8
Water and Solutions
Introduction to Chapter 25
This chapter explores the chemistry of acids and bases and explains the pH scale. In
the first Investigation, you will create a pH scale using a natural indicator solution.
You will learn about the applications of acid-base chemistry in household products,
medicine and the environment. In the second section and Investigation, you will
learn about and simulate the effects of acid rain.
Investigations to Chapter 25
25.1
Acids, Bases, and pH
Chapter 25
Acids
and
Bases
What is pH?
In this Investigation, a natural indicator and household chemicals are used to create
a color-based pH scale. You will use your pH scale to figure out the pH of additional
household chemicals and two mystery solutions.
25.2
Acid Rain
What is acid rain?
In this Investigation, you will model the effects of acid rain on a natural ecosystem.
You will observe the effect of different dilutions of an acid on the activity of water
fleas (Daphnia magna). Water fleas are an important source of food for fish and
other organisms in fresh water environments. In this Investigation, you will also
have the opportunity to research topics related to acid rain on the Internet.
427
Chapter 25: Acids and Bases
Learning Goals
In this chapter, you will:
D Describe pH as a way to measure the strength of acids and bases.
D Understand the definitions of an acid and a base.
D Identify pH of common household chemicals.
D Understand the cause of acid rain.
D Understand the environmental effects of acid rain.
D Demonstrate the effect of acid on a natural ecosystem.
Vocabulary
acid
acid precipitation
acid rain
428
base
electrolytes
neutral
pH
pH indicator
pH scale
smog
Chapter 25
25.1 Acids, Bases, and pH
In the Investigations, you measured several indicators of water quality, including the pH. You may
remember that pH values range from 0 to 14. Low values (0 to 6) indicate that a solution is acidic
whereas high values (8 to 14) indicate that a solution is basic. Your stomach secretes one of the stronger
acids, hydrochloric acid (HCl). As acidic solutions of digested food leave your stomach, other organs in
your digestive system secrete bicarbonate, a base. The added base neutralizes the strong, corrosive acid.
As you can see, acids and bases play a significant role in how your body works. What exactly are acids
and bases? What does pH mean?
Why is pH important?
The pH of water Water quality is evaluated using pH values for many reasons. For example, if the
indicates its pH of your tap water is too high, it might indicate that calcium or magnesium
quality deposits are forming in and may clog your water pipes. On the other hand, if the
pH is too low, the water may be corroding your pipes.
The pH of water is important to life. The pH of natural bodies of water also has to
be just right—neither too high nor too low. For example, at a pH of 4 or 5, fish
have trouble reproducing. At even lower pH values, they die.
Acids and bases The pH of water is related to the action of a class of chemicals called acids and
are defined by pH bases. A solution with a pH value that is less than 7 contains an acid, and a
values solution with pH values greater than 7 contains a base. You will learn how pH is
determined later on in this section.
What are acids and bases?
Acidic solutions
have more H+
ions, and basic
solutions have
more OH- ions
An acid is a chemical that contributes hydrogen ions, H+, to a solution. A base is a
chemical that contributes hydroxyl ions, OH-, to a solution. Therefore, solutions
can be described as “acidic” or “basic” according to the concentrations of H+ and
OH- ions in the solution. A solution with a high concentration of H+ ions and few
OH- ions is strongly acidic. A strongly basic (or alkaline) solution has a high
concentration of OH- ions and few H+ ions.
Figure 25.1: Acids and bases play
important roles in digestion.
25.1 Acids, Bases, and pH
429
Chapter 25
HCl is an acid Hydrochloric acid (HCl) is a very strong acid which can seriously irritate your
nose if you smell it and your skin if it spills on you! When HCl dissolves in water
(as indicated by the symbol “aq”), it ionizes to become H+ ions and Cl- ions:
An acid such as HCl ionizes almost completely when dissolved in water and
therefore contributes many H+ ions to a solution. Because of this, HCl is a known
as a strong acid.
