Learning Set 4 • How Does Air Pollution Affect Other Regions?
4.5 Investigate
How Do Scientists
Measure Acidity?
pH scale: a
measure of the
concentration of
hydrogen ions in a
substance.
neutral: a
solution with a pH
of 7. pH 7 has an
equal number of
hydrogen ions and
hydroxide ions.
acid: a solution
that tastes sour,
has more hydrogen
ions than pure
water, and has a
pH of less than 7.
base: a solution
with a bitter taste,
a slippery feel, and
a pH more than 7.
hydroxide ion:
one oxygen atom,
one hydrogen
atom, and an extra
electron.
You know that the more acidic the solution is, the higher the concentration
of hydrogen ions. Scientists communicate how acidic a solution is using
numbers on a pH scale. The pH scale tells you the concentration of
hydrogen ions in a solution. The name of this scale, pH, stands for “power
of hydrogen.” The scale ranges from 0 to 14. In the middle of the scale
is 7. Solutions with a pH of 7 are called neutral. Pure water, or distilled
water, is neutral and has a pH of 7. Keep in mind that water from a faucet
and rainwater are not pure water. They have substances dissolved in them,
so their pH is not 7. Acids are solutions that have a pH less than 7. The
lower the pH, the more acidic a solution is. This may sound confusing, but
it is important to remember. When pH is lower, a solution is more acidic.
Normal rainwater is slightly acidic, with a pH of about 5.6. The pH of acid
rain is closer to 4.3.
You might wonder about the numbers that are higher than 7 on the pH
scale. Solutions with a pH higher than 7 are called bases. Bases have a
bitter taste, and they tend to feel slimy or slippery. When placed in water,
a base produces hydroxide ions (OH–). A hydroxide ion consists of one
oxygen atom, one hydrogen atom, and an extra electron. The minus sign (–)
shows that the ion that has gained an electron and has a negative charge.
Pure water has fewer hydrogen ions than acids, and bases have fewer
hydrogen ions than pure water.
If a solution has a pH less than 7, then it has a higher concentration of
hydrogen ions than pure water. If it has a pH greater than 7, then it has
fewer hydrogen ions than pure water.
The pH scale ranges
from 0 to 14. Pure
water is neutral and
has a pH of 7. The
pH of an acid is
less than 7, whereas
the pH of a base is
greater than 7.
Project-Based Inquiry Science
pH 0 1
2
3
4
5
6
increasing acidity
7
8
neutral
AQ 208
9
10
11
12
13
decreasing acidity
14
4.5 Investigate
The interval between numbers on the pH
scale is different from the interval on a
number line. On a number line, the interval
between each number is one. On the pH
scale, each number indicates a change of ten
times. For example, a solution with a pH of
4 is ten times more acidic than a solution
with a pH of 5 and one hundred times more
acidic than a solution with a pH of 6.
Car battery acid
is a strong acid.
It measures
1 on the pH
scale. The acid
in oranges is a
weaker acid. It
measures 4 on
the pH scale.
Acid-base Indicators
You know that pH measures the acidity of a solution, but how do you
measure pH? After all, you cannot directly measure the concentration
of hydrogen ions. Scientists have found many different indicators for
measuring pH. An indicator is a tool that can be used to determine the
condition of something that cannot be measured directly.
Red Cabbage Juice
Boiled red cabbage juice has been used as an acid indicator for thousands
of years. People who dyed cloth in ancient times often used the juice from
boiled red cabbage leaves to produce a pale bluish-purple dye. They noticed
that when sour-tasting substances (acids) were added to the cabbage juice,
the dye turned red. When bitter-tasting substances (bases) were added to
the cabbage juice, the dye turned green. Red cabbage juice became one of
the first pH indicators. But it can determine only whether a solution is an
acid or a base. It cannot measure exactly how acidic or basic a solution is.
Red cabbage juice was one of the first
pH indicators.
indicator: a
tool that can
be observed to
determine the
condition of
something.
Acids turn red cabbage juice red. Bases
turn red cabbage juice green.
AQ 209
AIR QUALITY
Learning Set 4 • How Does Air Pollution Affect Other Regions?
lichen: two
distinct organisms,
a fungus and an
alga, living as one.
litmus paper: a
paper made from
wood, lichen, and
other compounds
that is used to
determine whether
a solution is acidic
or basic.
universal
indicator: a test
used to measure a
range of pH’s.
pH paper: a
universal indicator
that determines
how acidic a
solution is.
