LESSON 15.2
Key Objectives
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.
15.2
Homo
Homogeneous Aqueous Systems
CHEMISTRY
C
CH
Q: How can you make a pickle glow? Is it possible to read by the
light of a glowing pickle? Although it sounds absurd, an ordinary dill pickle from the deli can be a source of light! Metal
forks are inserted into the ends of the pickle and connected to a
source of alternating electric current. After a time, during which
the pickle becomes hot and produces steam, the pickle begins to
glow. The mechanism by which the light is generated is not fully
understood, but it is clear that conduction of electric current by
the pickle is an important factor.
Additional Resources
• Reading and Study Workbook, 15.2
• Probeware Laboratory Manual, Lesson 15.2
• Core Teaching Resources, Lesson 15.2
• Laboratory Manual, Labs 26, 27, 28, 29
• Small-Scale Chemistry Laboratory Manual,
Labs 22, 23, 24
• Laboratory Practicals, 15.1
Solutions
Key Questions
What types of substances
dissolve most readily in water?
Why are all ionic
compounds electrolytes?
Engage
&
CHEMISTRY
Y
YO
YOU
U Have students study the
photograph and read the text. Ask What is it about
the pickle that allows it to produce light? (It conducts
electricity.) Ask What are some other materials that
conduct electricity? (metals and ionic solutions)
Access Prior Knowledge
Explain that in this lesson students will rely on their
understanding of polarity, ionic compounds, covalent
compounds, and molar mass to understand the
concepts of aqueous systems.
National Science Education Standards
Y U
YO
&YOU
Why do hydrates easily lose
and regain water?
Vocabulary
tBRVFPVTTPMVUJPO t TPMWFOU
tTPMVUF t TPMWBUJPO
tFMFDUSPMZUF t OPOFMFDUSPMZUF
tTUSPOHFMFDUSPMZUF
tXFBLFMFDUSPMZUF
tXBUFSPGIZESBUJPO t IZESBUF
tBOIZESPVT t FGGMPSFTDF
tIZHSPTDPQJD t EFTJDDBOU
tEFMJRVFTDFOU
What types of substances dissolve most readily
in water?
Water dissolves so many of the substances that it comes in contact with that you won’t find chemically pure water in nature.
Even the tap water you drink is a solution that contains varying
amounts of dissolved minerals and gases. An aqueous solution
is water that contains dissolved substances.
Solvents and Solutes In a solution, the dissolving medium is
the solvent. The dissolved particles in a solution are the solute. A
solvent dissolves the solute, and the solute becomes dispersed in
the solvent. Solvents and solutes may be gases, liquids, or solids.
Recall that solutions are homogeneous mixtures. They are
also stable mixtures. For example, sodium chloride (NaCl) does
not settle out when its solutions are allowed to stand, provided
other conditions, such as temperature, remain constant. Solute
particles can be atoms, ions, or molecules, and their average
diameters are usually less than 1 nm (10Ź9 m). Therefore, if you
filter a solution through filter paper, both the solute and the solvent pass through the filter.
Substances that dissolve most readily in water
include ionic compounds and polar covalent compounds.
Nonpolar covalent compounds, such as methane, and compounds found in oil, grease, and gasoline, do not dissolve in
water. However, oil and grease will dissolve in gasoline. To
understand this difference, you must know more about the
structures of the solvent and the solute and what attractions
exist between them.
494 $IBQUFSt-FTTPO
A-1, B-2, E-2, F-3, F-4, F-6
Focus on ELL
1 CONTENT AND LANGUAGE Have small groups of students create flashcards with
the vocabulary terms by writing each term on one side of a card and a definition
and visual on the other. Allow students with limited English to use their native
languages for support.
2 FRONTLOAD THE LESSON Have students discuss aqueous systems they have made
or observed at home. Ask students to state whether the items in these aqueous
systems are solids, liquids, and/or gases.
3 COMPREHENSIBLE INPUT Have students bring in a grocery item that they believe
to be an aqueous system. Write the definition for homogeneous aqueous system on
the board, and have students place their items on a specified desk if they can defend
that their items satisfy the definition.
494
Chapter 15 • Lesson 2
Ľ
à
See solvation
animated online.
ET
KIN IC
ART
à
Foundations for Reading
BUILD VOCABULARY Have students make a concept
map using the following words: solvation, solute,
aqueous solution, solvent.
READING STRATEGY For each section in this lesson,
have students write a short summary identifying
the main idea of the passage. Students can refer to
these summaries when preparing for an assessment.
Ľ
à
Ľ
à
Ľ
Ľ
à
Ľ
à
Ľ
à
Ľ
à
à
Ľ
à
Ľ
à
Ľ
à
Ľ
Ľ
à
Ľ
à
Ľ
à
Ľ
à
Explain
Solutions
Surface of ionic solid
The Solution Process Water molecules are in continuous
motion because they have kinetic energy. When a crystal of
sodium chloride is placed in water, the water molecules collide
with the crystal. Remember that a water molecule is polar, with
a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. The polar solvent molecules
(H2O) attract the solute ions (Naà, ClŹ). As individual solute
ions break away from the crystal, the negatively and positively
charged ions become surrounded by solvent molecules and the
ionic crystal dissolves. The process by which the positive and
negative ions of an ionic solid become surrounded by solvent
molecules is called solvation. Figure 15.8 shows a model of the
solvation of an ionic solid such as sodium chloride.
