Areas of Science Human Biology & Health
Science With Your Smartphone
Time Required Average (6-10 days)
Prerequisites Ability to physically exercise (doing jumping jacks). Knowledge of acid-base reactions is helpful, but not required.
Material Availability Readily available
Cost Low ($20 - $50)
Safety Adult assistance recommended for construction and setup of respirometer. Use a safety valve in your experiment as recommended in the procedure to make sure that you do not accidentally suck up some of the indicator solution.
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You might know that your body needs oxygen to keep going, and that you breathe out carbon dioxide as waste. What happens when you exercise? You have probably noticed that you breathe faster, and your heart beats faster. What triggers your body to respond in this way? How does it "rev up" to keep your muscles going? In this project, you will get a peek into the fascinating science of exercise physiology and find out—with the help of a color changing reaction and Google's Science Journal app.


To measure changes in carbon dioxide levels in exhaled air before and after physical exercise.

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Andrew Olson, PhD, Science Buddies
Edited by Svenja Lohner, PhD, Science Buddies


  • Investigating CO2 in Breathing. Bronx High School of Science.
  • How Much Carbon Dioxide Is Produced During Exercise? Teacher's Notes. Faculty of Education, The Chinese University of Hong Kong.

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Effects of Exercise: Changes in Carbon Dioxide Output." Science Buddies, 20 Nov. 2020, Accessed 4 Mar. 2021.

APA Style

Science Buddies Staff. (2020, November 20). Effects of Exercise: Changes in Carbon Dioxide Output. Retrieved from

Last edit date: 2020-11-20


Every day, you need lots of energy for all the activities you do, even when you are just sleeping. To produce this energy, your body extracts energy from the food you eat. This process is called cellular respiration and takes place in the cells of your body. In a series of chemical reactions, the food you eat is broken down to glucose, which then reacts with oxygen from the air you breathe to carbon dioxide (CO2), water and energy as shown in Figure 1.

Cellular respiration diagram shows a cell taking in oxygen and glucose and creating carbon dioxide, energy and water
Figure 1. Cellular respiration—a process in which glucose reacts with oxygen to form carbon dioxide, water and energy—happens inside the cells of your body.

The chemical equation of cellular respiration is given in Equation 1.

Equation 1:

Water and carbon dioxide are waste products of cellular respiration that are usually not used by your body. Whereas the water ends up in your sweat or urine, the carbon dioxide is released back into the air when you exhale. In this project, you will learn a method for measuring the relative amount of carbon dioxide in the air that you exhale. To measure your carbon dioxide output, you will make use of the fact that carbon dioxide is an acidic gas. This makes it possible to use a colorimetric pH test. pH is a numerical (specifically logarithmic) measure of how acidic or basic (also called alkaline) something is. Technically, pH is the negative logarithm of the hydrogen ion concentration:

What this equation means is for each 1-unit change in pH, the hydrogen ion concentration changes tenfold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are basic (alkaline). If you want to know more about acids, bases, and pH, you will find more information here.

A colorimetric pH test means that the color of the solution changes when the pH changes. Here is how it works. When you add the pH indicator solution (specifically bromothymol blue) to plain water, it turns blue, or greenish blue, indicating that the pH is near 7. Carbon dioxide is very soluble in water. When it dissolves, it forms carbonic acid, which is acidic. This makes the pH of the water shift from neutral (7) to more acidic (somewhere near 6)—the pH indicator will change color to yellow. Figure 2, below, shows an example of the pH indicator solution bromothymol blue changing color over this pH range.

Three beakers are filled with liquids of different pH values and change color due to the pH indicator bromothymol blue
Figure 2. The pH indicator bromothymol blue changes color from yellow to blue over the pH range 6.0–7.6.

To compare your CO2 output under different conditions, you will exhale through a tube into a bottle partly filled with the pH indicator solution. The CO2 that you exhale will dissolve in the water, and gradually acidify it. You will be able to see the pH indicator change color as this happens. You can even record the color change of the solution using a phone equipped with Google's Science Journal app. By measuring how long it takes for the pH change to occur, you will have a relative measure of the amount of CO2 in your breath. The less time it took for the color change to happen, the more CO2 there was in your breath.

The two conditions you will test are before and after a short period of exercising. You probably notice that your breathing gets faster and your heart rate increases when you exercise. You can do your own research to look into how your body controls heart rate and breathing during exercise, which is a topic called exercise physiology. What do you think will happen to the amount of CO2 that you exhale after exercising? Will it increase, decrease, or stay the same? And what does your result mean with respect to the cellular respiration reaction in your body?

Terms and Concepts

  • Cellular respiration
  • Glucose
  • Oxygen
  • Carbon dioxide (CO2)
  • pH
  • Acidic pH
  • Basic pH
  • Neutral pH
  • Colorimetric pH test
  • pH indicator
  • Bromothymol blue
  • Heart rate
  • Exercise physiology


  • How is oxygen used and carbon dioxide produced in cellular respiration?
  • How do cells in the body obtain oxygen and get rid of carbon dioxide?
  • How does the body sense and respond to increased cellular respiration rate?
  • How can you measure the amount of carbon dioxide in your exhaled breath?
  • What is a colorimetric test and what can you use it for?


