use the caption information to predict the
answers to any questions associated with the figures.
Hydrogen
bond
Explain
Water in the Liquid State
Figure 15.3 Hydrogen Bonding
in Water
The polarity of the water molecule
results in hydrogen bonding. a. The
oxygen atom has a partial negative
charge. Each hydrogen atom has a
partial positive charge. b. Hydrogen
bonds form between the hydrogen atom
of one water molecule and the oxygen
atom of an adjacent water molecule.
Infer To form a hydrogen bond, what
must be true about hydrogen and
the element to which it is hydrogen
bonded?
See hydrogen bonds
animated online.
Ġà
ET
KIN IC
ART
2ĠĽ
Side view
Ġà
Liquid water
End view
Water and Aqueous Systems 489
Water Worth Drinking
START A CONVERSATION Tell students that,
although three-quarters of Earth’s surface is water,
only a small fraction of it can be used for drinking.
The majority of Earth’s surface water is in the
oceans and contains too high a concentration of
salts for the human body to tolerate.
USE VISUALS Have students study Figures 15.2
and 15.3. Ask What is meant by the term polarity?
(Polarity refers to the net molecular dipole resulting
from electronegativity differences between
covalently bonded atoms.) Ask What element in
a water molecule has a partial negative charge?
(the oxygen atom) Ask What element has a partial
positive charge? (the hydrogen atoms) Ask Predict
the type of substances you would expect to dissolve
in water. (polar substances, ionic compounds)
APPLY CONCEPTS Write the structural formula of
water on the board, or hold up a model of a water
molecule. Have students use Table 6.2 on page 181
to find and compare the electronegativity of oxygen
and hydrogen. Use VSEPR theory and the model
to show students how the oxygen’s non-bonding
electron pairs reduce the bond angle in a water
molecule to 105º. Point out how the net molecular
dipole and hydrogen bonding properties of water
are due to the large electronegativity difference
between hydrogen and oxygen. Note that hydrogen
bonding contributes to water’s unusually low vapor
pressure and high surface tension.
The year 2009 marked the 35th year of public health protection under the Safe
Drinking Water Act. The Act, passed in 1974 and amended in 1986 and 1996,
gives the Environmental Protection Agency (EPA) the authority to set drinking water
standards. Drinking water standards apply to public water systems.
Standards for safety are constantly being examined by the EPA and change as new
data become available. These standards set upper limits on potentially harmful
substances that could be present in water. These substances include pesticides,
household chemicals, and various microorganisms. Some microorganisms occur
naturally in drinking water sources, but others indicate contamination by sewage or
animal wastes. Two microorganisms are routinely monitored: coliform bacteria and
the Cryptosporidium parasite. Both organisms can cause gastrointestinal disorders
ranging from mild to severe. Cryptosporidiosis is potentially fatal for patients with
compromised immune systems.
Answers
FIGURE 15.2 oxygen
FIGURE 15.3 The hydrogen atom must be covalently
bonded to a very electronegative atom with
one or more lone pairs of electrons.
Water and Aqueous Systems
489
LESSON 15.1
ĠĽ
LESSON 15.1
Explain
USE VISUALS Direct students’ attention to Figure
15.4. To help explain why liquid water assumes
a spherical shape on many surfaces, explain that
nature tends to find the path of least resistance.
Guide students to understand that a spherical
shape provides the minimum surface area for a
given volume; since moving molecules takes work,
molecules expend the least energy possible to move
into a spherical arrangement while maximizing their
interactions with one another in the bulk liquid.
This energy efficiency creates the surface tension.
Explain that in order to produce any other shape,
more work would have to be done. So, the work
“saved” is the surface tension.
USE AN ANALOGY Convey that the skin-like
qualities of water are due to an exceptionally high
surface tension that is created by an extensive
network of hydrogen bonds. As an analogy,
describe the following scene. A crowd of autograph
seekers surrounds a celebrity. As each person
approaches as closely as possible, an impenetrable
circular barrier forms. This barrier will remain
intact as long as there is a net attraction toward
the circle’s center. The process is dynamic. As one
person wiggles closer to the celebrity, another is
forced to retreat, but the overall shape does not
change. Similarly, within a drop of water, attractions
between individual molecules may shift, but the
overall shape remains constant as molecules
continue to be drawn toward a central focal point.
