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Open-Ended Inquiry • Design Your Own Lab
Chapter 6 Lab
Acid Rain and Seeds
Big Idea
Problem
Humans affect regional and
global environments through
agriculture, development, and
industry.
How does acid rain affect seed germination?
Introduction
Every seed contains a tiny living plant and a food supply to nourish that
plant when it first begins to develop. The plant and the food supply are
enclosed in a protective coat, which keeps the contents of the seed from
drying out. The tiny plant can survive for weeks, months, or even years.
Factors such as temperature and moisture determine when the plant
within the seed will start to grow again. This resumption of growth is
called germination.
Before seeds can germinate, they must absorb water, which causes the
food-storing tissues to swell and crack the seed coat. The root is the first
part of the plant to emerge from the seed.
All rain is mildly acidic because carbon dioxide forms a weak acid
when it dissolves in water vapor. If the pH of rain falls below 5.0, the
rain is usually classified as acid rain. In this lab, you will design an
experiment to determine whether the pH of the water that a seed absorbs
can affect germination.
Skills Objectives
Students will be able to
• form a hypothesis about
the effect of acid rain on seed
germination.
• design an experiment to test
the hypothesis.
• do a serial dilution.
Preparation Time
30 minutes to prepare all the
solutions and 10 minutes to
prepare just the stock solution
Class Time
Part A: 25 minutes
Part B: 20 minutes to design,
15 minutes to set up, and
15 minutes to collect data and
clean up
Group Size
Small groups
Skills Focus
Design an Experiment, Organize Data, Measure, Graph
Materials
Materials
• 5 large test tubes
• test-tube rack
• glass-marking pencil
• 25-mL graduated cylinder
• 60 mL distilled water
• 20 mL vinegar solution
• food coloring
• pipette
• pH paper
• 120 dried beans
• paper towels
• zip-close plastic bags
• stick-on labels
• hand lens
• graph paper
If you are not using probes,
provide both wide range and
precision-range pH paper.
If you have test tubes with
screw tops, students can mix
the solutions by shaking.
Stopper test tubes without
screw tops if you store the
solutions overnight.
Advance Preparation
To make 400 mL of stock
solution with a pH close to 4,
mix 0.3 mL of vinegar with
400 mL of distilled water.
Time the initial setup for
Part B so that students can
collect data after 72 hours.
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Teaching Tip
To help review pH, ask
students whether they could
use vinegar to make a solution
with a pH of 8.0. (A solution
with a pH of 8.0 is a basic
solution. Even a very dilute
vinegar solution will still have
more hydrogen ions than
hydroxide ions. Its pH can
approach 7.0 but cannot be
equal to or greater than 7.0.)
Date
Safety
Wear goggles and a lab apron when you handle the vinegar. Rinse off
any solution that spills on your skin or clothing. If you use glass test
tubes or graduated cylinders, check for cracks or chips. Alert your
teacher if you break a glass object. Wash your hands thoroughly with
soap and warm water before leaving the lab.
Pre-Lab Questions
1. Design an Experiment What do you think the purpose is of adding
food coloring to the vinegar in Part A?
Sample answer: As the vinegar is diluted with water, the intensity of the
color will decrease. The food coloring provides a visual indicator for the
decreased concentration of acid (H ⫹ ions) in the solutions.
2. Infer How will you know that a seed has germinated?
Sample answer: The seed coat will crack and a root will be visible.
3. Use Models What do the solutions represent?
Sample answer: The solutions represent rain with different
concentrations of acid.
Procedure
For your experiment, you will need a set of solutions with different pH
values. Your teacher may have prepared the solutions in advance, or
you may have to prepare the solutions yourself. Either way, read
Part A of the procedure to understand how to do a serial dilution. A
serial dilution is the process that is used to make a set of solutions in
which the concentration decreases by the same increment from one
solution to the next.
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Part A: A Serial Dilution of Vinegar
The white vinegar sold in stores contains about 5 percent acetic acid
and has a pH of about 2.4. The stock solution of vinegar you will use
has a pH of about 4. You will use the stock solution to make a series of
solutions with different pH values. Each solution in the series will be
4 times as dilute as the previous solution.
1. Put on your safety goggles and apron. Place 5 large test tubes in a
rack. Label the test tubes Stock, 4×, 16×, 64×, and 256×.
2. Use the graduated cylinder to place 15 mL of distilled water in the
test tubes labeled 4×, 16×, 64×, and 256×. caution: Do not add
distilled water to the test tube labeled Stock.
3. Use the graduated cylinder to add 20 mL of the stock solution to the
test tube labeled Stock. Then add 4 drops of food coloring.
4. Use a pipette to transfer 5 mL of the stock solution into the test tube
marked 4×. You can mix the solution by quickly pumping a small
amount of solution into and out of the pipette a few times.
caution: Rinse off any solution that spills on your skin or clothing.