For information on respiration and exercise, carbon dioxide, and how the human lungs function, check out these websites:

To learn more about Google's Science Journal app, visit the website below:

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Materials and Equipment

  • Clear plastic bottle (ca. 500 mL) (2)
  • Water
  • Teaspoon
  • Bromothymol blue solution (0.04%). You can purchase bromothymol blue through an online supplier such as Amazon.
  • Aeration setup for de-acidifying the pH indicator solution. The setup should include an aquarium pump, tubing, and an airstone that can all fit together, although the airstone is optional. Such a setup may be purchased at a local aquarium store, or through an online supplier such as Amazon.
  • Scissors
  • Modeling clay
  • Straw
  • Safety valve for tube; available from Amazon
  • With option 1 in procedure: A small flashlight
  • With option 1 in procedure: A small box or books to lean your phone against
  • With option 1 in procedure: A smartphone or tablet to record your data
    This project uses Google's Science Journal app, a free app that allows you to gather and record data with a cell phone or tablet. You can download the app from Google Play for Android devices (version 4.4 or newer) or from the App Store for iOS devices (iOS 9.3 or newer).
    Note: This project was tested with the Android version of Science Journal in which light intensity is measured using the ambient light sensor and given in lux. The iOS version uses the phone's camera to measure brightness resulting in data expressed in EV (Exposure Value). Lux values and Exposure Values are not the same. Whereas Exposure Value is a base-2 logarithmic scale, the lux scale is linear. This might affect your data and result in different values and graphs when you are using an iOS version of the app—both versions will work for this project though. The graph examples given in the procedure show light intensity in lux.
  • With option 2 in procedure: Stopwatch or a clock or watch with a second hand
  • With option 2 in procedure: A helper to time you
  • Lab notebook
  • Pencil

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Experimental Procedure

Note: In this science project, you will investigate how much carbon dioxide you exhale under different conditions. You will use the Science Journal app to monitor the color of an indicator solution over time while its pH changes due to the exhaled carbon dioxide. The app creates a graph that will allow you to exactly determine the end point of the color change. If you do not have a phone, you can observe the reaction and use a stopwatch to time how long it takes for the indicator to change its color.

Building Your Respirometer

  1. Fill one clear plastic bottle a little less than one-third full with water and add one teaspoon (about 5 mL) of the 0.04% bromothymol blue solution. You should get a nice green or blue-green color (pH >= 7) as shown in Figure 3. If not, try using distilled water. This will be your diluted pH indicator solution.

Adding bromothymol blue solution to water to make a green or blue-green solution
Figure 3. Adding the bromothymol blue solution to the water should result in a green or blue-green solution.
  1. Cut about two inches from one side of the tube for the aquarium pump.
  2. Insert the safety valve in between both pieces of tube as shown in Figure 4. The tip of the red part inside the valve should be on the side of the longer end of the tube. Note: The safety valve will prevent you from accidentally sucking up indicator solution from the bottle.

Two ends of a tube are connected to a safety valve
Figure 4. The safety valve ensures that you do not accidentally suck up indicator solution.
  1. Cut a two-inch piece off your straw. You will use this as the outlet tube of your respirometer.
  2. Using the aquarium pump tube (with inserted valve and the airstone), the straw, and the modeling clay, set up your respirometer as shown in Figure 5. Make sure that the inlet tube reaches all the way to the bottom of the bottle. The outlet tube (the straw) should stay above the indicator solution.

Putty holds two tubes in the lid of a water bottle filled with a pH indicator solution
Figure 5. Set up respirometer with inlet and outlet tubes and diluted indicator solution.

Measuring Your Carbon Dioxide Output

  1. Fill the second bottle with the same amount of water as the first one and again add one teaspoon of bromothymol blue solution. Make sure the solution has the same color as the previous one and set the bottle aside as a control for color comparison.
  2. Now you are ready to begin. You can either use the Science Journal app (option 1) to monitor the color change of the indicator solution as described in step 3, or ask a helper to record the time of the color change with a stopwatch (option 2), described in step 4.

  3. Option 1: Using the Science Journal app
    Science Journal is an app that lets you record data using sensors that are built into many smartphones, including a light sensor, which measures light levels measured in lux (normally this sensor is used to automatically adjust the brightness of your phone's screen). To learn how to use the Science Journal app and how to use the light sensor, you can review the relevant tutorials on this Science Journal tutorial page. In this project, you can use the app to record the color change in your solution once the pH indicator turns from neutral (blue-green) to acidic (yellow-green).
    1. Make sure you know the location of the light sensor in your phone and test if it works as expected. The light sensor tutorial on the Science Journal tutorial page explains how to do this.
    2. Open the Science Journal app on your phone and select the light sensor. Make sure you clearly label your experiment and recordings.
    3. Lean the phone against a box or books with the light sensor facing sideways towards your respirometer. Then place the respirometer directly in front of the light sensor. Put the flashlight in front of the respirometer so it shines through the indicator solution directly onto the light sensor as shown in Figure 6. Note that the light sensor reading will be dependent on the position of the flashlight with respect to the phone. You might want to try different positions to get the highest readings. However, you should not move the flashlight or the phone while recording data.

