chapter6lab (doc)

Open-Ended Inquiry • Design Your Own Lab
Chapter 6 Lab
Acid Rain and Seeds
How does acid rain affect seed germination?
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 foodstoring 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 Focus
Design an Experiment, Organize Data, Measure, Graph
• 5 large test tubes
• test-tube rack
• glass-marking pencil
• 25-mL graduated
• 60 mL distilled water
• 20 mL vinegar
• food coloring
• pipette
• pH paper
• 120 dried beans
• paper towels
• zip-close plastic bags
• stick-on labels
• hand lens
• graph paper
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?
2. Infer How will you know that a seed has germinated?
3. Use Models What do the solutions represent?
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.
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.
Data Table 1: pH of Solutions
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.
10.Control Variables What will your independent variable be? What will
your dependent variable be?
Independent Variable:
Dependent Variable:
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:
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.
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.
Data Table 3
pH of Solution
Percentage of Seeds
2. Graph Use the results from Data Table 3 to make a graph. Plot pH on the xaxis and the percentage of seeds germinated on the y-axis. Ask your teacher
whether you should use graph paper, a graphing calculator, or graphing
3. Interpreting Graphs Does the graph support the hypothesis you made in
Part B of the procedure? Why or why not?
4. Infer At the time that bean plants and other seed plants evolved, what do
you think the pH of rainfall was, and why?
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.
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?
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.