Drop of water
Air
Figure 15.4
Surface Tension of Water
Water forms nearly spherical
drops on a leaf. Water molecules
at the surface of the water drop
cannot form hydrogen bonds with
molecules in the air, so they are
drawn into the body of the liquid.
Surface Tension Have you ever seen a glass so filled with water that the
water surface is not flat but bulges above the rim? Have you noticed that water
forms nearly spherical droplets at the end of a medicine dropper or on a leaf,
as shown in Figure 15.4? The surface of water acts like a skin. This property
of water’s surface is explained by the ability of water molecules to form hydrogen bonds. The water molecules within the body of the liquid form hydrogen
bonds with other molecules that surround them on all sides. The attractive
forces on each of these molecules are balanced. However, water molecules at
the surface of the liquid experience an unbalanced attraction. You can see
in Figure 15.4 that the water molecules are hydrogen bonded on only the
inside of the drop. As a result, water molecules at the surface tend to be drawn
inward. The inward force, or pull, that tends to minimize the surface area of a
liquid is called surface tension.
All liquids have a surface tension, but water’s surface tension is higher
than most. This is why, on some surfaces, water tends to bead up rather than
spread out. The surface tension of water tends to hold a drop of liquid in a
spherical shape. For example, you may notice that water tends to form beads
on the surface of a newly waxed car. The wax molecules are nonpolar, so there
is little or no attraction between the wax molecules and the polar water molecules. The drops are not perfect spheres because the force of gravity tends to
pull them down, causing them to flatten.
It is possible to decrease the surface tension of water by adding a surfactant. A surfactant is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension. Soaps and
detergents are surfactants. Adding a detergent to beads of water on a greasy
surface reduces surface tension, causing the beads of water to collapse and
spread out, as shown in Figure 15.5.
Figure 15.5 Effect of a Surfactant
Water drops bead up on some
surfaces. When detergent, a
surfactant, is added to water, the
drop spreads out.
Compare Which liquid drop
has a higher surface tension?
490 $IBQUFSt-FTTPO
Differentiated Instruction
L3 ADVANCED STUDENTS Explain that the structure of ice was determined using
X-ray diffraction, a method that was also used by Rosalind Franklin in her work with
DNA. This work contributed to the discovery of the helical structure of DNA by
James Watson and Francis Crick. Have students research how X-ray diffraction is
used to determine the structure of a substance.
L1 STRUGGLING STUDENTS Have students use prior knowledge and create a visual
of the water cycle. Ask them to explain changes of state. Have students refer to
their diagram as you teach the more unfamiliar properties of water.
490
Chapter 15 • Lesson 1
LPR LESS PROFICIENT READERS Allow students to use food coloring, cotton swabs,
and a protractor to visually grasp the concept of polarity and bond angle in water
molecules. Students can use the food coloring to stain one end of the cotton swab
green and the other end red to represent negative and positive charge and then
correctly orient their cotton swabs to form a 105º angle.
READING SUPPORT
Build Reading Skills:
Inference If water molecules
did not form hydrogen bonds
with each other, water would
have a much lower boiling
point. What do you think this
would mean for life as we
know it?
Boiling Point Molecular compounds of low molar mass are usually gases or
liquids with low boiling points at normal atmospheric pressure. Ammonia
(NH3), a molecular compound, has a molar mass of 17.0 g/mol and boils at
about Ź33°C. Water has a molar mass of 18.0 g/mol, but it has a boiling point
of 100°C. The difference between the boiling points of these two compounds
is due to hydrogen bonding, which is more extensive in water than in ammonia. It takes much more heat to disrupt the attractions between water molecules than those between ammonia molecules. If the hydrogen bonding in
water were as weak as it is in ammonia, water would be a gas at the usual temperatures found on Earth.
unusual surface property of
water that results from hydrogen bonding
Materials
rshallow dish or petri dish
rwater
rpaper clip
rrubber band, approximately
5 cm in diameter
rmicropipets or droppers (2)
rvegetable oil
rliquid dish detergent
Students may believe that the surface of a volume
of water contains a different kind of water molecule
than its interior. Remind students that surface
tension still exists even after you mix the water
sample to redistribute the molecules.