      A light sensor is used to measure the color change of an indicator solution in a water bottle

      A flashlight shines through a plastic bottle filled with an indicator solution towards the light sensor on a smartphone. As the color of the solution changes it will allow more or less light through, and the changes in light intensity can be measured.

      Figure 6. Experimental setup for recording the indicator color change with the Science Journal app. Make sure the flashlight is aligned with the light sensor of the phone.

    4. Confirm that the sensor readings are stable. Then, press the record button in the Science Journal app, take a deep breath and start exhaling through the inlet tube into the indicator solution for as long as you can. Try to exhale from your lungs and be careful to not block the sensor with the tube or the airstone! Inhale through your nose when necessary, and then continue blowing into the solution.
    5. Stop recording once the maximum sensor readings level off and are stable for more than 15 seconds. The sensor readings should start to stabilize once the indicator solution has changed color from blue-green to yellow-green (as shown in Figure 8). Your data should look similar to the graph in Figure 7. As the indicator solution changes from a darker color to a lighter color, more light can get through to the light sensor and the light intensity increases.

      Graph of measured light intensity while exhaling into a bottle of indicator solution

      In the example graph, the minimum lux value was 4,000, the average was 5,000 and the maximum value was 6,000 lux. The slope of the graph rose steadily with steeper increases in light intensity towards the end of the graph.

      Figure 7. Example graph of measured light intensity while exhaling into the indicator solution. The x-axis is time in minutes:seconds [min:s] and the y-axis shows light intensity in lux. Note, how the light intensity readings increase step-wise as more light reaches the light sensor due to the indicator changing from a darker color to lighter color.

    6. You can determine the endpoint of color change, or the time at which the light intensity first leveled off, from your graph. Dragging the cursor along the x-axis will give time and the measured value for each individual data point. In the example graph, shown in Figure 7, the color change is completed at 19.3 seconds, which is when the sensor reading stops changing.
    7. In your lab notebook, record the time that you determined from your graph.

  4. Option 2: Using the stopwatch
    1. Your helper should tell you when to start, and start the stopwatch.
    2. On your helper's command, take a deep breath and start exhaling through the inlet tube into the indicator solution for as long as you can. Do your best to maintain your current, comfortable breathing rate, inhaling through your nose and exhaling from your mouth through the tube.
    3. When the indicator solution in your respirometer has turned yellow-green (as shown in Figure 8) and the color does not change anymore, your helper should stop the stopwatch.
      Indicator solution in a plastic bottle turns yellow after fully saturated with carbon dioxide
      Figure 8. At the end of your reaction the indicator solution should be yellowish.

    4. In your lab notebook, record the number of seconds it took to change the color of the solution. You may need to practice this process with your partner several times before actual testing to determine the exact color at which to stop the time.

  5. Now aerate the pH indicator solution (in the respirometer) to return it to the starting pH. To do that, attach the inlet tube to the aquarium pump and switch it on (you can leave the safety valve attached) as shown in Figure 9. Aerate the solution until the solution matches the original color (compare to your control; it will take 5–10 minutes).

An aquarium pump is used to aeriate indicator solution in a plastic bottle
Figure 9. Aerating the indicator solution with an aquarium pump.
  1. When your respirometer solution is ready again, repeat step 3 or 4 depending on which option you chose the first time. Do this until you have at least three measurements at rest (more is better).
  2. Next, collect at least three measurements right after exercising (doing jumping jacks) for one minute. Test how long it takes you to change the color of the pH indicator immediately after you finished exercising, then rest for 10 minutes while you re-aerate the buffer. Then repeat the measurement until you have at least three data points for each condition (more is better).
  3. Average the results for each test condition and compare the results using graphs and data tables.
  4. How do your results compare to your expectations from your background research? Did the amount of carbon dioxide in your exhaled breath decrease or increase after exercising? Can you explain your results?

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  • Correlating CO2 Production with Other Measures. How does CO2 output correlate with other measures of increased physical activity such as breaths per minute and pulse rate?
  • Making Your Results More Quantitative. For a more advanced project, you can actually calculate the amount of CO2 produced. You can do this by adding a known amount of NaOH to the indicator solution, and calculating how much CO2 would be required to change the pH to 6. Then, measure how long it takes to acidify the indicator solution. Your measurements will allow you to calculate an estimate of the CO2 exhaled per second. Of course, to make this work you will have to work with solutions of known concentration and volume.
  • Effects of Training: Athletes vs. Non-Athletes. If you wanted to get really ambitious, you could see how conditioning affects CO2 output. Do conditioned athletes take longer to start producing additional CO2 with moderate exercise? Do they recover to normal levels faster after exercise?
  • Compare CO2 production after anaerobic and aerobic exercise. Compare respirometer results from subjects who run, walk, or bike for 4–7 minutes to those from subjects who do push-ups, lunges, or squats for the same duration.
  • For another Science Buddies project on exercise physiology, see: Heart Health: How Does Heart Rate Change with Exercise?.

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