Add carbon dioxide to water and observe changes to the pH of the water.
The carbon dioxide can come from exhaled human breath or from carbon dioxide gas canisters.
The acidity, or pH, of the water is measured with pH strips or bromothymol blue pH indicator.
Time: 15 minutes
Skill Level: Beginner
2 glass jars for water (with lids if using CO2 gas)
Drinking straw
pH strips or bromothymol blue pH indicator
CO2 from Soda Stream or CO2 cartridges (optional)
Specifics and purchasing links are on this spreadsheet Materials list for CCSAE experiments
The air we breathe in typically contains about 21% oxygen and 0.04% carbon dioxide.
The air that humans exhale after taking a normal breath contains about 16% oxygen and 4% carbon dioxide.
Source: https://byjus.com/biology/composition-gases-breathe/
Bromothymol blue changes color depending on the acidity of the solution. It is yellow at pH values below 6, blue at pH values above 7, and green at pH values between 6 and 7.6. Source: https://www.sciencecompany.com/Bromothymol-Blue-pH-Indicator-1-oz-P6363
Step 1
Fill one glass jar with ~ 500 ml of tap water (room temperature is fine) and leave the other jar empty.
Add 20 drops of bromothymol blue pH indicator to the jar with the water.
Step 2
Mix in the bromothymol blue and pour half of the water into the empty jar.
Step 3
Option 1: Add carbon dioxide from exhaled breath
Take a deep breath and exhale through the straw into the water. Exhale slowly and evenly, letting your breath bubble through the water. Observe the water and look for a change in color.
Continue taking deep breaths and bubbling your breath into the water until it changes color.
Step 3
Option 2: Add carbon dioxide from CO2 canister or Sodastream carbonator
Position the jar under the CO2 nozzle, keeping the nozzle out of the water. Wear safety goggles or put a towel around the top of the jar to catch splashed water. Carefully dispense a small amount of CO2.
Cover the jar and shake it to mix the gas in with the water. Observe changes to the water color.
Watch our example of these experiments:
Identifying common acids and bases using pH:
Use pH strips or bromothymol blue pH indicator to measure the pH of a variety of liquids:
lemon juice
Vinegar
cola soda
orange juice
tomato
black coffee
milk or chocolate milk
tap water
sea water
baking soda in water
dish soap (mild)
dish soap (grease cutting)
spray cleaners (Windex, whiteboard cleaner, Clorox Cleanup, etc.)
Suggestions:
Breathing into the water added carbon dioxide and made the water change color.
Putting carbon dioxide into the water made it change color.
Carbon dioxide in the jar got into the water and made the water change color
The water changed color because it became more acidic
The mechanism is CO2 absorption into water.
Gasses in the air, such as oxygen and carbon dioxide (CO2), dissolve in ocean water.
More CO2 in the air results in more CO2 in ocean water.
More CO2 in ocean water makes the water more acidic
Pathway for Ocean Acidification.pdfWe’d love to hear how you used this tool in your community, how the students responded, and your suggestions to improve the experiments and other resources. Contact us at ccsae@mit.edu.