Explore
Quick Lab
OBJECTIVE Af
After completing this activity, students
will be able to identify a property of water that
results from hydrogen bonding and will be able to
describe the effect of a surfactant on surface tension.
Quick Lab
Purpose To observe an
Misconception Alert
SKILLS FOCUS Observing, inferring, drawing
conclusions
Surface Tension
PREP TIME 10 minutes
Procedure
CLASS TIME 20 minutes
1. Thoroughly clean and dry the dish.
2. Fill the dish almost full with water. Dry
your hands.
3. Being careful not to break the surface,
gently place the paper clip on the water.
Observe what happens.
4. Repeat Steps 1 and 2.
5. Gently place the open rubber band on
the water.
6. Slowly add the vegetable oil drop by drop
onto the water encircled by the rubber band
until that water is covered with a layer of oil.
Observe for 15 seconds.
ADVANCE PREPARATION Gather paper clips, rubber
bands, detergent, and vegetable oil.
TEACHING TIPS Have students dry their hands
before touching the paper clips.
EXPECTED OUTCOME The paper clip floats. The
7. Allow one drop of dish detergent to fall onto the center of the
oil layer. Observe the system for
15 seconds.
rubber band takes on a circular shape when filled
with oil. Detergent destroys the surface tension and
causes the rubber band to return to its original,
noncircular shape.
ANALYZE AND CONCLUDE
1.
Analyze and Conclude
1. Observe What happened to the paper clip in Step 3? Why?
2. Predict If a paper clip becomes wet, does it float? Explain your answer.
3. Observe What shape did the rubber band take when the water inside it was
covered with oil? Why did it take the observed shape?
4. Describe What happened when a drop of dish detergent was placed onto the
layer of oil?
Water and Aqueous Systems 491
2.
3.
4.
The paper clip floated. The surface tension of
the water supported the paper clip.
No. The surface tension is broken, and the
paper clip sinks because it is made of a metal
that has a greater density than water.
The rubber band took a circular shape. The
surface tension of the surrounding water
pulling the rubber band radially outward was
not balanced by the surface tension of the oil.
The oil layer was broken, and the rubber band
returned to its original shape.
FOR ENRICHMENT Explain that some insects, such
Focus on ELL
4 LANGUAGE PRODUCTION Have students work in groups of four to complete the
lab. Make sure each group has ELLs of varied language proficiencies so that more
proficient students can help less proficient ones. Have students work according to
their proficiency level.
as a water strider, depend on the surface tension
of water for moving around and capturing food.
Tell students that detergents are increasingly found
as pollutants in bodies of water in nature. Have
students design an experiment to determine the
concentration of detergent at which water can no
longer support the weight of a water strider.
BEGINNING: LOW/HIGH Allow students to draw a picture of their observations.
INTERMEDIATE: LOW/HIGH Paraphrase the procedure and the Analyze and Conclude
Answers
questions for students.
FIGURE 15.5 The water droplet on the left has a
ADVANCED: LOW/HIGH Have these students assist those with lower English
proficiencies to complete the procedure and answer the questions.
higher surface tension.
READING SUPPORT Sample Answer: There would be
less water in the liquid state and more water in
the gaseous state.
Water and Aqueous Systems
491
LESSON 15.1
Vapor Pressure Hydrogen bonding between water molecules also explains
water’s unusually low vapor pressure. Remember that the vapor pressure of
a liquid is the result of molecules escaping from the surface of the liquid and
entering the vapor phase. An extensive network of hydrogen bonds holds the
molecules in liquid water to one another. These hydrogen bonds must be broken before water changes from the liquid to the vapor state, so the tendency of
these molecules to escape is low and evaporation is slow. Imagine what would
happen if it were not. All the lakes and oceans, with their large surface areas,
would rapidly evaporate!
LESSON 15.1
Water in the Solid State
How can you describe the structure of ice?
