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Alka-Seltzer & Gas Solubility (10 Favorites)

LAB in Solubility, Concentration, Acid Base Reactions, Titrations, Indicators, pH. Last updated March 22, 2022.


In this lab, students will use the reaction between Alka-Seltzer and water to investigate temperature and the solubility of carbon dioxide. They will use a neutralization reaction and an indicator to verify the amount of carbon dioxide produced at each temperature.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the following scientific and engineering practices:

  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Constructing Explanations and Designing Solutions


By the end of this lab, students should be able to:

  • Experimentally determine how temperature affects gas solubility.
  • Qualitatively carry out a titration.
  • Use a neutralization reaction to compare concentration.
  • Research the environmental relevance of carbon dioxide dissolving in water.

Chemistry Topics

This lab supports students’ understanding of

  • Solubility
  • Acid base reactions
  • Titrations
  • Indicators


Teacher Preparation: 15 minutes

Lesson: 40 minutes


For each group:

  • Alka-Seltzer tablet
  • 1.0-M NaOH solution (~5 mL)
  • Bromothymol blue indicator
  • 10-mL graduated cylinder
  • 25- or 50-mL graduated cylinder
  • Three 250-mL beakers
  • Thermometer
  • Three large test tubes
  • Test tube rack
  • Hot plate
  • Two thin-stem pipets
  • Stirring rod
  • Ice cubes
  • Balance
  • Weigh boat


  • Always wear safety goggles when working with chemicals.
  • When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

  • This lab demonstrates several chemistry topics in a very accessible manner: the temperature dependency of gas solubility, acid-base reactions, titrations, and indicators. Some of the real-world environmental effects of these topics (thermal pollution, ocean acidification, acid rain, etc.) provide a great hook to get students thinking about how chemistry is at work all around us, all the time. Students are given the opportunity to research one of these in the conclusion. (You may want to put a length requirement on the conclusion question.)
  • For this lab to be most impactful, students should probably be somewhat familiar with the concept of acids and bases. This lab can be used as an introduction to indicators and titrations on a qualitative level when the basic NaOH solution is used to neutralize the acidic carbonic acid solution and the indicator shows when the reaction is complete.
  • Name brand Alka-Seltzer is not necessary – any generic effervescent tablet will do, as long as it produces carbon dioxide.
  • If you don’t have enough balances for every group, you can distribute them at stations around the lab and have students weigh their Alka-Seltzer samples at the weigh stations.
  • Students will get practice creating a well-organized data table. Presenting data in a meaningful and easy-to-understand manner is a useful skill, and not one students get enough practice with.
  • The room temperature control (no added CO2 from the Alka-Seltzer) should be close to neutral, so the indicator should be greenish. When the Alka-Seltzer is added to both the hot and cold water, the carbonic acid produced shifts the color of the indicator to yellow. Since gases dissolve better in colder water, the cold water should be more yellow than the hot water, which should be closer to the original green. Adding the basic NaOH solution returns the color toward green as it neutralizes the acid in the solution (and into the blue range if they add too much!).
  • After the lab, you could do a fun mini-demo to show students what happens if you use a straw to exhale into the room temperature solution. You can demonstrate that you are exhaling CO2, citing that the solution turns yellow as it did when the Alka-Seltzer was added (though a student who is really paying attention might argue that it doesn’t definitively prove that it is CO2, as any acidic substance could turn the solution yellow!).
  • Set-up and clean-up are quick and straightforward. Most state and local regulations allow for dilute neutral solutions with small quantities of bromothymol blue to be disposed of down the drain when flushed with excess water. Check your local waste disposal regulations to confirm.

For the Student



In this lab, you will generate carbon dioxide gas by putting Alka-Seltzer tablets in water. When CO2 dissolves in water, some molecules react to form carbonic acid (H2CO3) according to the following reaction: H2O (l) + CO2 (g) → H2CO3 (aq). As a result, solutions containing carbon dioxide have a pH less than 7. During this experiment, you’ll monitor the pH of your solutions using an acid-base indicator called bromothymol blue.