In some ionic compounds, the attractions among the ions
in the crystals are stronger than the attractions exerted by water.
These compounds cannot be solvated to any significant extent
and are therefore nearly insoluble. Barium sulfate (BaSO4) and
calcium carbonate (CaCO3) are examples of nearly insoluble
ionic compounds.
Figure 15.9 shows that oil and water do not mix. What about
oil in gasoline? Both oil and gasoline are composed of nonpolar molecules. The attractive forces that hold two molecules in
oil together are similar in magnitude to the forces that hold two
molecules in gasoline together. Molecules in oil can easily separate and replace molecules in gasoline to form a solution. As a
rule, polar solvents such as water dissolve ionic compounds and
polar compounds; nonpolar solvents such as gasoline dissolve
nonpolar compounds. This relationship can be summed up in
the expression “like dissolves like.”
START A CONVERSATION Remind students that an
anion is any negatively charged atom or group of
atoms. A cation is any positively charged atom or
group of atoms. Have students suggest a memory
aid or a symbol to help them remember these
terms. (Sample answer: The first two letters of
anion, a and n, could represent “a negative”,
since anions are negative ions.)
USE VISUALS Have students study Figure 15.8.
Stress the idea that when a solid dissolves in water,
it breaks into small pieces. Ask What must happen
for an ionic solid to dissolve? (The molecules of the
solvent must be able to overcome the attractive
forces acting between ions and holding the solid
together.) Ask What part of a water molecule is
attracted to a negatively charged solute ion? (the
hydrogen atoms)
Extend
Connect to
Figure 15.9 Oil and Water
Oil and water do not mix. Oil is less dense than
water, so it floats on top. The colors result from
the bending of light rays by the thin film of oil.
Water and Aqueous Systems 495
Differentiated Instruction
LPR LESS PROFICIENT READERS Have students preview the section by looking for
vocabulary and other unfamiliar terms. Encourage students to write the terms, with
phonetic respellings and definitions, in their notebooks.
L1 SPECIAL NEEDS STUDENTS Have students explore the phrase “like dissolves like.”
Provide student pairs with pairs of liquids that are polar, pairs that are nonpolar, and
pairs that contain one polar and one nonpolar liquid. Have students mix the two
liquids in each pair and decide whether the liquids are like or unlike.
ADVANCED STUDENTS Ask students to determine a method for proving if a
bottled water product is pure water or if it is an aqueous solution. Have students
present bottled water products with misleading labels.
L3
PHYSIOLOGY
Remind students that solutions are homogeneous
mixtures containing a solvent and one or more
solutes. Explain that the solvent is typically defined
as the component in the system that is present in
the greatest amount. Point out that a water-soluble
solute can be a solid, liquid, or gas. Have students
identify some of the solutes found in blood. (Solutes
in blood are typically ions such as sodium, potassium,
calcium, chloride, hydrogen carbonate, and
phosphate. Glucose, a covalent compound, also is
dissolved in blood. Dissolved gases such as oxygen
and carbon dioxide are also present.)
Answers
FIGURE 15.8 The positive hydrogen end of the water
molecule orients itself toward the anion. The
negative oxygen end of the water molecule
orients itself toward the cation.
Water and Aqueous Systems
495
LESSON 15.2
Figure 15.8 Solvation of an Ionic Solid
When an ionic solid dissolves, the ions become
solvated, or surrounded by solvent molecules.
Infer Why do the water molecules orient
themselves differently around the anions and
the cations?
Solvated ions
LESSON 15.2
a
c
b
Extend
Electrolytes and Nonelectrolytes
START A CONVERSATION Display to the class the
warning labels on some small electrical appliances,
such as a hair dryer or an electric razor. Ask Why
do these appliances warn you not to use them near
water? (because there are electrolytes in tap water)
Ask Why does tap water conduct electricity? ( Tap
water comes from rivers, streams, and wells and
contains dissolved ionic particles from rocks and
river banks.)
SUMMARIZE Have students summarize in a chart
the types of compounds that strongly conduct
an electrical current, weakly conduct an electrical
current, and do not conduct an electrical current.
Ľ
à
à
2à
Ľ
Ľ
Ľ
à
Ľ
To (à)
electrode
à
To (Ľ)
electrode
Ľ
To (à)
electrode
Figure 15.10 Conductivity of Solutions
A solution conducts an electric current if it contains
ions. a. Sodium chloride, a strong electrolyte, is
nearly 100 percent dissociated into ions in water.
b. Mercury(II) chloride, a weak electrolyte, is
only partially dissociated in water. c. Glucose, a
nonelectrolyte, does not dissociate in water.
