Name
CHAPTER 9
Class
Date
Acids, Bases, and Salts
SECTION
1 Acids, Bases, and pH
KEY IDEAS
As you read this section, keep these questions in mind:
• What properties do acids have?
• What properties do bases have?
• How can pH tell you if something is an acid or
a base?
What Are the Properties of Acids?
Does the thought of eating a lemon cause your mouth
to pucker? You expect a lemon to taste sour. All the foods
in the figure below taste sour because they contain acids.
READING TOOLBOX
Outline Make an outline
of the material in this section. Use the headers in the
section to help you organize
your outline.
Citrus fruits, such as lemons, grapefruits, limes, and oranges, contain citric acid.
Apples contain malic acid. Grapes contain tartaric acid.
When acids dissolve in water, they ionize, or form
ions. When acids ionize, they form hydrogen ions, H+.
The hydrogen ions bond to water molecules, H2O, to
form hydronium ions, H3O+. An acid is a compound that
increases the number of hydronium ions when it dissolves
in water. Hydronium ions give acids their properties.
Some properties of acids are listed below.
• sour taste
• conduct electricity
• turn blue litmus paper red
• corrosive (able to destroy or eat away certain
materials)
READING CHECK
1. Define What is an acid?
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Acids, Bases, and pH continued
What Is the Difference Between Strong and
Weak Acids?
READING CHECK
2. Identify What determines
whether an acid is strong or
weak?
EHHDBG@<EHL>K
3. Identify What happens
to the total charge of the
solution when HNO3
dissolves?
All acids ionize when they are dissolved in water.
However, some acids ionize completely, while others do
not. The relative numbers of acid molecules that ionize in
water determine if the acid is strong or weak.
STRONG ACIDS
The figure below shows what happens to nitric acid
when it is dissolved in water. The arrow pointing to the
right shows that all of the nitric acid ionizes.
Nitric acid is a strong acid. When strong acids are dissolved in water, they ionize completely. This means that
all the dissolved acid molecules break apart into ions.
Nitric acid
Water
Hydronium ion
HNO3
4. Define What is an
electrolyte?
_
+
+
+
READING CHECK
Nitrate ion
+
H2O
H3O+
+
NO3–
When nitric acid ionizes, it forms hydronium ions and
nitrate ions. The ions move around freely in the water
and can conduct electricity. A substance that conducts
electricity when it is dissolved in water is called an
electrolyte. Strong acids are strong electrolytes. This
is because solutions of these acids contain as many
hydronium ions as the acid can possibly form.
WEAK ACIDS
Weak acids, such as acetic acid, do not completely
ionize. Only some of the molecules of acid ionize in
water. Examine the figure at the top of the next page.
The arrow pointing to the right shows that dissolved
acetic acid molecules break apart into ions. The arrow
pointing to the left shows that ions are also recombining
to form molecules. Therefore, a solution of a weak acid
contains fewer hydronium ions than a solution of a
strong acid with the same concentration.
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Acids, Bases, and pH continued
Acetic acid
Water
Hydronium ion
Acetate ion
_
+
+
+
CH3COOH
+
H2O
H3O+
+
CH3COO –
Because there are few ions in a solution of a weak
acid, the solution does not conduct electricity very well.
Thus, weak acids are weak electrolytes.
EHHDBG@<EHL>K
5. Infer Which of the two
beakers contains more ions?
How do you know?
6. Identify Which beaker
contains a strong acid?
Explain your answer.
Both of these beakers contain acids. One contains a weak acid
and one contains a strong acid.
ACID DANGERS
Although many products you use every day contain
acids, strong and concentrated acids can be very dangerous. For example, your stomach makes a very strong
acid called hydrochloric acid. You need this acid to help
you digest food. Normally, your stomach has a thick lining that protects it from the strong acid. However, if your
body makes too much acid, the acid can destroy your
stomach.
Even weak acids are not always safe to handle. Acids
can damage your skin. The vapors are also harmful to
your eyes, lungs, and mouth. This is why you should
always wear goggles, gloves, and a laboratory apron
when you work with acids. Never taste a chemical to
determine if it is an acid.
READING CHECK
7. List Identify three pieces
of safety equipment you
should use when you work
with an acid.
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Acids, Bases, and pH continued
Some Common Acids
Acid
Formula
Strength
Uses
Hydrochloric acid
HCl
strong
Cleaning
masonry;
treating metals
before plating
or painting;
adjusting pH of
swimming pools
Sulfuric acid
H2SO4
strong
making fertilizers;
the acid in car
batteries
Nitric acid
HNO3
strong
making fertilizers
and explosives
Acetic acid
CH3COOH
weak
making
chemicals,
plastics, and
medicines; the
acid in vinegar
Formic acid
HCOOH
weak
dying cloth; the
acid used by
stinging ants
Citric acid
H3C6H5O7
weak
making flavorings
and soft drinks;
the acid in citrus
fruits
EHHDBG@<EHL>K
8. Identify Which common
acid is found in car batteries?