Bromothymol Blue Indicator

Approximate pH










  • Alka-Seltzer tablet
  • 1.0-M sodium hydroxide solution (~5 mL)
  • Bromothymol blue indicator
  • 10-mL graduated cylinder
  • 25- or 50-mL graduated cylinder
  • Three 250-mL beakers
  • Thermometer
  • Three large test tubes
  • Test tube rack
  • Hot plate
  • Two thin-stem pipets
  • Stirring rod
  • Ice cubes
  • Balance
  • Weigh boat


  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids and bases, if any solution gets on your skin, immediately alert your instructor and thoroughly flush the area with water.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.


  1. Add 3–4 ice cubes to a 250 mL beaker and enough tap water to bring the volume to 200 mL.
  2. Add 200 mL of tap water to each of the other two beakers. Place one beaker on a hot plate (medium-high heat) and allow the other beaker to equilibrate to room temperature. (The room temperature beaker will be the experimental control.)
  3. Obtain two 1.0 gram samples of Alka-Seltzer by snapping off pieces of the tablet and weighing them in a weigh boat on the balance. The precise mass is not important, as long as the two samples are the same mass and approximately 1.0 g.
  4. When the water on the hot plate has reached 75–80 ºC, remove the beaker from the heat.
  5. Add two drops of bromothymol blue indicator to each beaker, including the room temperature control. Set the control aside.
  6. Simultaneously drop the two 1.0 gram samples of Alka-Seltzer into the hot and cold beakers. No Alka-Seltzer is added to the room temperature water sample. Carefully observe and compare all evidence of physical and chemical changes in the hot and cold beakers. When the Alka-Seltzer tablets have fully reacted, note the color and appearance of the solution in each beaker. 
  7. Measure the temperature of each solution, including the room temperature control, and label three large test tubes with the corresponding temperature. Rinse the thermometer in between measurements to avoid contamination.
  8. Pour 25 mL of solution from each beaker into a graduated cylinder and place each sample in the appropriate labeled test tube. (Use heat-protective gloves if the glass of the hot water beaker is too hot to touch.)
  9. Using a thin-stem pipet, add 1.0-M NaOH solution, one drop at a time, to the cold water reaction mixture. Count the number of drops of NaOH that must be added to the sample to match the color of the room temperature control solution. Stir or swirl the solution between drops to ensure thorough mixing. Record your data.
  10. Repeat the previous step with the hot water reaction mixture, again keeping track of how many drops of NaOH it takes to match the color of the room temperature solution. Record your data.
  11. Follow your teacher’s instructions for disposing of the solutions. Wash and return all glassware and clean your lab bench before the end of the period.


Create a data table to organize and record your data and observations throughout the investigation.


  1. Why did you add NaOH to the Alka-Seltzer solutions? What was the purpose of the control sample?
  2. From the results of this experiment, how does temperature affect the solubility of CO2 in water? Refer back to your data/observations and cite at least two specific pieces of evidence that support your conclusion.
  3. From your experience with carbonated beverages, do CO2 and H2O molecules have strong or weak attractions? Explain.
  4. Why does a carbonated drink “go flat” as it sits in an open room? How can you prevent carbonated drinks from going flat?


Research and respond to one of the following prompts:

  • Many power plants release warm water into streams and lakes. The effect is called thermal pollution. Briefly research this topic online. According to your research and the results of this experiment, explain why thermal pollution poses a threat to aquatic life and what can be done to avoid or repair the damage it causes.
  • Ocean acidification presents a variety of challenges to aquatic life. Do some research and describe the main causes of ocean acidification and its effects on marine life, as well as humans. How does ocean acidification relate to concepts examined in this lab? What are some ways to prevent and/or reverse damage caused by ocean acidification?
  • Related to ocean acidification, acid rain is also a threat to natural ecosystems, in addition to human health and infrastructure. Describe how issues surrounding acid rain relate to the concepts you studied in this lab. Research the causes and impacts of acid rain and what can be done to mitigate these effects and prevent future harm.