Predict Silver chloride dissolves only slightly
in water. If the electrodes were immersed in a
solution of silver chloride, would the bulb glow
brightly, dimly, or not at all?
To (Ľ)
electrode
To (à)
electrode
To (Ľ)
electrode
Electrolytes and
Nonelectrolytes
Why are all ionic compounds electrolytes?
Remember the glowing pickle? The pickle contained an electrolyte. An electrolyte is a compound that conducts an electric current when it is in an aqueous solution or in the molten
state. Conduction of an electric current requires ions that
are mobile and, thus, able to carry charges through a liquid.
All ionic compounds are electrolytes because they dissociate into ions. Sodium chloride, copper(II) sulfate, and
sodium hydroxide are typical water-soluble electrolytes.
Barium sulfate is an ionic compound that cannot conduct an
electric current in aqueous solution because it is insoluble, but
it can conduct in the molten state.
The experimental setup in Figure 15.10 can be used to
determine whether a solution contains an electrolyte. In order
for the bulb to light, an electric current must flow between the
two electrodes that are immersed in the solution. If ions are
present in the solution, they carry electrical charge from one
electrode to the other, completing the electrical circuit.
A nonelectrolyte is a compound that does not conduct
an electric current in either an aqueous solution or the molten state. Many molecular compounds are nonelectrolytes
because they are not composed of ions. Most compounds of
carbon, such as table sugar (sucrose) and the alcohol in rubbing alcohol (2-propanol), are nonelectrolytes.
496 $IBQUFSt-FTTPO
Check for Understanding
The Essential Question How do aqueous solutions form?
Assess students’ understanding of the difference between how electrolyte and
nonelectrolyte solutions form by asking students to give a thumbs up if they agree
with the following statement: Electrolyte solutions are characterized by charged
ions separated by molecules of water. Nonelectrolyte solutions are characterized by
uncharged molecules separated by water molecules.
ADJUST INSTRUCTION If students have difficulty with this concept, review Figure
15.10 with students, and then ask them to evaluate the statement again.
496
Chapter 15 • Lesson 2
NH3(g) à H2O(l)
CHEMISTRY
&YYOU
Q: Pickles contain table salt.
Why can electric current flow
through a pickle, causing it to
glow?
Explore
NH4à(aq) à OHŹ(aq)
Similarly, in an aqueous solution, hydrogen chloride produces hydronium
ions (H3Oà) and chloride ions (ClŹ). An aqueous solution of hydrogen chloride conducts an electric current and is therefore an electrolyte.
HCl(g) à H2O(l)
à
Class Activity
Ź
H3O (aq) à Cl (aq)
Not all electrolytes conduct an electric current to the same degree. In
Figure 15.10, the bulb glows brightly when the electrodes are immersed in a
sodium chloride solution. The bright glow shows that sodium chloride is a
strong electrolyte because nearly all the dissolved sodium chloride exists as
separate Naà and ClŹ ions. In a solution that contains a strong electrolyte, all
or nearly all of the solute exists as ions. The ions move in solution and conduct an electric current. Most soluble salts, inorganic acids, and inorganic
bases are strong electrolytes.
The bulb glows dimly when the electrodes are immersed in a mercury(II)
chloride solution because mercury(II) chloride is a weak electrolyte. A
weak electrolyte conducts an electric current poorly because only a fraction
of the solute in the solution exists as ions. Organic acids and bases are also
examples of weak electrolytes. In a solution of glucose, the bulb does not glow.
Glucose (C6H12O6) is a molecular compound. It does not form ions, so it is a
nonelectrolyte.
Electrolytes are essential to all metabolic processes. Your cells use electrolytes, such as sodium and potassium ions, to carry electrical impulses
internally and to other cells. These impulses are crucial to nerve and
muscle function. The kidneys help to maintain balanced electrolyte
concentrations in the blood. However, an electrolyte imbalance can
occur if you become dehydrated. For example, when you exercise,
you can lose water and electrolytes from your body through perspiration. The athlete in Figure 15.11 understands that it is important
to replenish these electrolytes by eating salty foods or by
drinking sports drinks.
&
CHEMISTRY
Y
YO
YOU
U Table salt, or sodium
chloride, is a strong electrolyte.
PURPOSE Students will compare electrolytes by
using a conductivity tester.
MATERIALS light bulb in a porcelain socket; 9V or
lantern battery; 2 copper metal strips; lamp cord;
alligator clips; 0.1M solutions of glucose (C6H12O6),
alanine (HC3H6O2N), glycine (HC2H4O2N), ascorbic
acid (HC6H7O6), malonic acid (H2C3H2O4), citric acid
(H3C6H5O7), acetic acid (HC2H3O2), and hydrochloric
acid (HCl); 8 beakers labeled 1–8; marking pen
Figure 15.11 Sports Drinks
It is important to replenish
electrolytes when exercising or
sweating. Sports drinks often
contain sodium and potassium.
The complete circuit consists of a light bulb (in a
porcelain socket), a 9V or lantern battery, and two
copper metal strips to be immersed in the test
solution. Make connections to the battery and light
bulb socket by using the lamp cord and alligator
clips. If possible, prepare eight such testers so that
students can directly compare the brightnesses of
the light bulbs.
