Hands-on Activity: Red Cabbage Chemistry
Educational Standards :
Pre-Req Knowledge (Return to Contents)
Have students complete the Introduction to Water Chemistry lesson before conducting this activity.
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For the teacher (to prepare group materials; instructions in Procedure section):
Introduction/Motivation (Return to Contents)
Red cabbage juice contains a natural pH indicator that changes colors depending on the acidity of the solution. The pigment in red cabbage that causes the red color change is called flavin (an anthocyanin).
Flavin is a water-soluble pigment also found in apple skins, plums and grapes. Very acidic solutions turn the indicator a red color, neutral solutions turn the indicator a purple color, and basic solutions turn the indicator a greenish-yellow color.
The pH of a solution expresses the concentration of hydrogen ions (H+). At a lower pH, more hydrogen ions are in solution, and therefore the solution is acidic. Many reactions in nature involves an increase or decrease in acidity. For example, as CO2 concentrations in the atmosphere increase, greater amounts are dissolved in the oceans, reacting with H2O to form carbonic acid (H2CO3). Carbonic acid quickly disassociates into bicarbonate(HCO3-) and hydrogen ion (H+).
Environmental and chemical engineers who focus on water quality, water treatment and water remediation need to measure, monitor and sometimes even adjust the pH of water. For example, in the water treatment process, important chemical reactions are affected by the pH of the water. Through today's activity, we will learn more about the pH of different liquids.
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
Before the Activity
With the Students
Attachments (Return to Contents)
Troubleshooting Tips (Return to Contents)
Double up on the paper cup with the Windex; otherwise, it tends to leak after a few minutes.
Assessment (Return to Contents)
Lesson Recap & Predictions: As a class, review the concepts presented in the associated lesson that relate to pH. Focus the conversation on topics such as acid rain and acid mine drainage. Have students predict which test liquids they think are acidic and which are basic.
Activity Embedded Assessment
Worksheet: Have students use the Red Cabbage Chemistry Worksheet to record their data and answer questions. Observe their written observations, data and answers to gauge their comprehension.
What's Going On? While students are conducting the lab, walk around and ask them questions to keep them engaged and on task, such as: Are the results from the indicator test what you expected? Why or why not?
Wrap-Up Discussion: At lab end, bring students together as a class and ask them the following questions. Make sure everyone understands the answers.
Activity Extensions (Return to Contents)
Have students test other liquids for their pH values and/or perform this lab activity at home. Consider testing items found in the kitchen, bathroom, laundry and garden.
Activity Scaling (Return to Contents)
where [aH+] is the concentration of hydrogen ions. Once students have completed the worksheet, have them manipulate this equation to solve for the concentration. Expect them to come up with the equation:
Have students use this equation for each liquid's pH value to determine the concentration of hydrogen ion in each of the seven liquids.
References (Return to Contents)
Helmenstine, Anne Marie. Chemistry. Red Cabbage pH Indicator - How to Make Red Cabbage pH Indicator. About.com: Accessed September 15, 2009. http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm
ContributorsJessica Ray, Phyllis Balcerzak, Barry Williams, Carleigh Samson
Copyright© 2010 by Washington University in St. Louis
Supporting Program (Return to Contents)GK-12 Program, School of Engineering and Applied Science, Washington University in St. Louis
Acknowledgements (Return to Contents)
This curriculum was developed with support from National Science Foundation GK-12 grant number DGE-0538541. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.