9. Identify Is the acid in
vinegar strong or weak?
All these products
contain acids.
What Are the Properties of Bases?
Baking soda is an example of a common base. It is
used to make cakes and cookies and may be found in
toothpastes and cleaning supplies. A base is a compound
that forms hydroxide ions, OH−, when it is dissolved in
water. Hydroxide ions give bases their properties. Some
of these properties are listed below.
READING CHECK
10. Identify What kind of
ions do bases form when
they are dissolved in water?
•
•
•
•
•
bitter taste
feel slippery
conduct electricity
turn red litmus paper blue
corrosive
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Acids, Bases, and pH continued
STRONG BASES
When strong bases are dissolved in water, they ionize
completely. Most strong bases are made up of metal ions
and hydroxide ions. These kinds of bases are called metal
hydroxides.
Sodium hydroxide is an example of a metal hydroxide.
Some drain cleaners contain sodium hydroxide. Sodium
hydroxide completely ionizes when it is put into water.
Sodium hydroxide and other strong bases are strong
electrolytes. That is, they conduct electricity well when
they are dissolved in water.
8g^i^XVa I]^c`^c\
11. Compare In terms of
ionization, how are strong
acids and strong bases
similar?
WEAK BASES
Ammonia is an example of a weak base. Ammonia,
NH3, does not contain hydroxide ions. Where do the
oxygen and hydrogen that make up the hydroxide ion
come from? Ammonia reacts with water to form OH−.
This reaction is shown in the figure below.
Ammonia
Water
Ammonium ion
Hydroxide ion
+
+
_
12. Explain How do bases
that do not contain hydroxide
ions form hydroxide ions?
+
+
NH3
READING CHECK
H2O
NH4+
+
OH–
The double arrow tells you that ammonia does not
ionize completely when it is dissolved in water.
Therefore, ammonia is a weak base and a weak
electrolyte.
DANGERS OF BASES
The table on the next page gives examples of common
bases. Many bases are used in soaps, dyes, and fertilizer.
However, concentrated bases can be very dangerous.
They may dissolve many materials, including skin. When
you work with bases, you should wear safety goggles,
gloves, and a laboratory apron to protect yourself.
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Acids, Bases, and pH continued
All these products
contain bases.
Some Common Bases
EHHDBG@<EHL>K
13. Identify Identify two
common bases used in
making soap.
Base
Formula
Strength
Uses
Potassium
hydroxide
KOH
strong
making soap; dyeing
products
Sodium
hydroxide
NaOH
strong
making soap; refining
petroleum; cleaning drains;
making synthetic fibers
Calcium
hydroxide
Ca(OH)2
strong
treating acid soil; treating
lakes polluted by acid rain;
making mortar, plaster, and
cement
Ammonia
NH3
weak
fertilizing soil; making
fertilizers; making nitric
acid; making cleaning
solutions
Methylamine
CH3NH2
weak
making dyes and
medicines; tanning leather
Aniline
C6H5NH2
weak
making dyes and varnishes;
used as a solvent
What Is the Difference Between Strong and
Concentrated?
Many people think the words concentrated and strong
mean the same thing. Similarly, many people think that
weak and dilute mean the same thing. In chemistry, however, these words have different meanings.
Recall that the concentration of a solution refers to
the number of solute particles in a particular amount
of solution. A solution with a large number of solute
particles is concentrated. A solution with a small number
of solute particles is dilute.
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STRONG AND DILUTE
Suppose a scientist added a very small amount
of hydrochloric acid to a liter of water. Because the
solution contains only a small amount of solute, you
could describe it as dilute. However, when any amount
of hydrochloric acid is dissolved in water, it ionizes
completely. At any concentration, hydrochloric acid is
a strong acid. Thus, a strong acid may form a dilute
solution.
8g^i^XVa I]^c`^c\
14. Infer Can a weak
base form a concentrated
solution? Explain your
answer.
What Is pH?
How can you tell if a solution is acidic or basic? One
way is to use an indicator. An indicator is a substance
that changes color if a solution is acidic or basic. Litmus
paper and pH paper are examples of indicators.
An indicator can tell you something is an acid or a
base. However, in many cases, you need to know how
acidic or basic a solution is. The pH of a solution is a
value that expresses how acidic or basic a solution is.
The pH scale ranges from 0 to 14. Solutions with a pH
less than 7 are acids. Solutions with a pH greater than 7
are bases. Solutions with a pH of 7 are neutral. A neutral
solution is not acidic or basic.
READING CHECK
15. Identify Is a solution
with a pH greater than
7 basic or acidic?