PROCEDURE Write the names and numbers of the
beakers on the board. Pour an equal volume of a
different solution into each labeled beaker. Remind
students that all the solutions have the same
concentration (0.1M). Immerse a pair of electrodes
in each solution and have students compare the
brightnesses of the light bulb. Direct students
to list the substances in order from strongest to
weakest electrolytes. Remind students that a
strong electrolyte is a substance that exists in solution
almost entirely as ions. A weak electrolyte is a
substance that only partly dissociates into ions
in solution.
Water and Aqueous Systems 497
EXPECTED OUTCOME Students infer that the
different substances are dissociated, or ionized,
to different extents.
Metabolic Process
Electrolytes are essential to all metabolic processes. Sodium and potassium ions
control nerve impulse transmission and muscle contraction. If renal function is
impaired or malabsorption from the gut disturbs optimum sodium and potassium
levels, then serious nervous-system problems arise. Loss of consciousness or
difficulty in maintaining muscle coordination could result. Any condition that causes
prolonged bouts of diarrhea can be life threatening because of the dramatic loss of
electrolytes. Electrolytes are excreted through the skin via sweat, and they must be
replenished or cramps and heat stroke may occur. Sports drinks are a good source of
electrolytes; they contain Na+, K+, and Ca2+.
Answers
FIGURE 15.10 The bulb will glow dimly.
Water and Aqueous Systems
497
LESSON 15.2
Some polar molecular compounds are nonelectrolytes in the pure state
but become electrolytes when they dissolve in water. This change occurs
because such compounds ionize in solution. For example, neither ammonia
(NH3(g)) nor hydrogen chloride (HCl(g)) is an electrolyte in the pure state.
Yet an aqueous solution of ammonia conducts an electric current because
ammonium ions (NH4à) and hydroxide ions (OHŹ) form when ammonia dissolves in water.
LESSON 15.2
Explain
Hydrates
Figure 15.12 Heating a Hydrate
Water can be driven from a
hydrate by heating it. a. Heating
of a sample of blue CuSO4 Ƃ5H2O
begins. b. After a time, much
of the blue hydrate has been
converted to white anhydrous
CuSO4.
USE VISUALS Have students examine Figure 15.12.
Ask What is a hydrate? (a compound that contains
weakly bound water molecules as part of its crystal
structure) Ask What does the solid dot between
the CuSO4 and the 5H2O in the compound formula
mean? (It indicates that water is an integral part
of the representative unit of the compound.) Ask
How can the presence of water in the structure of
the crystal lattice be inferred? (by the change in the
appearance of the hydrated substance when the
water is driven off)
Why do hydrates easily lose and regain water?
Teacher Demo
PURPOSE Students will observe the color change
MATERIALS CoCl2 • 6H2O solution, cotton swab,
white paper, hot plate, misting bottle, water.
PROCEDURE Use a cotton swab to draw a picture
or write a message on a piece of white paper by
using a solution of hydrated cobalt(II) chloride.
Place the paper on a warm (not hot) hot plate until
the writing dries. Use a misting bottle to mist water
onto the paper. The paper can also be used to
indicate the presence of humidity in the air.
b
Hydrates
Explore
when a hydrate gains and loses water.
a
Figure 15.13 Exposing a Hydrate
to Moist Air
Paper dipped in an aqueous
cobalt(II) chloride solution and
then dried is blue. In the presence
of moisture, the paper turns pink.
Infer How could you change the
pink paper back to blue?
When an aqueous solution of copper(II) sulfate is allowed to evaporate,
deep-blue crystals of copper(II) sulfate pentahydrate are deposited. The
chemical formula for this compound is CuSO4 Ƃ5H2O. Water molecules
are an integral part of the crystal structure of copper(II) sulfate pentahydrate and many other substances. The water contained in a crystal is called
the water of hydration or water of crystallization. A compound that contains
water of hydration is called a hydrate. In writing the formula of a hydrate, use
a dot to connect the formula of the compound and the number of water molecules per formula unit. Crystals of copper(II) sulfate pentahydrate always
contain five molecules of water for each copper and sulfate ion pair. The
deep-blue crystals are dry to the touch. They are unchanged in composition
or appearance in normally moist air. However, when the crystals are heated
The forces holding the
above 100°C, they lose their water of hydration.
water molecules in hydrates are not very strong, so the water is easily lost
and regained. Figure 15.12 shows how the blue crystals of CuSO4 Ƃ5H2O
crumble to a white anhydrous powder that has the formula CuSO4. A substance that is anhydrous does not contain water. If anhydrous copper(II) sulfate is treated with water, the blue pentahydrate is regenerated.
CuSO4 Ƃ5H2O(s)
àheat
Źheat
CuSO4(s)à5H2O( g)
Another compound that changes color in the presence of moisture is
cobalt(II) chloride. A piece of filter paper that has been dipped in an aqueous solution of cobalt(II) chloride and then dried is blue in color (anhydrous
CoCl2). As you can see in Figure 15.13, when the paper is exposed to moist air,
it turns pink because of the formation of the hydrate cobalt(II) chloride hexahydrate (CoCl2 Ƃ6H2O). The blue paper could be used to test for the presence
of water.