Explain
Water in the Solid State
USE VISUALS Direct students’ attention to
Table 15.1. Ask What relation do you notice
between temperature and density? (The density
of liquid water increases over most of the range
of decreasing temperature.) Ask Why does this
trend not continue below 4ºC? (Below 4ºC, the
kinetic energy of the molecules is insufficient to
overcome hydrogen bonding, which holds the
water molecules in fixed positions.) If students have
difficulty discerning the pattern, use the overhead
and plot the data in the table on a coordinate grid;
place temperature on the x-axis and density on the
y-axis.
Table 15.1
Density of Liquid Water and Ice
Temperature (°C)
Density (g/cm3)
100 (liquid water)
0.9584
50
0.9881
25
0.9971
10
0.9997
4
1.0000
0 (liquid water)
0.9998
0 (ice)
0.9168
CRITICAL THINKING Have students explain why
ice, which has a smaller mass per volume ratio
than liquid water, results in a lower relative
density. (Density is based on mass per volume; the
substance with the smaller mass per volume ratio
has the lower density.)
Misconception Alert
Make sure students understand that hydrogen
bonding always involves hydrogen, but it is not a
type of chemical bond. Rather, it explains a type of
intermolecular attractive force. Explain that polarity
is a type of intermolecular force—a force that
occurs between molecules. Covalent bonding—the
bond between hydrogen and oxygen in a water
molecule—is a type of intramolecular force.
You have seen that water in the liquid state exhibits some unique properties. The same is true for water in the solid state. For example, ice
cubes float in your glass of iced tea because solid water has a lower
density than liquid water. This situation is not usual for liquids. As a
typical liquid cools, it begins to contract and its density increases gradually. The density increases because the molecules of the liquid move
closer together so that a given volume of the liquid contains more molecules and thus more mass. If the cooling continues, the liquid eventually solidifies with a density greater than the density of the liquid. A
typical solid sinks in its own liquid because the density of the solid is
greater than that of the corresponding liquid.
As water begins to cool, it behaves initially like a typical liquid.
It contracts slightly and its density gradually increases, as shown in
Table 15.1. Notice that at 4°C, the density of water is at its maximum
of 1.0000 g/cm3. When the temperature of the water falls below 4°C,
the density of water actually starts to decrease. Below 4°C, water no
longer behaves like a typical liquid. Ice, which forms at 0°C, has about
a 10 percent lower density than liquid water at 0°C. You may have
noticed that ice begins to form at the surface of a pond when the temperature reaches 0°C, but the ice does not sink. It floats at the surface,
making ice skating and ice fishing possible. Ice is one of only a few solids that floats in its own liquid.
Why is ice less dense than liquid water? As you can see in
Figure 15.6, hydrogen bonds hold the water molecules in place in the
The structure of ice is a regular open framework of
solid phase.
water molecules in a hexagonal arrangement. When ice melts, the
framework collapses. Looking back at Figure 15.3, you can see that the
water molecules pack closer together in liquid water, making it more
dense than ice.
Figure 15.6 Structure of Ice
The unique properties of ice are a result of hydrogen bonding. a. Extensive
hydrogen bonding in ice holds the water molecules farther apart in a more
ordered arrangement than in liquid water. b. The hexagonal symmetry of a
snowflake reflects the structure of the ice crystal.
Compare and Contrast How are the structures of liquid water (shown in
Figure 15.3) and ice similar? How are they different?
a
b
Hydrogen
bond
Ice
492 $IBQUFSt-FTTPO
Check for Understanding
The Essential Question How do the interactions between water molecules
account for the unique properties of water?
Assess students’ knowledge about the importance of hydrogen bonding in water by
asking them to write a paragraph that explains how hydrogen bonding is responsible
for one of the following properties of water: high surface tension, high boiling point,
low vapor pressure, or high melting point.
ADJUST INSTRUCTION If students are having difficulty with this exercise, have them
reread the portion of this lesson associated with their chosen property. Then provide
students with the opportunity to revise their paragraphs.
492
Chapter 15 • Lesson 1
CHEMISTRY
Y
&YOU
Q: What properties
operties of
water that result from
hydrogen bonding make it
essential to life on Earth?
Figure 15.7 Ice Floats in Liquid Water
Many organisms that live in water
would not survive if ice were more
dense than liquid water.
