IOWA 4-9 SCIENCE PROJECT
TEACHER GUIDE for: pH and Acid Rain Lew Stoneking
Ecology.890 Grades 8-9
CONCEPT OBJECTIVES:
Throughout the learning cycle the students are to become
familiar with the testing procedures for pH, to learn that
the pH scale is a logarithmic scale, to be able to design
and analyze an experiment, to gain insight into how to solve
the problems that are associated with acid rain.
PROCESS OBJECTIVES: Thinking skills developed in the cycle
include observing, measuring, recording, and inferring.
TEACHER BACKGROUND:
The acid rain problem is becoming paramount in the
Northeastern part of the United States. However, with
increasing industrial wastes and changing wind patterns, the
threat of a changing soil pH in the fertile agricultural
growing regions of the country may eventually lead to
unsuitable growing areas.
It should be noted that the Mid west region at present
does NOT have a large problem with acid rainfall.
EXPLORATION:
The students will be given various solutions to determine
their pH value. Do remember, this is an exploration phase
and no real concept needs to be emphasized, though the
student should realize that different solutions give
different pH readings.
* Prepare solutions for testing- examples; dilute NaOH
(lye), saliva, fresh river or creek water, milk, rain water,
orange or tomato juice, soda pop, vinegar, dilute HCL
(hydrochloric acid), etc.
* Supply pHydronion paper 1-14 (strips 3 cm in length)
* Have the students do the pH lab as described in Student
Activity One entitled "pH ... AN INTRODUCTION". (see
student sheets)
As the students finish the lab, ask if they have learned
anything about the various substances or if they see any
particular patterns or sequence to the pH values of the
various solutions.
CONCEPT DEVELOPMENT:
Activity 2 (See Student Sheets) introduces the pH scale and
how it relates to acids an bases. Have the students
indicate on the scale provided where the solutions tested in
the exploration activity would be located.
Activity 3 concentrates on the concept of the pH
logarithmic scale. The students are to make serial
dilutions from a stock solution of pH 2. The process is
further outlined in Student Activity Three. (see student
sheets) The student should understand the concept of the
lab is to know the movement from one level of pH to another
level of pH is NOT a linear move, but through a LOGARITHMIC
scale. The example given in the student material is that a
solution of a pH of 6 has ten (10) TIMES the amount of
acidic concentration as a solution of a pH of 7. The
ability to control the pH of a solution will be useful in
the following application activity.
APPLICATION:
Have the students develop a DOUBLE BLIND EXPERIMENT that
would test the effect of acid rain on either daphina, bean
seeds or bean plants.
The DOUBLE BLIND EXPERIMENT could be set up like the
example Student Activity Four included in the packet. (see
student sheets)
EVALUATION:
You, the teacher, have many places to observe and evaluate
the activities done by the students in these activities.
Exploration - observe if the students can make the
connection that what they are measuring is the
concentrations of H+ ions in solution and they should
realize that the pH differs in different substances.
Concept Development- as the students compared different
solutions made by serial dilutions they should understand
that pH is a logarithmic scale that moves from level to
level in 10X the amount of acid or base, depending on the
direction.
Application - producing their own lab design... stating the
problem clearly, hypothesis formation, putting procedural
steps together that will result in a controlled lab design
that will give good data, etc.
STUDENT ACTIVITY ONE pH ... AN INTRODUCTION
Materials needed: prepared solutions, strips of pH indicator
paper
You are now going to test various solutions with a
special indicator paper. Place one drop of a solution on a
strip of pH paper provided and immediately compare the color
of the paper against the color chart.
Read and record the corresponding pH number and the name
of the solution on the chart below. Then repeat the process
until you have found the pH value all solutions.
Unknown Sample pH Solution
1 _____ _________________________
2 _____ _________________________
3 _____ _________________________
4 _____ _________________________
5 _____ _________________________
6 _____ _________________________
STUDENT ACTIVITY TWO
The pH, a numerical value used to describe the strength
of an acid, is determined by a mathematical formula based on
a solution concentration of hydrogen ions (H+). Acids are
solutions whose hydrogen ion concentration is greater than
that for pure distilled water. Bases are solutions whose
hydrogen ion concentrations is less than that for pure
distilled water.
The pH scale ranges from a numerical value of 0 to 14. A
value of pH 1 is very acid (battery acid), where a pH of 7
is neutral (distilled water), and a pH of 13 is very
alkaline (lye).
pH SCALE
-- 14 --
^ -- 13 --
|
-- 12 --
Increasingly
Basic -- 11 --
-- 10 -- desert pool water
-- 9 --
----------------------------------------------------------
-- 8 -- sea water L
human blood I
NEUTRAL POINT--> -- 7 -- saliva F
E
-- 6 --
acid rain water E
-- 5 -- X
I
-- 4 -- acid water in peat bogs S
orange juice T
----------------- -- 3 -- cola drink ----------- S
-- 2 -- stomach acid contents
Increasingly
Acidic -- 1 --
|
v -- 0 --
Indicate on the pH scale above the locations of the
solutions you tested in the previous activity.