NaOH is a base Sodium hydroxide (NaOH) is a base commonly used for rigorous housecleaning
and for unclogging drains. As you can see from the chemical formula, NaOH will
release OH- ions when it dissolves in water:
Since NaOH ionizes almost completely when dissolved in water and contributes
many OH- ions to a solution, it is known as a strong base.
Weak acids and Many acids and bases do not ionize completely in water and are known as weak
bases acids and bases.
Vinegar, also known as acetic acid, is a weak acid because it contributes a few H+
ions to solution. Vinegar is sometimes used for cleaning but is not very irritating to
the skin. Your nose may not like it, however!
A weak acid or base ionizes incompletely when dissolved in water as shown by the
double arrows in the chemical equation above.
Household ammonia is an example of a weak base because it does not completely
ionize in water.
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Hydronium Ions
In an acid solution, separate
hydrogen ions (H+) do not
exist by themselves for very
long. Each hydrogen ion is
attracted to the oxygen end of
a water molecule. The two
combine to form a hydronium
ion. In chemistry, it is
common to refer to H3O+ ions
as H+ ions.
Chapter 25
pH and the pH scale
pH is based on the
concentration of
H+ ions in
solution
Just as centimeters describe length, pH describes the exact concentrations of H+
ions in a solution. Most instruments that measure pH use a pH scale that runs from
0 to 14. At pH 1, a solution is strongly acidic. At pH 14, a solution is strongly
basic, or alkaline.
pH 7 means equal A solution that has a pH of 7 has equal numbers of H+ and OH- ions and is called
H+ and OH- ions a neutral solution.
Low pH means Everyone is familiar with using a ruler to measure things. However, a pH scale is
high H+ different from a ruler in that the more H+ ions there are in a solution, the lower the
concentration pH value. Conversely, fewer H+ ions in a solution means a higher pH value.
The pH scale is A pH scale is based on powers of ten. A solution that is pH 1 has 10 times more
based on powers hydrogen ions than a solution of pH 2. In turn, the pH 2 solution has 10 times more
of 10 hydrogen ions than a solution of pH 3.
Another way of looking at the scale is to see that as the pH numbers increase, the
amount of H+ ions decreases by powers of 10. Figure 25.2 illustrates the pH scale.
For example, a solution at pH 1 has 10 million times more hydrogen ions than a
solution at pH 7, and a solution at pH 7 has 10 million times more hydrogen ions
than a solution at pH 14!
You now know that the amount of H+ in a solution at pH 6 is 10 times the amount
in a solution at pH 7. But a solution at pH 7 is neutral. What is the amount of OHions in a neutral solution? What is the amount of OH- ions in a solution at pH 6?
Acids have more
H+ ions, bases
have more OHions
The answers to these questions are that the amount of OH- ions in a neutral
solution is equal to the amount of H+ ions. If you go down the pH scale to 6, the
amount of OH- ions decreases by a power of 10 whereas the amount of H+
increases by a power of 10. Remember:
•
•
pH values below 7 means H+ ions outnumber OH- ions, and
pH values higher than 7 means OH- ions outnumber H+ ions.
Since each pH is an increase or decrease by tenfold, there is a big difference in ion
concentration between each pH value.
Figure 25.2: The pH scale is
based on the concentration of
hydrogen ions in solution.
25.1 Acids, Bases, and pH
431
Chapter 25
The ionization of water
Water is both a Water plays an important role in acid and base chemistry. Water itself is both a
weak acid and a weak acid and a weak base. A water molecule can split or ionize into H+ and OHweak base ions. Because the H+ ions immediately attach themselves to water molecules, the
equation for the ionization of water is written as:
2H2O ↔ H3O+ + OHWhat does the The double-headed arrow in the equation means that water, if left alone, is always
double-headed in equilibrium with its ions. The rate at which water breaks down into ions is the
arrow mean? same as the rate at which the ions re-form into water. However, water ionizes so
slowly that most water molecules exist whole, not as ions.
