Where in the World is Carbon Dioxide?

Modified with permission from:

Global Climates - Past, Present, and Future, S. Henderson, S. Holman, and L. Mortensen (Eds.), EPA Report No. EPA/600/R-93/126. U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC, pg. 65 - 76.

The Greenhouse Effect: Global Climate Change, M. C. MacCracken, W.E. Pence, and S.J. Armstrong (Eds.), Lawrence Livermore National Laboratory, Department of Energy, Livermore.

In this multi-part activity, students will set up experiments to help them better understand the atmospheric portion of the carbon cycle.


Carbon dioxide () provides the bubble in your soda pop and the "rise" in your baked goods. But it is also a very significant greenhouse gas. Carbon dioxide is important in maintaining the earth's average temperature of about 15°C (59°F). The traps infrared energy emitted from the earth's surface and warms the atmosphere. Without water vapor, , and methane (the three most important naturally produced greenhouse gases), the earth's surface would be about -18°C (0°F). At this temperature, it is doubtful that complex life as we know it would ever have evolved.

Where does come from? Plants and animals give it off when they extract energy from their food during cellular respiration. Carbon dioxide bubbles out of the earth in soda springs, explodes out of volcanoes, and is released when organic matter burns (such as during forest fires).

Over geologic time, sources and sinks generally balance. In today's atmosphere, however, levels are climbing in a dramatic and easily measurable fashion, providing evidence that there are now more sources than sinks.

What are the sources for this 'extra' ? Human activities are thought to be primarily responsible for the observed increases. Of the human sources of :

There are natural sources of as well. Plants and animals give off while alive and respiring and when dead and decaying (bacteria that consume the dead bodies respire too, after all). Carbonate rocks contain that can be released by exposure to acid and/or weathering. Certain naturally carbonated spring waters (for example, Perrier water) contain because the water has passed though carbonate rocks on its way to the surface. Volcanoes are also a source of . However, these geological sources are insignificant when compared to the human sources.

Plants (both terrestrial plants and marine phytoplankton) are the most important carbon sinks, taking up vast quantities of through the process of photosynthesis. To a lesser extent, atmospheric can also be dissolved directly into ocean waters and thereby be removed from the atmosphere. While plants also release through the process of respiration, on a global, annual basis, the amount of taken up by plants through photosynthesis and released through respiration approximately balances out. Thus, the released from human activities is truly the 'extra' .

Scientists typically monitor the concentration of in atmospheric samples by using sensitive devices called infrared gas analyzers. These devices pass a beam of infrared (IR) light through a sample of gas. The amount of IR that reaches a detector on the other side can be used to determine the amount of in the sample. A worldwide network of monitoring stations currently tracks the earth's rising levels.

Carbon dioxide has another characteristic that enables students to detect themselves. When dissolved in water, carbon dioxide forms a weak acid, called carbonic acid. The chemical bromothymol blue (BTB) is a sensitive indicator of the presence of acid. When gas containing is bubbled through a BTB solution, carbonic acid forms and the indicator turns from dark blue to green, yellow, or very pale yellow depending on the concentration (lighter colors mean higher concentrations).

This activity has multiple parts:

Note: For the activity to be most effective, students should have a working knowledge of the carbon cycle. Activity 15 provides a good overview of the cycle.

Learning Goals

  1. Students will be able to explain the concept of 'sources' and 'sinks' as they relate to .

  2. Students will understand the use of an indicator solution (BTB) to reveal the presence of .

  3. Students will understand the qualitative differences between animal and fossil fuel sources of global .

Alignment to National Standards

National Science Education Standards

Benchmarks for Science Literacy, Project 2061, AAAS

Grade Level/Time


For each two-student team


This activity requires significant preparation. This section addresses teacher preparation only. Detailed procedural instructions for the three parts of the activity are in the student guide. It is strongly suggested that teachers read the student guide prior to implementing this activity.


BTB is available in either concentrated liquid or powdered form. Do the following to prepare the BTB solution.

One liter of working solution should serve a class of 30 students, in two-person teams.

In Part 1, the students will conduct an experiment designed to detect the presence of . When combined, baking soda and vinegar produce pure . In this experiment, the BTB will dramatically change color (from bright blue to yellow) when introduced to the . This basic experiment will form the basis of the experiments to follow.