STUDENT ACTIVITY THREE
The pH values on the scale used in the previous
activity are logarithmic... that is, they increase by a
factor of 10 between each pH value. Therefore, a solution
with a pH of 6 is 10 times more acidic than a solution with
a pH of 7. Thus a solution with a pH of 5 is 100 times more
acidic than a solution with a pH of 7: hence, pH 4 is 1000
times more acidic: pH 3 is 10,000 times more acidic: and so
on...
In this activity, you will be making serial dilutions
in order to make various concentrations of acid solutions.
#
#Materials: a spot plate or small test tubes, test tubes (
20 ml capacity), stock solution bottle (containing pH 2
acid), beaker of distilled water (250 ml), pH paper (narrow
range 3 - 6.5), graduate cylinders (10 ml)
A serial dilution is done by using a small sample from
a stock solution and diluting it with distilled water (pH 7)
to make a reduction from the original sample. In this
activity, you will be using a stock acid solution that has a
pH of 2. To make solution A take 1 ml. of stock solution ad
mix it with 9 ml. of distilled water, label this Solution A.
Solution be is made with 1 ml. of Solution A and 9 ml.
of distilled water. Continue making serial dilutions until
you have a sample of Solution A - D.
After you have made all four solutions. Test the pH of
each and record below:
SOLUTION COMPONENTS pH VALUE
* Did you see any pattern result from the serial
dilutions?
* How are the pH values related when solutions are mixed
in the 10 to 1 ratio that your serial dilutions were made
in?
Further Dilutions (OPTIONAL)
Experiment further to see if you can get different pH
values that fit in between those that you made..... What is
the proper amount of stock versus distilled water for a
1. pH 3.5 solution? ______ ml. stock ______ ml. water
2. pH 5.5 solution? ______ ml. stock ______ ml. water
3. pH 7.0 solution? ______ ml. stock ______ ml. water
STUDENT ACTIVITY FOUR The Effect of Acid Rain
Precipitation is defined as being acidic if the pH is less
than 5.6, which is the pH of pure rain water. It is thought
that the slight natural acidity of normal rain is controlled
by carbonic acid, which is formed by the reaction of
atmospheric carbon dioxide with water. Under unpolluted
conditions, the pH of "pure" rain water was probably not
controlled by carbonic acid alone. Release of sulfur
dioxide from volcanoes might have caused a higher natural pH
in other areas.
In this activity you will try to investigate the possible
effects of "acid rain" on living organisms. As a member of
a small group you will design a controlled experiment to
test the effects of varying concentrations of acid on either
daphnia or the germination of bean seeds. The actual lab
will vary from group to group.
The type of lab design you are to use is called a DOUBLE
BLIND EXPERIMENT. A double blind experiment is a valuable
tool in science. It is planned by a designer ad conducted
by some other experimenter. The experimental results are
then given back to the designer for analysis. Since this
lab will be designed and carried out by different groups
(maybe in another class period) it is very IMPORTANT that
the design be as thorough and as accurate as possible. You
should exercise caution in your lab design because of the
potential hazards involving concentrated acids. Your lab
should include the following parts;
PROBLEM - In order to state the problem you must be familiar
with the background of what is to be investigated. The
problem statement should be in question form. For
example... How does agent orange affect the growth of living
cells?
PROCEDURE - Since another group will be conducting your lab,
it is important that your procedural steps be sequential and
detailed.
HYPOTHESIS - This is actually a prediction, in other words,
what you think will result from the experiment. Then,
explain your reasoning behind your prediction. For
example...I predict that agent orange will impair the
growth of living cells due to the fact that it is a
synthetic organic compound that will interact with cell
chemistry causing an interference with normal cell
processes.
OBSERVATION - This is room for the experimenters conducting
the investigation to record any observations. The designers
should include any data tables that are necessary.
CONCLUSIONS - Questions should be included here to help the
experimenters analyze the information gained in the
investigation.
After completion of the lab the following questions should
be answered and given with the lab back to the experimental
designer group. Be as complete as possible in your
responses.
EXPERIMENTAL CRITIQUE
1. What variables did you investigate in the lab?
2. Would you, based on the results of the lab, accept or
reject you hypothesis?
3. What experimental error, if any, could have taken place
in the lab?
4. Were all the materials available to do the experiment?
5. Were the directions and procedures clear and
understandable?
6. What are the good points of this experimental plan?
7. Was the time allowed within proper limits?
8. Is there an experimental control? Is it adequate?
9. How could this experiment be re-designed to better test
the problem statement?