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Creating Carbon Dioxide

Based on an activity in Global Warming and the Greenhouse Effectt
Published by Lawrence Hall of Science, 1990
Great Explorations in Math and Science (GEMS)

Teacher Page

Hypothesis:
Human respiration produces carbon dioxide.
 
Materials:
Bromothymol Blue solution (BTB)
2 Balloons of different color of at least 7 inch diameter
2 Drinking straws
Bicycle pump
Masking tape
2 50 mL graduated cylinders
 Procedure:
    1. Pour 25 mL of BTB into each of the two graduated cylinders and set them aside.

      2. Students should be familiar with the properties of BTB solution.  The liquid is normally blue but will change to a golden yellow color in the presence of an acid.  A weaker acid may create a green color.  When carbon dioxide is in water, it creates carbonic acid.  You may want to demonstrate this by adding a small amount of vinegar to a BTB solution.
    2. Put a straw into the neck of each balloon and secure with masking tape.
    3. Inflate a balloon by blowing through the straw and pinch the balloon closed at the neck.
    4. Use the bicycle pump to inflate a second balloon of a different color and pinch the balloon closed at the neck.
    5. Release the amount of air to make it the same size as the first balloon and then seal with a twist tie.
    6. Place the straw of one balloon into the BTB in the graduated cylinder so the opening of the straw is in the liquid of the cup.
    7. Release the gas in the first balloon  to allow the gas to bubble through the BTB.
    8. Repeat steps 6 and 7 with the second balloon.

Observations:
Compare the color of the liquid in the two graduated cylinders.  Record your observations in the space below.
Graduated cylinder with gas from air pump- Little or no change.
Graduated cylinder with gas from exhalation- A color change should be observed. The degree of change will vary but should at least be noticeably green.
Conclusion:
Did your observations support your hypothesis? Defend your answer.
A change in color indicates the presence of carbon dioxide therefore the gas collected in the balloon from exhaling contains carbon dioxide.
Follow up:
Name one variable in this experiment that you could investigate in a follow up experiment. Describe how you could carry out this experiment.
A follow up  experiment could be done by testing gas from different sources, different amounts of BTB, or by re-breathing into the balloon. Allow students to try their own investigation. The assessment rubric is used to grade the follow up for student understanding.
 
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