Speaking of Carbon Dioxide

Summary: Students talk into a bottle of water containing Bromothymol blue. The color of the water changes from blue to green to yellow.

Estimated Time: 10 minutes

Materials Needed:

1 clear water or soda bottle with cap (.5 L or smaller)
Bromothymol Blue
tap water
Straw (optional)

Safety Notes: Do not ingest any materials used in the lab.

Introduction: We have seen one way that carbon dioxide is created. It is made when we react vinegar and baking soda. Where else do you think we can find carbon dioxide?

Procedure:

Fill the bottle 1/2 - 2/3 full of water.

Add a couple of drops of Bromothymol blue to the bottle .

What color is the water?

Wrap your hand around the mouth of the bottle and talk into the bottle through your hand. Repeat "carbon dioxide" several times. Cap the bottle and pass it to a student. Have the student repeat "carbon dioxide" over and over again until a change occurs. [Optional: If working with older students, provide one bottle set up per group. Have students place a straw in the bottle and blow into it until the color change occurs. This is not recommended for young students as Bromothymol blue should not be ingested.]

What color is the water now?

Will it change more if we keep doing this? Why?

Continue the process until satisfied that the color has stabilized.

What do you think the color change means?

What do you think might have caused the color to change?



About Us Green Chemistry Educators Meet Scientists Activities Carbon Dioxide Polymers Pictures The Science House	Speaking of Carbon Dioxide Summary: Students talk into a bottle of water containing Bromothymol blue. The color of the water changes from blue to green to yellow. Estimated Time: 10 minutes Materials Needed: 1 clear water or soda bottle with cap (.5 L or smaller) Bromothymol Blue tap water Straw (optional) Safety Notes: Do not ingest any materials used in the lab. Introduction: We have seen one way that carbon dioxide is created. It is made when we react vinegar and baking soda. Where else do you think we can find carbon dioxide? Procedure: Fill the bottle 1/2 - 2/3 full of water. Add a couple of drops of Bromothymol blue to the bottle . What color is the water? Wrap your hand around the mouth of the bottle and talk into the bottle through your hand. Repeat "carbon dioxide" several times. Cap the bottle and pass it to a student. Have the student repeat "carbon dioxide" over and over again until a change occurs. [Optional: If working with older students, provide one bottle set up per group. Have students place a straw in the bottle and blow into it until the color change occurs. This is not recommended for young students as Bromothymol blue should not be ingested.] What color is the water now? Will it change more if we keep doing this? Why? Continue the process until satisfied that the color has stabilized. What do you think the color change means? What do you think might have caused the color to change? Think About It: Elementary Level: Bromothymol blue is an indicator. The indicator is blue in solutions that are basic or neutral. Water is neutral. It turns green and yellow in solutions that are acidic. Some examples of acidic solutions are soda (carbonated beverages), vinegar and lemon juice. It is green in solutions that are barely acidic and yellow in solutions that are more acidic. What color did our water turn? It turned green, then yellow. This means our water became acidic. How do you think we changed our water? What did we put into the bottle? We put our breath into the water. What is in our breath? What is in the air we breathe out? A lot of what we breathe out is carbon dioxide. The carbon dioxide is what made the water become acidic. We just provided evidence that we breathe out carbon dioxide. Middle/High School Level: Bromothymol blue is an acid-base indicator. It is blue in basic and neutral solutions. It gradually turns to green and yellow as acidity increases. Our water was yellow in the end, which means it was an acid. What do you think made our water turn to an acid? What did we add to the water? When we blew or spoke into the water we added our breath. Our breath contains carbon dioxide. The carbon dioxide reacted with the water to create an acid. What do you think this reaction was? H₂O (l) + CO₂ (g) --------> H₂CO₃ (l) The carbon dioxide created carbonic acid. This reaction occurs all of the time. It happens as rain falls. It happens any time you expose water to air. When else could this happen? After you open a bottle of water and start to drink it, what happens to it? Do you think this a dangerous acid? Does all the water change to acid? How do you know that this does not happen? Teaching Tips: We suggest completing this activity as a class demonstration or having one student perform the activity in each group. The amount of water is approximate and depends on the bottle used. The amount of water also changes how much carbon dioxide is needed to change the color to yellow. It is recommended that you try it once before doing the activity with students to get a feel for how much water you want to use. Bromothymol blue can be purchased from a store that sells aquarium supplies. You may wish to omit acid/base discussion for young students. It is not recommended to use straws and blowing bubbles for young students as Bromothymol blue should not be ingested.