Figure 25.3: .Water ionizes into
H+ and OH- ions. Therefore, it acts
like both a weak acid and a weak
base. Pure water has a neutral pH
(pH 7) because the concentrations of
H+ and OH- ions are equal.
Examples of acid and base chemistry
Acids and bases Many reactions, such as the ones that occur in your body, work best at specific pH
play a role in values. For example, acids and bases are very important in the reactions involved
digestion in digesting food (see figure 25.1). As you may know, the stomach secretes
hydrochloric acid (HCl), a strong acid (pH 1.4). The acidity of our stomachs is
necessary to break down the protein molecules in our food so that they can be
absorbed. A mucus lining in the stomach protects it from the acid produced. As
food and digestive fluids leave the stomach, however, other organs in the digestive
system also need to be protected from the acid. This is accomplished by two parts
of the system—the pancreas and liver. These two organs secrete bicarbonate to
neutralize the stomach acid before it reaches other organs.
Acids and bases The reactivity of acids and bases means that these chemicals have many uses.
have many uses They react with each other and other chemicals. Metals and glass can be etched
with corrosive acids. Lye (pH 13) is used to unclog drains, and sulfuric acid (pH 1)
is used throughout industry. Millions of tons of sulfuric acid are produced each
year (40 million tons in 1990). Its uses range from making rayon to cleaning
impurities from gasoline.
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Figure 25.4: Carbon dioxide gas
(CO2) escaping from carbonated
water. The CO2 molecules are chains
of three atoms, and the water
molecules have a triangular shape.
CO2 makes soda fizzy and reacts with
water to form carbonic acid, which
gives the soda a sharp, or acidic,
taste. This and other acids in soda,
like phosphoric acid, can dissolve
tooth enamel. Be sure to brush your
teeth after drinking soda!
Chapter 25
Electrolytes and nonelectrolytes
Electrolytes Current is the flow of charge. When a solution contains dissolved ions (charged
conduct current particles), it can conduct current. Chemicals that conduct current when dissolved
in water are called electrolytes. These chemicals form ions when dissolved.
Salt dissociates in Ionic compounds, molecular compounds, and even atoms can contribute ions to a
water solution. When an ionic compound is dissolved in water, the polar ends of the
water molecule attract the positive and negative ions in the solution. In previous
sections, you learned how water dissolves table salt (NaCl). Recall that the
attraction of the polar ends of the water molecule is strong enough to break the
weak ionic bonds of NaCl. When an ionic compound is brought into solution by
water it is said to dissociate. The term ionization is used if a molecular compound
or atom forms an ion.
For example, when NaCl dissolves in water we say it dissociates. When the
element Na loses an electron, it ionizes to Na+.
Acids and bases All acids and bases are electrolytes because they contribute ions to a solution.
are electrolytes Some chemicals, like salt (NaCl), dissociate to form ions in solution but are not
acidic or basic. Acids, bases, and salt water are examples of electrolytes.
Non-electrolytes Other chemicals do not form ions when they are dissolved in solution. They are
do not have ions called non-electrolytes. Non-electrolytes are not acidic or basic. Sugar dissolved
and are not acidic in water is an example of a non-electrolyte.
or basic
Electrical appliances and water
Because tap water contains small amounts of dissolved ions, it is an
electrolyte. Remember that even a small amount of current is dangerous if
it enters your body directly? Water provides a way for electric current to
enter your body, so always take care when using electrical appliances near
water!
Electrolytes and your
body
When you perform a
strenuous activity, your body
cools itself by sweating.
Sweat contains water and
dissolved
salts
(or
electrolytes) like sodium and
potassium. Before, during,
and after exercising, you can
replenish fluids and your
body’s
electrolytes
by
drinking diluted fruit juice,
slightly salty water, or by
consuming a sports drinks.