Litmus Paper
Ancient people also observed that
sour substances changed the color
of a substance called litmus. Litmus
is a dye extracted from lichen. Lichen
is an odd kind of organism. It is both
a fungus and an alga, living as one.
Lichen was often called “dyer’s weed,”
and people used it to dye cloth. Early
scientists also used litmus to indicate
if a solution was acidic or basic.
They knew that litmus changed color
according to the acidity of a substance.
In the 1800s, a scientist developed
litmus paper made from wood, lichens,
and other compounds. An acid turns
blue litmus paper red. A base turns
red litmus paper blue. Litmus paper is
still used today to determine whether a
solution is acidic or basic.
pH Paper: A Universal Indicator
Litmus paper and red cabbage juice
can indicate whether a solution is an acid, but they cannot measure
the strength of the acid. A different type of test is needed for that.
Modern scientists have developed what they call universal indicators.
A universal indicator is a mixture of substances that each changes
color at different pH values. You test how acidic a solution is by dipping
the universal indicator in the solution and observing what color it turns.
An indicator for pH can be a liquid or a solid. The most commonly used
universal indicator for pH is called pH paper. When you dip pH paper
pH paper turns
different colors
to indicate
the pH of a
solution.
Project-Based Inquiry Science
Lichen, a fungus and an alga living as
one, are used to make litmus paper.
AQ 210
4.5 Investigate
into a solution, it changes color to
indicate the pH of the solution.
pH Meters
pH meters can also test the
strength of an acid or a base. A
pH meter measures hydrogen ions
in the solution tested. On most
pH meters, the pH of the solution
is displayed on a digital readout.
This makes pH meters the most
accurate way to measure pH.
A pH meter is the most accurate way
to measure the pH of a solution.
How Acidic Is Each Solution?
Materials
•well plate
Each group in the class will measure the pH of distilled water and 5 different
liquids. You will use pH paper or a pH meter to measure the pH of each
of the liquids. Then you will
report to the class.
•pH paper
Predict
•tea in dropper
bottle
Record the names of the
5 solutions you will be testing
in the first column of your
Acids and Bases page. You
are familiar with some of these
solutions. For each of your
solutions you are familiar with,
predict where it will fall on the
pH scale. Use information you
know about the pH scale and
about each solution to make
your decision. Keep in mind
the characteristics of acids and
bases while you are making
your predictions. Record your
predictions on your Acids and
Bases page.
•coffee in
dropper bottle
•tape
•water in a
dropper bottle
•baking powder
(dissolved
in water) in
dropper bottle
•milk in dropper
bottle
•power drink in
dropper bottle
•pickle juice in
dropper bottle
•window cleaner
in dropper
bottle
•stain remover
(liquid) in
dropper bottle
•liquid antacid in
dropper bottle
AQ 211
AIR QUALITY
Learning Set 4 • How Does Air Pollution Affect Other Regions?
Materials
•aspirin (dissolved
in water) in
dropper bottle
•buffered aspirin
(dissolved
in water) in
dropper bottle
Procedure
1. One member of your group should take the well plate to the solution
station. Place 4–5 drops of water in well A1. Place 4–5 drops of each
of your solutions into wells A2 through A6 in Row A of your well plate.
2. Each member of your group
should have 6 pieces of pH
paper. Take turns testing the
pH of each of the solutions
in your well plate. Use a
fresh strip of pH paper for
each solution. After you see
the color the paper turns,
match its color to the colors
on the pH scale, and record
the pH of each solution on
your Acids and Bases page.
•clear carbonated
beverage in
dropper bottle
•household bleach
in dropper bottle
•vinegar in
dropper bottle
•mineral water in
dropper bottle
•liquid soap in
dropper bottle
•rubbing alcohol
in dropper bottle
•hydrogen
peroxide in
dropper bottle
•liquid shampoo
in dropper bottle
•mouthwash in
dropper bottle
•safety glasses
•Acids and
Bases page
Some of these
solutions can burn
your eyes or skin.
Some are poisonous.
Be sure to wear
safety glasses. Do
not taste any of the
solutions. When
you are finished,
allow your teacher
to dispose of the
solutions.
Analyze Your Data
3. Allow your test strips to dry,
and then tape them on your
Acids and Bases page.