Some familiar hydrates are listed in Table 15.2. Each one contains a fixed
quantity of water and has a definite composition. To determine what percent
by mass of a hydrate is water, first determine the mass of water in one mole of
hydrate. Then determine the molar mass of the hydrate. The percent by mass
of water can be calculated using the following equation:
SAFETY AND DISPOSAL Wear gloves. Cobalt
chloride is toxic if ingested. Collect the cobalt chloride
solution, evaporate it to dryness, place the cooled
residue in a bag, and label the bag for disposal.
EXPECTED OUTCOMES The picture or message will
appear blue when the paper dries and pink when
the paper is misted with water.
mass of water
Percent by mass H2Oä
ò100%
mass off h
hydrate
d t
498 $IBQUFSt-FTTPO
Check for Understanding
Why are all ionic compounds electrolytes?
Assess students’ knowledge about electrolytes by asking them to signal true or false
to the following statements. Have students signal true with a thumbs up and false
with a thumbs down.
• All solutions are conductors of electricity. (false)
• Degree of electrical conductivity is directly related to kinetic energy. (false)
• Ionic compounds conduct electricity because they contain ions. (true)
• Molecular compounds conduct electricity because they do not contain ions. (false)
ADJUST INSTRUCTION If students are having difficulty answering, direct them to
reexamine Figure 15.10 and reread the text that follows. Then ask them to respond
to the statements again.
498
Chapter 15 • Lesson 2
Some Common Hydrates
Formula
Chemical name
Common name
MgSO4 Ƃ7H2O
Magnesium sulfate heptahydrate
Epsom salt
Ba(OH)2 Ƃ8H2O
Barium hydroxide octahydrate
CaCl2 Ƃ2H2O
Calcium chloride dihydrate
CuSO4 Ƃ5H2O
Copper(II) sulfate pentahydrate
Blue vitriol
Na2SO4 Ƃ10H2O
Sodium sulfate decahydrate
Glauber’s salt
KAl(SO4)2 Ƃ12H2O
Potassium aluminum sulfate dodecahydrate
Alum
Na2B4O7 Ƃ10H2O
Sodium tetraborate decahydrate
Borax
FeSO4 Ƃ7H2O
Iron(II) sulfate heptahydrate
Green vitriol
H2SO4 ƂH2O
Sulfuric acid hydrate (mp 8.6°C)
Efflorescent Hydrates The water molecules in hydrates are held by
weak forces, so hydrates often have an appreciable vapor pressure. If a
hydrate has a vapor pressure higher than the pressure of water vapor in
the air, the hydrate will lose its water of hydration or effloresce. For example, copper(II) sulfate pentahydrate has a vapor pressure of about 1.0 kPa
at room temperature. The average pressure of water vapor at room temperature is about 1.3 kPa. Copper(II) sulfate pentahydrate is stable until
the humidity decreases. When the vapor pressure drops below 1.0 kPa,
the hydrate effloresces. Washing soda, or sodium carbonate decahydrate
(Na2CO3 Ƃ10H2O), is efflorescent. As the crystals lose water of hydration, they effloresce and become coated with a white powder of anhydrous
sodium carbonate (Na2CO3).
Figure 15.14 Desiccator
A desiccator may contain calcium
chloride. Substances that must be
kept dry are stored inside.
moist air
Demonstrate some of these properties of hydrates
by setting out watch glasses containing fresh sodium
hydroxide pellets, anhydrous calcium chloride, and
anhydrous copper sulfate. Advise students not to
touch the compounds. Point out the compounds that
are hygroscopic and those that are deliquescent. Many
students may have seen rice in salt shakers at
restaurants or at home. Ask What purpose does the
rice serve? (Rice acts as a desiccant.) Explain why
hygroscopic agents are preferred over deliquescent
agents for use as desiccants, or drying agents.
Emphasize the reversible properties of hydration.
Point out that desiccants can be recycled by
heating to drive off absorbed water. If possible,
show students a desiccator, and explain why
desiccators are used in the laboratory to store
hygroscopic chemicals.
USE VISUALS Display Table 15.2 on an overhead
projector. Review the nomenclature used to name
salts and the use of prefixes to indicate the number
of water molecules in the crystalline hydrate.
Ask Do you know some of the familiar uses for
the compounds in the table? (Epsom salt is used
as a laxative; Glauber’s salt is used as a laxative
or diuretic; alum is used as an astringent, as an
emetic, and in the manufacture of baking powder,
dyes, and paper; borax is used in the manufacture
of glass, enamel, artificial gems, soaps, and
antiseptics.)
Hygroscopic Hydrates Hydrated ionic compounds that have a low
vapor pressure remove water from moist air to form higher
hydrates. These hydrates and other compounds that remove
moisture from air are called hygroscopic. For example, calcium chloride monohydrate spontaneously absorbs a second
molecule of water when exposed to moist air.