The water in these fluids
helps your body continue to
cool itself so that you don’t
get overheated. By replacing
electrolytes, you may be
helping your body speed up
resorption of fluids. Diluted
fruit juice or a sports drink
contains small amounts of
carbohydrates to give your
body the energy boost it may
need
during
strenuous
exercise.
25.1 Acids, Bases, and pH
433
Chapter 25
The pH of substances you use or consume
Many foods are
acidic and many
cleaning products
are basic
Table 25.1 contains a list of some common chemicals and their pH values. What
do you notice about this list of substances? Where would you find acids in your
kitchen? Where would you find bases?
Table 25.1: The pH of some common chemicals.
Household chemical
lemon juice
vinegar
soda water
baking soda
bar soap
ammonia
Acid or base
acid
acid
acid
base
base
base
pH
2
3
4
8.5
10
11
It turns out that many of the foods we consume or use for cooking are acidic. On
the other hand, many of our household cleaning products are basic.
A pH indicator In the Investigation, you will be testing the pH of common chemicals using
another item that you may find in your kitchen. You will measure pH using a pH
indicator—a chemical that changes color at different pH values.
Common foods can also be used as pH indicators. In the Investigation, the
indicator you will use is made from the juice of a red cabbage.
Acids, bases, and taste
Our taste buds are sensitive to acids and bases. We taste acids as sour and bases
as bitter. Lemon juice is strongly acidic, and soap is strongly basic. Acids that
are stronger than lemon juice and bases that are stronger than ammonia are so
reactive that they can harm your skin and damage clothing.
Figure 25.5: The pH scale
showing common substances.
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Chapter 25
Health Perspective: pH and your blood
It is very important for your blood pH to stay within the normal range. At higher or lower pH
values, your body does not function properly. Fortunately, you can regulate the pH of your blood
simply by breathing!
Blood is a watery solution that contains many solutes including the dissolved gases carbon
dioxide and oxygen. Carbon dioxide appears in your blood because it is produced by respiration.
Recall that respiration is the combustion of sugar by your body. You breathe in oxygen to get
this process going. The end products of this reaction are energy, water, and carbon dioxide.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
The rate at which you breathe controls the concentration of carbon dioxide in your blood. For
example, if you hold your breath, more carbon dioxide enters your blood. If you hyperventilate,
you blow off carbon dioxide, so that significantly less is in your blood. These two processes
influence blood pH. The equation below illustrates how carbon dioxide dissolves in an aqueous
(watery) solution like blood:
CO2
carbon dioxide
+
+
H2O
water
↔ H2CO3
↔ carbonic acid
↔ H+
↔
+ HCO3hydrogen ion
+
bicarbonate ion
When CO2 dissolves, H+ ions are produced in solution. Therefore, the more CO2 in your blood,
the more acidic your blood will become. If you breathe slowly, the added CO2 makes your blood
more acidic. However, if you breathe too often and too quickly (hyperventilating), the loss of
CO2 makes your blood more basic. You can offset this effect by breathing into a paper bag. This
forces you to re-breathe carbon dioxide.
When you breathe normally, your blood pH ranges between 7.35 and 7.45.
Figure 25.6: Some causes of low
blood pH (acidosis) include holding
your breath, excessive lactic acid
produced during heavy exercise, and
production of ketone acids because of
fasting or having low levels of insulin
(diabetes). High blood pH (alkalosis)
can be caused by hyperventilating.
Under normal conditions, your blood
pH ranges between 7.35 and 7.45.
25.1 Acids, Bases, and pH
435
Chapter 25
25.2 Acid Rain
You have probably heard about acid rain. You might know that acid rain harms the environment. But
what is acid rain?
Life occurs within certain ranges of pH. Most plants and animals function best when pH is nearly
neutral. At the extreme ends of the pH scale, where you find strong acids and strong bases, many life
processes will not occur. For example, fish have trouble reproducing when the pH of their watery
environment is too acidic.