&
CHEMISTRY
Y
YO
YOU
U Ice is less dense than
liquid water because of hydrogen bonding. As a
result, ice floats on the surface of water providing a
layer of insulation for the water beneath. Many
organisms are able to survive underwater in the
winter because the water is not frozen.
Extend
Connect to
SOCIAL STUDIES
NLIN
PR
S
E
O
Have students compare areas of population density
and the distribution of water. Provide students with
two different maps. One map should show the
locations of cities and towns. The other should be a
regional contour map showing water courses, rivers,
lakes, and reservoirs. Have students determine why
most large cities or historical sites are located near
water sources, and have them explain the effects
when such places are not near water. Ask students
to examine the vast irrigation networks and water
channels that crisscross the nation’s agricultural
areas. Have students determine the ways in which
water is transported to arid regions and to population
centers with insufficient fresh water.
M
OBLE
15.1 Lesso
LessonCheck
1.
Review What causes the high surface tension, low vapor pressure, and high boiling point
of water?
2.
Describe How are water molecules
arranged in ice?
3. Explain Why does water form spherical drops
on some surfaces?
4. Relate Cause and Effect What effect does a
surfactant have on the surface tension of water?
Evaluate
5. Infer Water (H2O) and methane (CH4) have
similar molar masses. Methane changes from a
liquid to a gas at Ź161°C. Water becomes a gas at
100°C. What could account for the difference?
6. Apply Concepts What causes water pipes to
break in freezing weather?
BIGIDEA BONDING AND INTERACTIONS
7. Describe how hydrogen bonding accounts for the
properties of water.
Water and Aqueous Systems 493
Lesson Check Answers
1.
2.
3.
Water molecules are hydrogen
bonded to each other but not to air
molecules. Net attraction is inward,
minimizing the water surface area.
Hydrogen bonding makes it more
difficult for water molecules to
escape from the liquid phase to the
vapor phase.
Ice has a honeycomb-like structure
of water molecules.
The surface tension of a liquid
tends to hold a drop of liquid in a
spherical shape.
4.
5.
6.
7.
Surfactants lower the surface
tension of water by interfering with
hydrogen bonding.
Water has intermolecular hydrogen
bonding between its molecules;
methane does not.
Water expands as it freezes.
Water molecules at the surface of
a water drop are drawn into the
body of the liquid producing water’s
unusually high surface tension.
Hydrogen bonds also hold water
molecules to one another resulting
in water’s unusually low vapor
pressure.
Informal Assessment
Ask Why is the surface tension of water so high
compared to that of other liquids? (Water molecules
form a large number of hydrogen bonds, in
addition to dipole-dipole forces between molecules.)
Ask Why does water form a meniscus in a narrow
tube? (Water molecules have a greater attraction to
the molecules on the surface of the glass than they
do to each other.) Then have students complete the
15.1 Lesson Check.
Reteach
Have students explain the property of water that
allows ice to float in its liquid phase. (Ice has a lower
density than liquid water. Because there is more
space between water molecules in the solid state
than the liquid state, there are fewer molecules in
any given volume. Less mass per volume results in a
lower relative density of ice as compared to water.)
Answers
FIGURE 15.6 There are two hydrogen atoms
attached to one oxygen atom in both
structures. The molecules in liquid water
are further apart and have a less ordered
arrangement than ice.
Water and Aqueous Systems
493
LESSON 15.1
The fact that ice floats has important consequences for organisms.
A layer of ice on the top of a body of water, such as the one shown in
Figure 15.7, acts as an insulator for the water beneath, preventing the
water from freezing solid except under extreme conditions. The liquid
water at the bottom of an otherwise frozen body of water is warmer than
0°C, so fish and other aquatic life are better able to survive. If ice were
denser than liquid water, bodies of water would tend to freeze solid during the winter months, destroying many types of organisms.
Ice melts at 0°C, which is a high melting temperature for a molecule
with such a low molar mass. A considerable amount of energy is required
to return water molecules in the solid state to the liquid state. The heat
absorbed when 1 g of water at 0°C changes from a solid to a liquid is
334 J. This heat is the same amount of energy that is needed to raise the
temperature of 1 g of liquid water from 0°C to 80°C.