Carbon Dioxide Concentrations

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Updated on May 02, 2013

Carbon dioxide is a greenhouse gas. Because carbon dioxide concentrations in the atmosphere have been increasing rapidly since the Industrial Revolution, the gas is of primary concern to the problem of global warming. One major source of carbon dioxide is from transportation vehicles. In this experiment, students will use bromothymol blue to compare the relative carbon dioxide concentrations of various gases, including the exhaust of different vehicle types. Bromothymol blue is an acid-base indicator that turns yellow in acidic solutions and blue in alkaline solutions. Carbon dioxide bubbled into water forms carbonic acid. Thus, the more carbon dioxide in a gas sample, the more acidic and yellow the bromothymol blue solution becomes.


Student compares the relative carbon dioxide concentrations of various gases, including different types of vehicle emissions.


  • 10-20 10-inch balloons
  • 10-20 drinking straws
  • 10-20 twist-ties
  • Measuring tape
  • Baking soda
  • Vinegar
  • Bicycle pump
  • File folder or funnel
  • Bromothymol blue solution (acid-base indicator available for purchase online) 0.04% (Aqueous), 1 L Bottle (1-2 bottles)
  • Vase, flask, or other thin-necked vessel
  • 10 clear glasses or cups of the same size (small clear plastic party cups work well)
  • Permanent marker


  1. Find three to five vehicles of different types to test for carbon dioxide concentrations. Examples: car, truck, SUV, ATV or four-wheeler, lawn mower, etc. Create a hypothesis to explain which gas will contain the most carbon dioxide and why: human breath, air, or one of the vehicles you will test.
  2. Collect the gas samples. Make sure to fill each balloon with its sample to a volume such that the balloon measures 4 inches in circumference (use the measuring tape). Seal each balloon with a twist-tie; do not tie the balloon in a knot. Write the sample name on the balloon with a permanent marker.
  3. Human breath: Breathe into a balloon to fill it to the correct volume and seal with twist-tie.
  4. Air: Use a bicycle pump to fill the balloon to the correct volume and seal it with a twist-tie.
  5. Pure Carbon Dioxide: Place several spoonfuls of baking soda into a vase or flask.Pour a half cup of vinegar onto the baking soda.The chemical reaction between the baking soda and vinegar will produce pure carbon dioxide. Pull a balloon around the top of the vase or flask to collect the carbon dioxide. Repeat if necessary to fill the balloon to the correct volume and seal with a twist-tie.
  6. Vehicles: Seek adult help. For each vehicle you wish to test, start the vehicle. Quickly and carefully fit a funnel around the tailpipe. If you do not have a funnel or it does not fit properly, you can make one from a file folder shaped into a cone. Take care to avoid breathing the exhaust or burning yourself on the pipe. Quickly fill a balloon to the correct volume and seal it will the twist-tie. Repeat for all vehicles you wish to test.
  7. Pour equal amounts (about 1 cup) of bromothymol blue into each of the cups you will use to test the gases (1 cup per gas tested). If you do not have enough solution, you may dilute it with a little water. Just be sure to use distilled water and dilute the entire supply only slightly. Write the name of the gas to be tested on each cup. For each gas-filled balloon: Fit the drinking straw into the opening of the balloon without untying the twist-tie. Seal the balloon over the straw with tape. Make sure that the gas you are testing matches the one written on the cup and the balloon. Carefully place the other end of the straw into the cup of solution. When everything is in place, untwist the twist-tie to allow the gas to escape the balloon and bubble into the bromothymol blue solution. Note the color of the solution and put the sample aside to compare to the others later. Repeat until each gas sample has been tested. Compare the color of the solutions. The closer the color is to yellow, the more carbon dioxide present. Arrange the solutions from most yellow to most blue. Write down the order and their colors. Create a continuum line (like a time line, but based on color) of the gases.
  8. Draw your conclusion by comparing your hypothesis to the results. Which gases had the most carbon dioxide? Which had the least?
Lynsey Peterson is a science education writer with research experience in plant ecology. She has enjoyed many years of teaching biological, environmental, and earth sciences to middle and high school students.

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