Also, plants have trouble growing if soil is acidified below 5.1, and aquatic animals have difficulty
surviving in water below pH 5.5. It was recently discovered that lakes and forests in certain parts of the
world were not as healthy as they once had been. The cause was higher acidity in the lakes, resulting
from rainfall, snowfall, or fog that was highly acidic. How can rain or snow be harmful to the
environment?
The definition of acid rain
Rain and snow are Rainfall, snowfall, or any precipitation happens to be naturally acidic. This is
naturally acidic because the rain, snow, or fog mixes with carbon dioxide, a gas present in air, and
forms small amounts of carbonic acid. The pH of precipitation before the
Industrial Revolution (which started in the mid-1700’s) was probably around 5.6.
Therefore, the normal pH of rain and other forms of precipitation is considered to
be 5.6. Any rain, snow, or fog that has a pH lower than 5.6 is called acid rain or
acid precipitation.
Acid rain results
from gases
produced during
combustion
reactions
The chief indicators of acid rain in the atmosphere are the gases sulfur dioxide
(SO2), sulfur trioxide (SO3), nitrogen oxide (NO), and nitrogen dioxide (NO2). A
large part of SO2 in the atmosphere is created by the burning of coal and oil that
contain sulfur, and by industrial processes such as metal purification. Nitrogen
oxide is a leading by-product of fuel combustion from traffic and power plants.
Sulfur trioxide and nitrogen dioxide are created from the reactions of sulfur
dioxide and nitrogen oxide, respectively, with oxygen-containing compounds in
the atmosphere. Both of these reactions speed up due to chemicals present in
particulate matter, a common pollutant released by traffic and industrial processes.
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Figure 25.7: If a body of water
has a pH of 4.5 or lower, almost
nothing can grow in it. Other animals
that live near the water, such as frogs
and insects, often die as well.
Figure 25.8: Ideally, fish require
water within the 6 - 8 pH range.
Chapter 25
Sulfur and
nitrogen gases
mix with water to
form sulfuric and
nitric acids
Sulfur oxides in the air mix with water, ozone, or hydrogen peroxide to form
sulfuric acid (H2SO4). This strong acid is the number one cause of acid rain. Even
before acid rain is formed, the four gases that cause acid rain (SO2, SO3, NO, and
NO2) create health problems in cities. They are irritants to the respiratory system
and may increase the incidence of asthma and other respiratory ailments.
Nitrogen oxides react with components in the atmosphere to form nitric acid
(HNO3), the second greatest cause of acid rain.
Nitric and sulfuric oxides are produced by cars and industry. These
airborne chemical react with water to form nitric and sulfuric acids.
Weather patterns (i.e., wind) can transport these acids to other areas where
they will fall as precipitation. Acid precipitation can affect the health of
people, trees, and aquatic life and erodes buildings and statues.
Figure 25.9: You can help reduce
the emission of acid rain gases by
using these alternative means of
transportation.
25.2 Acid Rain
437
Chapter 25 Review
Chapter 25 Review
Vocabulary review
Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms.
Set One
Set Two
1. acid
a. A chemical that contributes H+ ions to a
solution
1. acid rain
a. The naturally occurring rock that protects some
areas of the country from acid rain
2. base
b. A solution that has no H+ or OH- ions
2. acid precipitation
b. Rain with a pH lower than 5.6
3. pH
c. A solution that has equal numbers of H+ and
OH- ions
3. smog
c. The process of turning an acidic or basic
solution into a neutral (pH 7) solution
4. neutral
d. A chemical that conducts current when
dissolved in a solution
4. acid shock
d. A sudden increase of acid precipitation into a
lake or other ecosystem
5. electrolyte
e. The measurement of the amount of H+ and
OH- ions in a solution
5. neutralization
e. A combination of gases produced by industry
and automobiles; a combination of smoke and
fog
f. A chemical that contributes OH- ions to a
solution
438
f. Rain, snow, or fog with a pH lower than 5.6