CaCl2 ƂH2O(s)
START A CONVERSATION Many of the terms used
to describe the properties of hydrates may be
unfamiliar to students. Review the definitions and
pronunciations of the terms effloresce, hygroscopic,
desiccant, and deliquescent. Have students write
the definition for each term in their vocabulary
notebooks.
CaCl2 Ƃ2H2O(s)
Calcium chloride is used as a desiccant in the laboratory.
A desiccant is a substance used to absorb moisture from the
air and create a dry atmosphere. Anhydrous calcium chloride
can be placed in the bottom of a tightly sealed container called
a desiccator, which is shown in Figure 15.14. Substances that
must be kept dry are stored inside. A solid desiccant such as
calcium sulfate (CaSO4) can also be added to a liquid solvent,
such as ethanol, to keep it dry. The calcium sulfate does not dissolve appreciably in the solvent but absorbs water from the ethanol.
When a desiccant has absorbed all the water it can hold, the compound can be returned to its anhydrous state by heating.
Water and Aqueous Systems 499
Desiccants
Cameras and electronic equipment made in Japan and other Asian countries are
usually shipped to the United States in the holds of ships. Because ocean air contains
so much moisture, the packaging for this equipment usually contains small packets
of a desiccant. The desiccant absorbs the water vapor from the air, preventing it
from condensing on the delicate wiring and circuit boards of the equipment, possibly
causing corrosion and short circuits.
Answers
FIGURE 15.13 Heat it gently to drive off the water.
Water and Aqueous Systems
499
LESSON 15.2
Table 15.2
LESSON 15.2
CHEM
TU
Explain
TOR
Sample Problem 15.1
Finding the Percent by Mass of Water in a Hydrate
Sample Practice Problem
Determine the percent by mass of water in calcium
chloride dihydrate to the nearest tenth:
CaCl2 2H2O. (24.5%)
·
Extend
Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate
(Na2CO3 Ƃ10H2O).
—Analyze
List the known and the unknown. To
determine the percent by mass, divide the mass of
water in the one mole of the hydrate by the molar
mass of the hydrate and multiply by 100%.
KNOWN
formula of hydrate äNa2CO3 Ƃ 10H2O
˜Calculate
percent H2Oä%
Solve for the unknown.
After students have studied Sample Problem 15.1,
have them work in groups to find the percent by
mass of water in the compounds listed in Table 15.2.
·
·
·
·
·
·
For every 1 mol of Na2CO3 Ƃ 10H2O,
there are 10 mol of H2O.
·
(MgSO4 7H2O, 51.1%; Ba(OH)2 8H2O, 45.7%;
CaCl2 2H2O, 24.5%; CuSO4 5H2O, 36.1%;
Na2SO4 10H2O, 55.9%; KAl(SO4 )2 12H2O, 45.5%;
Na2B4O7 10H2O, 47.2%; FeSO4 7H2O, 45.3%;
H2SO4 H2O, 15.5%)
·
·
UNKNOWN
Determine the mass of
10 mol of water.
Determine the mass of
1 mol of the hydrated
compound.
Calculate the percent
by mass of water in the
hydrate.
mass of 10 mol H2Oä10 [(2 ò1.0 g) á16.0 g] ä180.0 g
molar mass of Na2CO3 Ƃ 10H2Oä(2 ò23.0 g) á12.0 g á(3 ò16.0 g) á180.0 g
ä286.0 g
percent by mass H2Oä
ä
mass of water
ò100%
mass of hydrate
180.0 g
ò100%ä62.94%
286.0 g
™Evaluate
Does the result make sense? The mass of the water accounts for more than
half the molar mass of the compound, so a percentage greater than 50 percent is expected.
8. What is the percent by mass
of water in CuSO4 Ƃ5H2O?
In Problem 8, start by
determining the mass of
5 mol of water and 1 mol
of the hydrate.
9. If you need 5.00 g of anhydrous Na2CO3 for
your reaction, how many grams of
Na2CO3 Ƃ10H2O could you use instead?
You know from the Sample
Problem above that 62.94%
of the hydrate is water, so
37.06 g out of every 100 g
of the hydrate is Na2CO3.
500 $IBQUFSt-FTTPO
Foundations for Math
ORDER OF OPERATIONS AND THE CALCULATOR Point out that finding the percent by
mass of water in hydrates involves several uses of the order of operations. Students
must use the correct order of operations each time they find the molar mass. If
students use a calculator, encourage them to use grouping symbols when they
enter the values in the calculator.
In Sample Problem 15.1, students should enter the calculation for the molar mass
of Na2CO3 as (2 × 23) + 12 + (3 × 16) to ensure that the calculator follows the correct
order of operations.
500
Chapter 15 • Lesson 2
a
Evaluate
Figure 15.15 Sodium Hydroxide
Deliquescent substances, such as
sodium hydroxide, can remove water
from the air. a. Sodium hydroxide
pellets absorb moisture from the air.
b. Eventually a solution is formed.