Acidic: pH < 7
Basic: pH > 7
Neutral: pH = 7
Increasing acidity
1
2
3
Lemon
juice
4
5
6
Milk
Soft
drink
Human
saliva
7
8
9
10
11
Sea Detergents
water
12
13
Household
ammonia
Tap water
Acid rain
EHHDBG@<EHL>K
Increasing basicity
16. Identify Is a soft drink
more acidic or less acidic
than milk?
17. Analyze Relationships
What happens to acidity as
pH increases? What happens
to basicity as pH increases?
Clean rain
Human stomach contents
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RELATIVE CONCENTRATIONS OF IONS
READING CHECK
18. Describe Describe the
relative concentrations of
hydronium and hydroxide
ions in an acidic solution.
The pH of a solution indicates the concentration of
hydronium ions. An acidic solution has a greater concentration of hydronium ions than hydroxide ions. A basic
solution has a greater concentration of hydroxide ions
than hydronium ions. A neutral solution has equal concentrations of hydronium ions and hydroxide ions.
CONCENTRATION AND pH
Recall that you can use molarity (M) to describe the
concentration of a substance in a solution. You can use
the concentration of hydronium ions in a solution to
determine the solution’s pH.
In a solution, strong acids like HCl and HNO3 produce
one hydronium ion for each dissolved acid particle. The
concentration of a strong acid indicates how many particles are in the solution. Because every particle of a strong
acid that dissolves will ionize, the hydronium ion concentration equals the acid concentration.
Writing the H3O+ concentration in scientific notation
can help you determine pH. When the concentration of
hydronium ions is one times ten to a power, the pH is the
negative of the power of 10. For example, pure water has
a hydronium ion concentration of 0.0000001 mol/L, or
1 × 10–7 M. Thus, pure water has a pH of 7.
CALCULATING pH
A hydrochloric acid solution has a H3O+ concentration
of 0.0001 M. What is the pH of the solution?
Math Skills
19. Calculate HNO3 is a
strong acid. What is the pH of
a 0.01 M solution of HNO3?
Unknown:
pH
Step 1: List the given and unknown
values.
Given:
concentration of
HCl = 0.0001 M
Step 2: Write the molar
concentration of hydroxide ions in
scientific notation.
concentration of H3O+
Step 3: The pH is the negative
of the power of 10 in the H3O+
concentration.
= 0.0001 M = 1 × 10-4 M
concentration of H3O+ ions
= 1 × 10-4 M
pH = -(-4) = 4
The solution has a pH of 4.
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INTERPRETING pH DIFFERENCES
As you have seen, pH is based on powers of 10.
Because of this, small differences in pH reflect big differences in hydronium concentrations. For example, the pH
of apple juice is about 3. The pH of coffee is about 5. The
difference in pH of the two liquids seems small. However,
the difference of two pH units is actually quite large.
Because each pH unit represents a power of 10, a pH
difference of two represents a difference in acidity of 102,
or 100. Thus, apple juice is 100 times more acidic than
coffee. Antacid tablets are basic. When antacid tablets
are dissolved in water, the solution they form has a pH of
about 8. Thus, coffee is 103, or 1,000 times, as acidic as an
antacid tablet solution.
Math Skills
20. Calculate Solution A
has a pH of 9. Solution B has
a pH of 3. How much more
acidic is solution B than
solution A?
MEASURING pH
Indicators such as litmus paper do not measure pH
very precisely. To measure pH precisely, you can use a pH
meter. Because ions in a solution have electric charge,
they can conduct electric current. A pH meter measures
pH by determining the electric current created by the
movement of ions.
EHHDBG@<EHL>K
21. Apply Concepts Is the
tomato acidic or basic?
Explain your answer.
A pH meter can measure the
H3O+ concentration precisely.
22. Infer How is the pH
value shown on the pH
meter more precise than pH
values you have seen so far?
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SECTION VOCABULARY
acid any compound that increases the number
of hydronium ions when dissolved in water
base any compound that increases the number
of hydroxide ions when dissolved in water
electrolyte a substance that dissolves in water
to give a solution that conducts an electric
current
indicator a compound that can reversibly
change color depending on conditions such
as pH
pH a value that is used to express the acidity or
basicity of a system; each whole number on
the scale
1. Write Equations Write an equation showing the ionization of hydrochloric acid in
water.
2. Explain How do a strong acid and a weak acid behave differently when each is
dissolved in water?
3. Identify Relationships What is the relationship between the strength of an acid or
base and the strength of the electrolyte that it forms?
4. Classify Suppose you have a solution that has 4 times as many hydronium ions as
hydroxide ions. Is this solution acidic, basic, or neutral?
5. Arrange Arrange the following substances in order of increasing acidity: vinegar
(pH = 2.8), stomach acid (pH = 2.0), and a soft drink (pH = 3.4).
6. Graph Trends Create two graphs to describe how pH changes as acidity and
basicity increase. Be sure to label the axes of your graphs.
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