This activity is taken from the Global System Science Series developed by Cary Sneider and Richard Golden and developed at the Lawrence Hall of Science, University of California, Berkeley, CA 94720
Which source do you think has a greater concentration of carbon dioxide: your breath, exhaust from a car, or the air in the classroom? Discuss this with your teammates and write down your prediction. Then determine whether or not you were right by first taking gas samples and then testing the samples to determine which has a greater concentration of carbon dioxide.
Each investigating team of 3-4 students will need:
5 clear vials
1 graduated cylinder (100 ml)
1 cup of water
1 bottle of BTB solution with a dropper (Bromthymol Blue)
1 bottle of ammonia solution with a dropper
1 narrow-necked bottle (such as a wine bottle)
4 balloons of different colors with twist ties
1 teaspoon of baking soda
100 ml of vinegar
3 sheets of blank paper
1 piece of string, about � meters in length
Part 1. Chemical Test for Carbon Dioxide
Teams will need to work together in this experiment, because most of the steps require more than one pair of hands. For example, one person will need to hold the bottle in place while another person slips the balloon over the end.
Make a sample of pure carbon dioxide (CO2). Use the graduated cylinder to pour 100 ml of vinegar into the narrow-necked bottle. Use a scrap of paper to make a funnel. With the funnel, put 1 teaspoon of baking soda into the bottle. Let the mixture bubble for 1 second to drive the air out; then slip a balloon over the neck of the bottle. The balloon should inflate to a 7-10 cm diameter. (If it doesn�t, add more baking soda and try again.)
Secure and measure Sample A (pure carbon dioxide). Twist the rubber neck of the balloon, and fasten it shut with a twist tie. Record the color of the balloon on the first sheet of paper. Call it �Sample A.� Measure the circumference of the balloon with a string. This will be the standard volume of the other samples of gas that you will collect and test.
Make Sample B (Room Air). Blow up a second balloon using a bicycle pump (do not use your breath). Measure its circumference with a string. Add more air or allow some to escape unit it is the same size as the sample of pure carbon dioxide . Secure it with a twist tie. Record the color of the balloon on the data sheet under Sample B.
Prepare the test vials. Use the graduated cylinder to measure and pour 15 ml of BTB solution into each of three vials. Place the vials on the data sheet in the circles marked A, B, and C. Vial C will be the control vial. This vial will remain untouched; no gas will be bubbled through it.
Test Sample A. ( This step requires two people working together.) Insert the end of the straw into the neck of the balloon with gas Sample A. Wrap the neck of the balloon around the straw so it makes a tight seal, but do not remove the twist tie yet. Insert the other end of the straw into the bottom of vial A. Remove the tie and slowly untwist the balloon so the gas will escape through the straw. It is important to untwist slowly so that the gas in the balloon does not come out too quickly and splash the liquid out of the vial. Allow all the gas in the balloon to bubble through the BTB solution in vial A.
Observe color changes. Observe the color of the solution as you slowly bubble the gas through it. What shades of color does it pass through before reaching a final color when all of the gas has been bubbled through it? Record the final color of the solution on the data sheet.
Test Sample B. Bubble Sample B from its balloon through the BTB solution in vial B. Compare its color with the blue of the control vial. Record the color of the solution on the data sheet. Save the solutions for later comparison.
Draw conclusions. Compare the colors of vials A, B and C (the control). What do your observations tell you about how BTB can be used as a chemical test for the concentration of carbon dioxide in a sample of gas. Write down your suggestions on the data sheet.
Collect samples of Human Breath and car exhaust and test against the control using the same methods. These will be vials D and E.
Part 3. Measuring Carbon Dioxide
In the previous experiment, we were able to determine which sample had a larger concentration of carbon dioxide by comparing the color of BTB solutions after equal-sized samples of gas were bubbled through them. In this part of the activity, you will measure the percentage of carbon dioxide gas in each of the samples.
Slowly add dilute ammonia to Vial D, drop by drop. Count the number of drops needed to return the solution in vial D to the same color as the control vial �. Shake the contents of the vial when the solution is close to the blue color of the control vial to thoroughly mix the contents. When the solution remains the same deep blue of the control vial after shaking, record the number of drops. This process is called titration.
Use the titration procedure on vials A, B, and E in the same way. Add the ammonia, drop by drop, to the vials that are very yellow in color last. They may require up to 100 drops to turn them back to the same color as the control vial.
Note: If the vial is about to overflow, pour the contents into a larger clean container, and continue adding drops. In that case, the color of the solution will become very pale. Add water to the BTB control vial so that the volumes of the two solutions are about the same. Then you will be able to compare the color of the test solution with the color of the control vial.
Analyze the results. The number of drops of ammonia required to neutralize the solution is proportional to the concentration of carbon dioxide in the gas sample that was bubbled through it. For example, if it required 90 drops to neutralize the vial from Sample A (pure Carbon Dioxide), then a sample that required only 45 drops to neutralize had a concentration of about 50% carbon dioxide. Write down the percentage concentration of carbon dioxide in each of the samples.
Graph you results. When you have finished testing vials A, B, D, and E, make a bar graph showing how many drops were required to turn each of these solutions back to the same color blue as the control vial. Be certain to label both axes of the graph.
Draw conclusions. Discuss these questions with your teammates, and write down what you think are the best answers.
a. If a gas sample is suspected to contain carbon dioxide but its presence is not indicated by the BTB test, what conclusion can you draw?
b. What does this experiment tell you about the concentration of CO2 emitted in animal breath and vehicle exhaust? (compare them.)
c. What additional information would you need to judge how much CO2 animals and vehicles each contribute to the total atmospheric concentration of CO2?
d. Currently there are about 180,000,000 gasoline-burning vehicles on the road in the U.S. alone. In the future, there is likely to be many more. How do you think this may affect the concentration of carbon dioxide gas in the atmosphere?