Classify What is the solvent? What
is the solute?
b
Informal Assessment
To assess students’ understanding of the properties
of aqueous solutions, show students the following
activity and ask them to answer each question.
Dissolve a small amount of nickel(II) chloride in
500 mL of distilled water.
1. What is the solute and what is the solvent?
(Water is the solvent and nickel(II) chloride is
the solute.)
2. What type of electrolyte is nickel(II) chloride?
How do you know? (Nickel(II) chloride is
an ionic compound and is soluble in water;
therefore, it is a strong electrolyte.)
3. What types of substances will dissolve in water
to form aqueous solutions? (ionic compounds
and polar covalent molecules)
Then have students complete the 15.2 Lesson Check.
NLIN
PR
S
E
O
Reteach
M
OBLE
15.2 Lesso
LessonCheck
10.
Identify What types of substances dissolve
most readily in water?
15. Compare Is the percent by mass of copper in
CuSO4 Ƃ5H2O the same as in CuSO4? Explain.
11.
Review What property of all ionic compounds make them electrolytes?
16. Compare and Contrast Distinguish between
efflorescent and hygroscopic substances.
12.
Explain Why do hydrates easily lose water
when heated and regain water when exposed to
moisture?
BIGIDEA BONDING AND INTERACTIONS
13. Classify Identify the solvent and the solute in
vinegar, a dilute aqueous solution of acetic acid.
14. Calculate What is the percent by mass of water
in iron(II) sulfate heptahydrate (FeSO4 Ƃ7H2O)?
Project models of water molecules, cations, and
anions on an overhead projector to show how water
molecules orient their dipoles to solvate ions. Point
out that cations and anions are attracted to different
ends of the water molecule. Stress that the classification
of water-soluble substances as strong electrolytes,
weak electrolytes, or nonelectrolytes is determined by
the relative number of ions in solution. Explain that
nonelectrolytes do not produce ions.
17. Which of the following substances dissolve to a
significant extent in water? Explain your answer
in terms of the interactions between the solvent
and solute.
a. CH4
d. MgSO4
b. KCl
e. Sucrose (C12H22O11)
c. He
f. NaHCO3
Water and Aqueous Systems 501
Lesson Check Answers
10. Ionic compounds and polar covalent
compounds
11. They dissociate into ions.
12. The forces holding the water
molecules in hydrates are not very
strong, so the water is easily lost
and regained.
13. The solute is acetic acid. The solvent
is water.
14. 46%
15. The percents of copper in copper
sulfate and copper sulfate
pentahydrate are different because
the formula masses of the two
compounds are different.
16. Efflorescent compounds such as
certain hydrates lose water to the
air. Hygroscopic compounds remove
moisture from air, sometimes
forming hydrates.
17. a. insoluble, nonpolar
b. soluble, ionic
c. insoluble, nonpolar
d. soluble, ionic
e. soluble, polar
f. soluble, ionic
Answers
8. 36.1%
9. 13.5 g Na2CO3 ? 10H2
FIGURE 15.15 The solvent is water and the solute is
sodium hydroxide.
Water and Aqueous Systems
501
LESSON 15.2
Deliquescent Compounds Have you ever noticed the small packets of
silica gel that are often packaged with electronic equipment and leather
goods? Although the structure of silica gel is not the same as a hydrated
salt, it is a hygroscopic substance used to absorb moisture from the surrounding air to prevent damage to sensitive equipment and materials.
Some compounds are so hygroscopic that they become wet when exposed
to normally moist air. These compounds are deliquescent, which means
that they remove sufficient water from the air to dissolve completely and
form solutions. Figure 15.15 shows that pellets of sodium hydroxide are
deliquescent. For this reason, containers of sodium hydroxide and other
deliquescent chemicals should always be tightly stoppered and the chemicals should never come in contact with your skin. The solution formed by
a deliquescent substance has a lower vapor pressure than that of the water
in the air.
CHEMISTRY & YOU
CHEMISTRY
Y
&
CHEMISTRY
Y
YO
YOU
U Students should be
familiar with the process of osmosis in biological
systems and understand that water flows across a
membrane toward an area of high concentration.
In contrast, reverse osmosis requires water to flow
away from an area of high concentration. Just as in
a biological system, the process of moving water
away from a highly concentrated solution toward
a more dilute one requires an input of energy. In this
case, a high-pressure differential is used to force the
water through a semi-permeable membrane. It is
this requirement of high pressure that creates a
problem for reverse osmosis: the higher the
efficiency of the process, the higher the pressure
provided must be, thus increasing the energy costs.
Pose the following question to students: Considering
both its benefits and problems, why is reverse osmosis
a highly desirable solution for water filtration? You
may need to assist students in the following ways:
• Reverse osmosis can be used to filter any water,
not just saltwater.
• A reasonably high efficiency of reverse osmosis
can be achieved only in large filtration plants,
which can provide the high pressure needed.
• Reverse osmosis can be performed on a small
scale, even in faucet and aquarium filters.
• Membranes used for reverse osmosis must be
frequently replaced.
