47 g
Acids, Bases, and the pH Scale
readi
n
CHALLENGE
W
herever chemical solutions are involved, pH matters. Some
important chemical reactions, such as those involved in
corrosion of iron or digestion of food, will only take place within a
specific pH range. In agriculture, the soil’s pH affects crop yields of fruits,
vegetables, and grains, such as wheat, barley, and corn. The control of pH
is also important within your body. For example, the pH of your blood is
maintained at a slightly basic level of approximately 7.4. In your stomach
an acidic pH between 2 and 4 helps you digest your food. The pH of water
in the environment is also important because it affects the survival of
fish and other organisms. Scientists measure pH when they study acid
rain and when they work to maintain drinking water quality.
What is pH, and how does it affect the quality of water?
Populations of brook trout have been drastically reduced by the effects of acid rain on freshwater
lakes and streams in the northeastern United States.
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Acids, Bases, and the pH Scale • Activity 47
Materials
For each student
1Student Sheet 47.1, “Directed Reading Table:
Comparing Acidic, Basic, and Neutral Liquids”
concept map from Activity 38, “Dissolving Duel”
Reading
Use Student Sheet 47.1, “Directed Reading Table: Comparing Acidic, Basic, and
Neutral Liquids,” to guide you through the following reading.
The pH scale
0.0
1.0
2.0
3.0
Very
acidic
5.0
6.0
7.0
Common range
for most
natural waters
8.0
9.0
10.0
11.0
12.0
13.0
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4.0
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As you observed in the last activity, the pH of a solution is a number that
expresses how acidic or basic it is. The pH scale ranges from 0 for very
acidic to 14 for very basic. At the middle of the scale, 7, is a neutral liquid,
such as distilled water. The scale is not linear with equal increments like
a ruler or a thermometer. The pH scale is a lot like the Richter scale used
to measure the strength of earthquakes. An 8.0 earthquake is 10 times
stronger than a 7.0 earthquake, and a pH of 3 is 10 times more acidic than
a pH of 4. For every increase or decrease in pH of 1.0 unit, the acidity or
basicity of the solution changes tenfold.
14.0
Very
basic
pH Scale
2561 LabAids SEPUP IAPS SB
Figure: PhysSB C 47.04
LegacySansMedium 10/11.5
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Activity 47 • Acids, Bases, and the pH Scale
The color change of a pH indicator results from a chemical reaction between
the indicator and the acid or base dissolved in water. An acidic solution con­tains acid and turns pH paper deep orange or red. A basic solution contains
a base, and turns pH paper blue or violet. Acids are recognizable by their
sour taste and because they can dissolve some metals. Bases—from soap
to bleach to drain cleaner—have other properties. They tend not to react
with metals and are bitter in taste. They are slippery to the touch because
they react with your skin oils to form a substance a lot like soap. However,
both acids and bases can be corrosive to human skin, if the pH is below 2
or above 12.
Acids, Bases, pH, and Industry
Controlling pH is important in the production of foods, paper products,
and chemicals. The low pH of vinegar preserves foods, such as pickles,
because it slows the growth of bacteria and other microbes. Because they are
chemically reactive, acids and bases are also important in the manufacture
of many products. For example, sulfuric acid is by far the largest single
product of the chemical industry and is used to make fertilizer, refine petro­
leum, and clean and process metals. Nitric acid is used to make explosives
and dyes. The bases sodium hydroxide and potassium hydroxide are used to
make soaps. Sodium hydroxide is also the main ingredient in drain cleaners.
Ammonia, a base that contains nitrogen, is used both as a fertilizer and to
produce other fertilizers.
Orange juice and vinegar are two familiar
liquids that contain acids.
C-86
Acids, Bases, and the pH Scale • Activity 47
Drain cleaner and household ammonia are two familiar
liquids that contain bases.
pH and the Environment
If the pH of water in rivers, streams, and lakes becomes too high or too low,
many animals and plants cannot survive. Wastes released by human activity
may disturb the acid–base balance in bodies of water. Even a small change
from the normal range can harm some aquatic life and cause changes in
ecosystems that are hard to predict. Most organisms that live in freshwater
are healthiest at pH values from 6 to 9. Variations in pH outside of this range
reduce populations of organisms by decreasing survival of adults and
reducing reproductive rates.
pH and Drinking Water
The federal government’s standards for drinking water require a pH
between 6.5 and 8.5. Acidic or basic tap water may react chemically with
water pipes and release contaminants into the drinking water. The biggest
hazard is that acid may dissolve lead out of the metal of some pipes into
the water flowing through them. Lead can have serious health effects that
vary with dosage and among different individuals. Young children are
much more sensitive than adults, but adults can also be affected. Lead
acts on the brain, nervous system, and other organs, causing effects from
behavioral and learning problems to seizures and even death.
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Activity 47 • Acids, Bases, and the pH Scale
Analysis
1. In what ways can the pH or acidity of water affect:
a. living things, such as fish?
b.people?
2. Fill in the blanks of the following sentences with the correct number.
a. A solution with a pH of 5 is ___ times as acidic as a solution with a
pH of 6.
b. A solution with a pH of 4 is ___ times as acidic as a solution with a
pH of 6.
3. Add the following words to your concept map from Activity 38,
“Dissolving Duel.”
acid
base
neutral
pH
4. Compare and contrast acids and bases by completing a Venn diagram.
In your science notebook, make a larger version of the Venn diagram
shown below. Compare acids and bases by recording common features
in the space where the circles overlap. Contrast acids and bases by
recording unique features of each kind of substance on the far side of
each circle.
Acids
Bases
IAPS
5. 2561
WhatLabAids
do youSEPUP
predict
willSB
happen to pH when an acid and a base are
Figure: PhysSB C 47.05
mixed
together?
Explain
your
prediction.
LegacySansMedium 10/11.5
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