5. Transfer 5 mL of the 4× solution into the 16× test tube and mix.
6. Repeat the transfer of 5 mL of solution from one text tube to the next
until 5 mL of solution has been added to the test tube labeled 256×
and the solution mixed.
7. Remove and discard 5 mL from the text tube labeled 256×.
8. With your teacher’s guidance, select the proper equipment to
measure the pH of each solution—either pH paper or a pH probe. If
you will be using a probe, see your teacher for instructions. Record
the results in Data Table 1.
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Step 7
At the end of this step students
should have 15 mL of solution
in each test tube.
Data Table 1: pH of Solutions
Dilution
pH
Stock
4×
16×
64×
256×
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Part B: Design an Experiment
9. Form a Hypothesis How will changing the pH of a water-based
solution affect the percentage of seeds that germinate? Record the
hypothesis you will test to answer this question.
Hypothesis: Sample answer: As the pH of the solution increases, the
percentage of seeds that germinate will increase.
10. Control Variables What will your independent variable be? What
will your dependent variable be?
Independent Variable pH of solution
Dependent Variable: percentage of seed germination
Step 11
Students may be concerned
that seeds need light to
germinate. Remind them that
the built-in food supply will
sustain the plants until they
can produce food through
photosynthesis.
11. Describe Your Plan Record the details of your plan. You will have
120 dried beans and the solutions from Part A. Which variables will
you need to control? Before you begin, have your teacher review
your plan.
Experimental Plan: Students will need to control the number of seeds,
the amount of solution, the time, and where the seeds are stored.
Students can wrap the seeds in a paper towel, soak the towel with
15 mL of solution, and store the towel in a labeled zip-close plastic bag.
Make sure there is no standing water in the bag and that there is air in
the bag before it is sealed. The seeds can be stored in a warm location
without direct sunlight.
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12. Organize Data Construct a data table in the space below. You will
need columns for the pH values of the solutions being tested and for
the number of seeds that germinate. You might want to include a
third column for other observations.
13. Disposal After you gather your data, follow your teacher’s
instructions for cleanup and disposal of materials. Then wash your
hands thoroughly with soap and hot water.
Sample Data
Data Table 2: Part B Results*
pH
Number of Seeds
Germinated
Observations
4.0
0
Seeds absorbed less of the solution as
compared to the other trials.
4.6
2
Seeds absorbed less of the solution as
compared to the other trials.
5.2
11
5.8
15
6.4
10
Most roots were > 1 cm in length.
Most roots were < 1 cm in length.
*The sample data is from trials with Great Northern white beans. Each
trial used 20 beans. The data was collected after 72 hours.
Analyze and Conclude
1. Calculate Use your data to calculate the percentage of seeds that germinated at
each pH. Record the results in Data Table 3.
Sample Data
Data Table 3
pH of Solution
Percentage of Seeds
Germinated
4.0
0
4.6
10
5.2
55
5.8
75
6.4
50
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2. Graph Use the results from Data Table 3 to make a graph. Plot pH on the x-axis
and the percentage of seeds germinated on the y-axis. Ask your teacher whether
you should use graph paper, a graphing calculator, or graphing software.
3. Interpreting Graphs Does the graph support the hypothesis you made in Part B
of the procedure? Why or why not?
Sample answer: The percentage of seeds does initially increase as the pH increases,
but at pH 6.4 the percentage starts to decrease again. So my hypothesis is only
partially supported.
4. Infer At the time that bean plants and other seed plants evolved, what do you
think the pH of rainfall was, and why?
Sample answer: I think the pH of rainfall was greater than 5.2 and less than 6.4
because the highest percentage of seeds germinated within that range of pH.
5. Evaluate Identify at least one way that this lab was not a perfect model for the
effect of acid rain on the germination of seeds.
Sample answer: Acid rain contains acids such as nitric acid and sulfuric acid, not acetic
acid. Students may also say that in nature the rain would be absorbed by soil, not by
a paper towel.
6. Relate Cause and Effect Explain why acid rain could harm animals as well
as plants. Which types of animals do you think would be most at risk from
acid rain?
Students are likely to say that animals also could be harmed because water is an
essential resource for animals as well as plants. Animals that live in water would be
most at risk. (Animals could also be harmed if the plants they need to consume are
harmed.)
Extend Your Inquiry
Is the average pH of rain the same in all parts of the United States? Look at a recent
map of pH data collected from field stations across the lower 48 states. Identify any
pattern in the data and try to explain this pattern.
Provide a recent national trends map from the National Atmospheric Deposition
Program. Students should note that pH values for rain at field stations east of the
Mississippi are lower than those for field stations west of the Mississippi. Possible reasons
for this trend are the location of power plants and other industries that burn fossil fuels;
the effect of population density on the number of homes heated with fossil fuels and the
amount of traffic; and the fact that winds tend to move air from west to east.
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