Y
TECHNOLOGY
&YOU:
Reverse Osmosis Desalination
Ninety-seven percent of the world’s water
is saltwater. Unfortunately, humans cannot
drink saltwater. However, a process called
reverse osmosis desalination can turn
saltwater into drinkable water.
High pressure
Low pressure
Seawater
Treated water
Salt
Contaminants
Membrane
Brine
1 Seawater supply
Seawater moves from the
ocean to the desalination
plant through large pipes.
2 Pretreatment system
Incoming water is treated and filtered
to remove debris, sediment, and
other microscopic particles.
Brine
3 Reverse osmosis process During a process called
reverse osmosis, high pressure is used to force the
seawater through semi-permeable membranes. These
membranes only allow water to pass through, leaving
salts and contaminants behind in a concentrated
seawater solution, called brine. The brine is then
released back into the ocean.
502 $IBQUFSt$IFNJTUSZ:PV
21st Century Learning To be successful in the 21st century, students
need skills and learning experiences that extend beyond subject matter mastery. The
following project helps students build the following 21st Century Skills: Financial,
Economic, Business, and Entrepreneurial Literacy; Creativity and Innovation; Critical
Thinking and Problem Solving; Communication and Collaboration; Information
Literacy; and Initiative and Self-Direction.
Pose the following challenge to students: A local coastal government is trying to
decide between distillation and reverse osmosis for a city’s new water filtration plant.
Form a team of 3–5 students to represent a local citizens’ group that favors reverse
osmosis over distillation, and prepare an advocacy report supporting your views on
this technology. Presentations should be prepared and submitted electronically and
should include a suggested site for the plant. Students should include information on
the pros and cons of the technology, its potential environmental impact, and its
economic feasibility.
502
Chapter 15 • Chemistry & You
Advantages of Reverse Osmosis Desalination
Disadvantages of Reverse Osmosis Desalination
Explain
Expands drinking water resources Some
areas of the world do not have abundant
sources of drinking water. Reverse osmosis
desalination provides drinking water in cases of
drought, water shortages, or national disasters.
Low environmental impact The conversion of
saltwater to drinking water via reverse osmosis
desalination uses minimal chemicals.
Low start-up costs The cost to install a reverse
osmosis desalination facility is relatively low.
High energy consumption The cost of
running a reverse osmosis desalination plant is
high compared to other methods for obtaining
drinking water.
Low efficiency The volume of drinking water
produced is low compared to the volume of
seawater treated.
Potentially harmful to sea life The process of
removing seawater and returning brine to the
ocean can disturb marine environments.
MAKE A CONNECTION One of the key difficulties
Extend
5 Drinking water storage
The treated drinking water
is transferred to a tank and
stored until needed.
Connect to
6 Drinking water supply
The tap water you drink
may have originally come
from the sea!
4 Post treatment system
The treated water goes through
more filters. In addition, chemical
disinfectants are usually added.
in setting up reverse osmosis facilities is the high
pressure required to efficiently process saltwater.
Remind students that the principle of osmosis is
ultimately the reason for the requirement of a
high-pressure differential. In a situation where the
solute concentrations are drastically different on
the two sides of a membrane, there is naturally a
relatively high pressure exerted toward the higher
concentration. This causes difficulty during the process
of reverse osmosis because not only must the water
be forced through the membrane but the system must
also overcome the natural pressure of osmosis.
ENVIRONMENTAL SCIENCE
The principal environmental concern from reverse
osmosis desalination is the fact that an inevitable
byproduct of the process is a concentrated mix of
salts and impurities known as brine. The brine has
a much higher salinity than ocean water and is
therefore poisonous to marine wildlife. It is especially
harmful to bottom-dwelling animals because brine
sinks due to its higher solute concentration. Have
students brainstorm potential ways in which the
environmental impact due to the release of brine could
be minimized.
Take It Further
1. Classify Would you consider seawater to be a
homogeneous aqueous system or a heterogeneous aqueous
system? Explain.
2. Infer Brine is more dense than seawater. How might
the release of brine back into the ocean affect the marine
environment?
Water and Aqueous Systems 503
Differentiated Instruction
L1 STRUGGLING STUDENTS Have students work in small groups to produce a
poster demonstrating the process by which a desalination plant filters saltwater.
Posters should include the saltwater intake, flow through the semi-permeable
membrane, and water and brine outflow.
LPR LESS PROFICIENT READERS Help students understand the term desalination
by explaining the connection between the term saline and saltwater. Point out that
desalination can be referred to as the process of “de-salting” water.
L3 ADVANCED STUDENTS Divide students into pairs. Have the pairs research
how desalination has evolved over the years and what advantages reverse osmosis
desalination has over earlier methods. Have students present their findings to the
class. Encourage creativity in the presentation medium.
Answers
1.
2.
Seawater is a homogeneous aqueous system
because it is an aqueous solution.
Since brine is more dense than seawater,
it will sink to the bottom of the ocean.
The brine could affect marine life that live on
the ocean floor.
Water and Aqueous Systems
503
CHEMISTRY & YOU
Pros & Cons