The students will analyze the from car exhaust (which will represent fossil fuel), their own breath (representing animals), and the outside air by bubbling a known amount of each gas though a standard volume of BTB. They will first simply compare how the different sources change the color of the BTB solution as they did for pure in Part 1.

To make a meaningful comparison, it is important that students collect equal volumes of gases. We suggest using rubber balloons blown up to the same diameter from each source as collectors. To do this, make a simple balloon diameter template with a piece of cardboard or half of a manila folder. Draw a circle about 7.5 cm in diameter in the middle. Cut out the circle and discard, saving the frame for use as a template.

You will need one of these templates for each group of students. As they collect samples, the students can use these to make sure that the samples are of approximately equal volumes. The templates can be re-used.

A. Automobile exhaust collection

Important note: Teachers should provide students with balloons full of car exhaust. It is not recommended that students participate in filling the balloons with car exhaust. An adult assistant (or two) is necessary, however.

Materials needed for collecting car exhaust:


  1. Blow up and allow the balloons to deflate. This will stretch the rubber and make them easier to fill with the relatively low-pressure exhaust.

  2. Prepare a cone to collect the car exhaust by rolling up a manila folder lengthwise. One end must be larger than the opening for the car's tail pipe and the other end must be small enough for the balloon to fit over it.

    Use plenty of tape to hold the cone in shape and to make the sides of the cone fairly airtight. Note: the paper funnel will work for several fillings without burning. DO NOT use a plastic funnel. As the exhaust pipe heats up, the plastic may melt. You may use a metal funnel, but be VERY careful to avoid any skin contact with the hot metal.

  3. Have an assistant turn on the car (make sure brake is on).

  4. Put the balloon on the small end of the cone.

  5. Using the heat resistant mitts, approach the exhaust pipe from the side. Place the large end of the cone over the tail pipe. Use the gloved hand to help form a seal between the cone and the exhaust pipe. DO NOT BREATHE THE EXHAUST. The balloon should fill quickly; if not, have your assistant step lightly on the accelerator.

  6. When the balloon is filled, have an assistant use a twist tie or two to tightly seal the balloon. Do this by twisting the neck several times and doubling it over once, then place the twist tie around the constricted area.

  7. You will want to have at least one balloon for each group of students. It is useful to prepare a few extra filled balloons.

B. Animal carbon dioxide collection

Students will fill up balloons with their own exhalations according to the instructions in the student guide. Emphasize to the students that they should hold air in their lungs for a few moments to allow plenty of exchange between being absorbed and being released in their lungs. Breaths that are too rapid will contain less than normal exhalations.

C. Outside air collection

Students will collect outside air using an air pump (or bicycle or sportsball pump) to blow up a balloon. The sample collection must be done out-of-doors as inside air can be enriched from breath.

At this point, your student teams will each have three balloons, one of car exhaust, one of their own breath, and one of outside air. They will bubble the gases through a BTB solution in test tubes, observing the color changes, according to the student directions. They should clearly observe the rapid and dramatic change with the car exhaust, the less significant change with their own breath, and the minor change with room air. The students will save these test tubes for Part 3.


As is bubbled through a BTB solution, it reacts with the water to form carbonic acid. The more in the gas, the more acid is formed. As the pH of the solution goes down (becomes more acid), the BTB changes from blue to green to yellow. In order to measure (approximately) how much actually reacted with the solution, we can use a procedure called 'titration.'

To do this titration, we add small volumes of a basic (high pH) solution (such as household ammonia) to the BTB mixture and record how much of this solution it takes to make the BTB return to its original blue color. The more that was bubbled through the BTB solution, the more ammonia will be required to restore the original color.

Students will be provided with dropper bottles of ammonia and will be instructed to slowly and carefully, drop-by-drop, add ammonia to the tubes, until the original color returns. In this way they can compare content of the sources they tested.

Assessment Ideas

In this activity, the students have examined several sources of . Ask them the following questions:

Modifications for Alternative Learners

When you're finished with the activity, click on To Student Guide or Back to Activities List at the top of the page.