Purpose: To compare the amount of Carbon Dioxide (CO2) in four different sources of gases.
(Enough for each team of two or four students)
5 vials or test tubes
A graduated cylinder
A funnel straw
A marble-size piece of modeling clay
4 different colored balloons
A narrow-necked bottle (the neck should be narrow enough for a balloon to fit over it)
A dropping bottle of bromthymol blue indicator solution
A dropping bottle of dilute household ammonia (1 part ammonia to 50 parts distilled water)
100 mL vinegar
5 mL baking soda
Safety goggles for wear at all times
Teacher's Lab Notes:
The students will be filling balloons with pure carbon dioxide, exhaled air, and ambient air. For safety reasons, you should fill the balloons with automobile exhaust. You should wear thick gloves to protect your hands from being burned. Fill the balloons in an open area and when a slight breeze is blowing to keep the exhaust gases away from your face. Place a balloon over the narrow end of a metal funnel and place the wide end of the funnel over the exhaust pipe of a running car. When inflated, the balloons should be about 7.5 cm in diameter. It may be easier to overinflate the balloon and then let a little gas escape. Twist and tie the balloon. Repeat the procedure with the same color balloon until you have one for each lab group.
The ambient air solution in vial A will not turn yellow. The level of CO2 in ambient air is too low to affect bromthymol blue.
Students will need around 60 drops of the diluted ammonia to neutralize the solution in vial D (vinegar-baking soda reaction). The other two vials should require between 7 and 40 drops. Caution students to add the drops slowly and shake solutions between drops so they can get a careful record of when the color changes back to the same color blue as the control.
Since the students will have to add a relatively large amount of ammonia to the solution in vial D, the color of this sample may be affected by dilution. To equalize this effect, you can have students add some water to the other samples to make the volume in each sample equal. This is easiest to do if sample D is titrated last.
1. Add 15 mL of water and 10 drops of bromthymol blue indicator solution to each vial or test tube. Label the vials A, B, C, D, and Control.
2. Fill each balloon until it has a 7.5 diameter.
Sample A (Ambient Air) - Use a tire pump to inflate the balloon to the required diameter. Twist the rubber neck of the balloon and fasten it shut with a twist tie. The tie should be at least 1 cm from the opening of the balloon. Record the color of the balloon used for this sample.
Sample B (Human Exhalation) - Have one team member blow up a balloon to the required diameter. Twist and tie the balloon, and record balloon color.
Sample C (Automobile Exhaust) - Your teacher will supply you with this balloon. Record the color.
Sample D (Nearly pure CO2) - Put 100 mL of vinegar in the narrow-necked bottle. Using a funnel, add 5 mL of baking soda. Let the mixture bubble for 3 seconds to drive the air out, then slip the balloon over the neck of the bottle. Inflate the balloon to the proper diameter. Twist, tie, and record the color.
3. Soften the clay and wrap it around one end of the straw to make a small airtight collar that will fit into the neck of a balloon. The collar should look like a cone with the straw in its middle, and should be large enough to plug the neck of the balloon.
4. Pick up Balloon A. Keeping the tie on it, slip the balloon's neck over the clay collar and hold it against the collar to make an airtight seal. Place the other end of the straw into the vial of water and bromthymol blue labelled A. Have another partner remove the tie on the balloon and slowly untwist the balloon. Keeping the neck of the balloon pinched to control the flow of gas, gently squeeze the balloon so the gas slowly bubbles through the solution.
5. Repeat the same procedure with the other balloons and their respective vials. In some cases, the bromthymol blue solution will change color, from blue to yellow, indicating the presence of carbonic acid formed from CO2.
6. Analyze each of the samples by titrating them with drops of dilute ammonia. Ammonia neutralizes the carbonic acid. The bromthymol blue will return to a blue color when all the acid has reacted. Add drops of ammonia to each of the samples that turned yellow, carefully counting the number of drops needed until they are about the same color as your control. Record the results.
Post Lab Discussion:
Make a chart on the board to pool each group's results. Ask the students which samples had the most and the least carbon dioxide. Why didn't the ambient air sample not turn yellow? (The test isn't sensitive enough to detect low concentrations of CO2.) Carbon dioxide is a natural part of our atmosphere, but too much CO2 could make the Earth warmer through an increased greenhouse effect. Why is automobile exhaust a concern? What ways could you reduce the amount of CO2 you create? How could a city reduce the amount of CO2 they emit? What's more important, to develop and adapt cars with a new fuel that's safe for the environment or to improve public transportation systems? What alternative power sources could be used with cars? (Solar, electric, methanol.) Why might it be difficult for the public to start using an alternative source? (Car industry not mass-producing new cars, expense of buying new car, less power/speed than gas- powered car.)
This activity is used with the permission of Climate Protection Institute and the Global Systems Science (GSS) project at Lawrence Hall of Science, University of California, Berkeley. To receive more information about GSS and other activities visit www.lawrencehallofscience.org/gss. This activity originally appeared in The Science Teacher, May 1989.