Most science teachers know that red cabbage juice can be used as a natural pH indicator in acid base experiments. The pigment found in red cabbage changes color depending on the acidity of a solution, making it a simple and inexpensive indicator for classroom labs.
In our biology classes, we discovered another interesting use for this natural indicator. Instead of using red cabbage juice for a traditional acid base lab, we used it to detect the carbon dioxide produced during cellular respiration.
Normally we perform this respiration lab using phenol red as an indicator. When our supply of phenol red started running low, we needed an alternative. The answer turned out to be something very simple: red cabbage juice indicator.
If you would like a ready to use version of this activity, I have created a biology lab that includes procedures, data tables, analysis questions, and teacher guide. You can view the full Cellular Respiration Lab Using Red Cabbage Juice Indicator here.
Why Red Cabbage Works as a pH Indicator
Red cabbage contains natural pigments called anthocyanins. Anthocyanins are water soluble pigments that occur naturally in many fruits and vegetables such as grapes, plums, and apples.
Anthocyanins act as natural acid base indicators. When dissolved in water, the pigments change color depending on the pH of the solution. This is why red cabbage juice works as a natural pH indicator in science experiments.
In neutral solutions, red cabbage indicator appears dark purple.
In acidic solutions, the indicator turns pink or red.
In basic solutions, the indicator turns greenish yellow.
Because these color changes are easy to observe, cabbage juice works very well as a natural pH indicator in science experiments.
How to Make Red Cabbage Juice Indicator
Preparing cabbage juice indicator is very simple, and students can even help prepare the indicator themselves.
Chop a head of red cabbage into small pieces.
Place a small handful of cabbage leaves into a beaker.
Cover the cabbage with water.
Boil the cabbage for about 15 minutes.
As the cabbage boils, the pigment is extracted from the leaves and the water turns a deep purple color.
Remove the cabbage pieces with a slotted spoon or pour the solution through filter paper. The purple liquid that remains is your red cabbage pH indicator.
Detecting Carbon Dioxide Using Red Cabbage Indicator
In our cellular respiration lab, students use cabbage juice indicator to detect the presence of carbon dioxide. Carbon dioxide reacts with water to form carbonic acid. When this reaction occurs, the pH of the solution decreases.
Because red cabbage indicator changes color when the pH changes, the solution turns pink or red when carbon dioxide is present.
Students can observe this reaction very easily. When students blow through a straw into the cabbage indicator solution, the carbon dioxide in their breath reacts with water to form carbonic acid. The indicator quickly changes from purple to pink, showing that carbon dioxide has been produced.
This simple demonstration helps students understand how gas exchange occurs during cellular respiration.
Cellular Respiration Lab Using Germinating Seeds
In the second part of the lab, students investigate whether plants also release carbon dioxide during cellular respiration.
The lab setup contains three test tubes filled with cabbage juice indicator.
One tube serves as the control.
The second tube contains germinating seeds, which have a high rate of cellular respiration.
The third tube contains dry seeds, which are alive but dormant.
Over time, the germinating seeds release carbon dioxide as they carry out cellular respiration. When the carbon dioxide dissolves in the indicator solution, carbonic acid forms and the solution turns pink.
Dry seeds carry out respiration at a much slower rate, so only a slight color change occurs.
After about 24 hours, students can clearly observe that germinating seeds produce more carbon dioxide than dormant seeds, providing evidence that cellular respiration is taking place.
A Simple and Engaging Biology Lab
This lab is easy to set up, easy to clean up, and provides a very visual way for students to observe cellular respiration. Students enjoy seeing the dramatic color change in the cabbage indicator solution, and the activity helps reinforce the connection between gas exchange, respiration, and cellular metabolism.
If you would like a ready to use version of this activity, you can find the full lab here:
Cellular Respiration Lab Using Cabbage Juice Indicator
More Cellular Respiration Activities
If you are teaching a unit on cellular respiration, you may also find these related blog posts helpful:
- Cellular Respiration PowerPoint
- Easy Way to Measure the Rate of Respiration
- Measuring the Rate of Cellular Respiration
- Lab: Gas Exchange in Cellular Respiration
- Lab: Use of Glucose in Cellular Respiration
Happy Teaching!
This is a fantastic practical and I can vouch for the fact that it works really well. I also used very dilute ethanoic acid to give another colour when 'calibrating' the indicator - it gives a lovely bright magenta colour. Also if you use bungs containing thermometers, you can observe the heat energy given off by the germinating peas as they respire. JES, London (UK)
ReplyDeleteThanks so much for your comments! Adding the thermometer is brilliant, and I will definitely give that a try!
DeleteI look forward to using this activity next fall. Can you use dry peas from the grocery store or do you need actual seed peas?
ReplyDeleteHi Leslie. You need to use peas that will germinate. I have not tried peas from the grocery store, and I don't think they would work very well. Try to find seed peas if you can. Thanks for visiting my blog!
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