Open-Ended Inquiry Design Your Own Lab. Chapter 6 Lab Acid Rain and Seeds. Problem. Introduction. Skills Focus

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1 Chapter 6 Lab Acid Rain and Seeds Open-Ended Inquiry Design Your Own Lab Problem 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 Focus Design an Experiment, Organize Data, Measure, Graph Big Idea Humans affect regional and global environments through agriculture, development, and industry. 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 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 Materials 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. 39

2 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.) 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. 40

3 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 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 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. 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 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. 42

5 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 Observations Germinated Seeds absorbed less of the solution as compared to the other trials Seeds absorbed less of the solution as compared to the other trials 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

6 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. 44