Work with your group to analyze your data, and make sure you all agree on
the pH of each solution. If group members determined a different pH for
any of the solutions, measure the pH of those solutions again. Retest the
liquids until you are satisfied that you have accurately determined their pH.
1. Which of your solutions were neutral? How do you know?
2. Which of your solutions were acidic? How do you know? Which was
most acidic? How do you know?
3. Which of your solutions were basic? How do you know? Which was
most basic? How do you know?
4. Compare your predictions with your results. Which solutions were you
able to predict accurately? Which solutions were difficult to predict?
Why?
Project-Based Inquiry Science
AQ 212
4.5 Investigate
Communicate
Share Your Results
As a class, arrange all the solutions in order from most acidic to most
basic. Each group measured the pH of different solutions. Each member
of the group measured the pH of each of the group’s solutions. As
you are arranging the solutions in order, compare the results of those
measurements. For each liquid for which members of the group recorded
different pH results for the same solution, retest the liquid until you are
satisfied that you have accurately determined the pH.
Earlier in the Unit, you saw that, when you diluted colored water, you
produced solutions that had fewer parts per million of the dye. Dilution is
also a way to change the acidity of a solution. Choose one of your solutions,
and predict what would happen to its pH if you added water to the solution.
What Is the pH of Acid Rain?
You cannot directly measure the acidity of acid rain, because you most
likely do not have a sample of acid rain in front of you. You can, however,
model acid rain and measure the acidity of the water in your model. You can
model acid rain by capturing smoke from combustion and dissolving it in
water. It will not have all of the products of combustion from a coal-burning
power plant, but it will have some of them. Your teacher has burned some
paper, captured the smoke, and then dissolved the smoke in water. You will
measure the pH of the resulting solution.
Procedure
Slowly open your bag containing the solution of smoke dissolved in water,
and pour the solution into the cup. Dip a strip of pH paper in the water.
Record your observations.
Materials
Analyze Your Data
•bag with smoke/
water solution
•pH paper
•clear plastic cup
1. What was the color of the pH paper after it was dipped in the
smoke/water solution? Use the pH scale to determine the pH of the
smoke/water solution.
2. How did the pH of the smoke/water solution differ from the pH of the
distilled water you tested in the previous investigation? What does that
tell you about the smoke/water solution?
AQ 213
AIR QUALITY
Learning Set 4 • How Does Air Pollution Affect Other Regions?
Reflect
1. Compare the acidity of the smoke/water solution to the acidity of the
household liquids you tested earlier. Which liquid is closest in acidity to
the smoke/water solution?
2. Normal rainwater is slightly acidic (pH = 5.6). How much more acidic is
the smoke/water solution? Why do you think rainwater that contains the
products of combustion is more of a problem for the environment than
normal rainwater?
3. Your smoke/water solution had only some of the products of combustion
dissolved in it. How do you think the pH of the smoke/water solution
compares to the pH of acid rain? Why?
4. Earlier in the Unit, you learned about measuring concentration using
parts per million (ppm) and parts per billion (ppb). pH is also a measure
of concentration. How do you think the concentration of hydrogen ions
in ppm or ppb is different when pH is higher than when it is lower?
Update the Project Board
Acid in the air is a big problem in the Adirondacks. As air pollutants mix
with water, acid rain is formed. Add your understanding of acids and acid
rain to the Project Board in the What are we learning? column. Be sure to
include evidence from your reading and from your investigations that help
you to support your learning. You may also have some new questions to
add to the Project Board. For example, you still do not know exactly how it
causes damage to the plants and animals.
What’s the Point?
All substances can be classified as acids, bases, or neutral. Acids produce
hydrogen ions when placed in water. The concentration of hydrogen ions
in a solution is a measure of its acidity. Hydrogen ions cannot be measured
directly, so indicators are used to determine whether a solution is an acid,
a base, or neutral. Acid-base indicators include red cabbage juice, litmus
paper, and pH paper. A color change in the indicator shows the presence
of an acid or a base. No color change indicates a neutral substance. The pH
of a substance is a measurement of the concentration of hydrogen ions in a
solution. The pH scale has a range of 0-14 and indicates the strength of an
acid or a base. When the pH changes by one number on the scale, the level
of acidity changes by a factor of ten.
Project-Based Inquiry Science
AQ 214