Introduction: (Initial Observation)
If you have ever cooked a red cabbage, you certainly remember the dark blue-purple color of the cooked red cabbage and its juice. If you add a few drops of lemon juice to the dark purple cabbage, it immediately changes color and becomes red. This happens because lemon juice is an acid (Mostly Citric Acid and some ascorbic acid).
You may do this experiment and make you own pH indicator and use it to identify acidic and alkaline substances at your home.
Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.
Acid-base indicators are organic compounds that change colors at various pH values. They are useful to the biologist because they produce characteristic color changes at various degrees of acidity or alkalinity. Most indicators are organic dyes with complex formulas that are synthesized in laboratories using reactions that are also complicated. Litmus paper is an example of an acid-base indicator commonly used in the middle school laboratory. Some indicators, however, can be derived from easily obtainable plants. In this activity you will extract one to these natural acid-base indicators and use it to find the approximate pH values of some substances you might find at home or at school.
Here are a couple of web sites that provide the explanation and procedure for the project:
And here are some sites that show sample projects (including teacher’s notes):
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
Can you use cabbage extract to test the pH of different household substances?
The purpose of this project is to make pH indicator using cabbage. You will then test different household items to determine if they are acidic, neutral or alkaline.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
After you make your pH indicator using cabbage, use it to test different household items and determine if they are acidic or alkaline.
Independent variable (also known as manipulated variable) is the type of household materials you test. Possible values are Milk, Lemon Juice, Liquid detergent.
Dependent variable (also known as responding variable) is the acidity. Possible values are acid, neutral, base.
Controlled variables are temperature and light. We make sure that all our experiments will be performed under the same temperature and light conditions.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis:
Since cabbage extract will change color in different pH, we can use it to make pH indicators for the range of pH that color change of cabbage extract is visually detectable.
Among household items, vinegar and lemon juice are acidic. Soaps and detergents are alkaline. My hypothesis is based on my gathered information indicating that vinegar is acetic acid and lemon juice contains citric acid. Also detergents are made using strong alkaline.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Acid-base indicators are organic compounds that change colors at various pH values. They are useful to the biologist because they produce characteristic color changes at various degrees of acidity or alkalinity. Most indicators are organic dyes with complex formulas that are synthesized in laboratories using reactions that are also complicated. Litmus paper is an example of an acid-base indicator commonly used in the middle school laboratory. Some indicators, however, can be derived from easily obtainable plants. In this project you will extract one to these natural acid-base indicators and use it to find the approximate pH values of some substances you might find at home or at school.
Part 1: Preparation of the acid-base indicator
The acid-base indicator is an extract of the juices from red cabbage.
- Cut a head of red cabbage into quarters.
- Take 1 quarter and slice it into 5 or 6 smaller chunks and put them into the blender.
- Add about 400 ml of H2O and blend using 2-3 30 second intervals. (Try to get the cabbage fairly well chopped so the water is really purple)
- Filter the chopped cabbage extract through a coffee filter.
Part 2: pH Standards
Note: There are several options available for generating pH standards. One option is to use an inexpensive pH meter. Another option would be to use standards prepared using hydrochloric acid (HCl) and sodium hydroxide (NaOH). A final option is to prepare standards using commonly available chemicals. As with all chemicals please handle with extreme care.
- Label test tubes as follows: A- pH 2, B-pH 3, C-pH 5, D-pH 7, E-pH 8, F-pH 9, G-pH 12, H-pH 14
- Add 3 ml of the indicator solution prepared in part 1 to each of the tubes.
- Add the following: A- 10 ml of fresh squeezed lemon juice, B-10 ml of white vinegar, C-10 ml of boric acid solution (avoid contact with skin), D-10 ml of H2O, E-10 ml of sodium bicarbonate solution, F-10 ml of borax solution, G-10 ml of sodium carbonate solution, H-10 ml of drain cleaner solution.
Preparation of solutions used in part 2:
- To make boric acid solution dissolve 2 g of boric acid in 200 ml of H2O.
- To make sodium bicarbonate solution dissolve 2 g of baking soda in 200 ml of H2O.
- To make borax solution dissolve 2 g of sodium borate in 200ml of H2O.
- To make sodium carbonate solution dissolve 2 g of sodium carbonate in 200ml of H2O
- To make drain cleaner solution dissolve 2 g of solid drain cleaner (Drano) in 200 ml of H2O. BE CAREFUL! Do not get any on your skin or clothing. Use with caution.
Part 3: Testing the pH of common household solutions
Note: Be creative when looking for substances to test. Clear or light colored liquids show better results.
- Pour 40 ml of the extract into a 200-ml beaker.
- Add 40 ml of the acetic acid.
- Compare the resulting solutions color to that of your prepared standards. Use the pH’s indicated above in Part 2, number 1. Record your results.
- Follow steps 1,2 and 3 above using 40 ml of the substances you want to check (Sodium Hydroxide, bleach, ammonia, lemon-lime soda, lemon juice, or vinegar each done separately) instead of the acetic acid. Record your results.
- Try adding a piece of dry ice to 40 ml of the extract. Record the results.
Materials and Equipment:
- 100 ml Graduated Cylinder
- Several 200 ml beakers
- Coffee Filter
- Red Cabbage
- Chemicals for pH standards (optional)
- Boric acid
- Sodium bicarbonate (baking soda)
- Sodium borate (borax)
- Sodium carbonate
- Drain cleaner (Drano)
- Lemon juice
- Suggested test solutions (40 ml each)
- Sodium Hydroxide
- Acetic Acid
- Household bleach (Clorox)
- Lemon-lime Soda (7-up or Sprite)
- Lemon Juice
- Vinegar (white)
- Any other relatively clear household cleaner or juice you might want to check
- Small amount of dry ice (solid CO2) if available
Results of Experiment (Observation):
Here are the approximate colors your red cabbage juice indicator should give for various pH levels. The neutral purple (pH 7) is about where the juice will be when first prepared. Then as you mix with other substances the color may change.
Changes toward the red will indicate acidic substances, with stronger acids producing a deeper red. Changes in color toward the green will indicate base substances.
If you do not have all the measurement equipment such as graduated cylinder, you may need to do some calculations or scale, you might need to do some calculations to convert your measurements to what is described here, otherwise no calculations will be needed.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
In your conclusion write if you were able to make pH indicator using cabbage. Also write the results of your pH test of different household items.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Using Raspberries As A pH Indicator
Although red cabbage is the most popular homemade indicator, blackberries can also be used to make a somewhat less sensitive version of litmus paper.
Simply place a half cup of blackberries in a small bowl and use a fork to mash them into a pulp gooey pulp. Next, add a small quantity of water and mix thoroughly so that you have a thin but heavily colored berry juice. Then cut some strips off of a sheet of white construction paper (¼” wide by 3″ long would be good) and drag them through the berry juice first on one side and then the other until they are soaked through and through. Slide the strip through your fingers to rake off any pulp that stuck and lay the strips side by side on a paper towel to dry. Once dried, examine both sides to make sure there are no pieces of pulp still stuck on (pick them off if there are) and your homemade pH paper is ready to use.
Your blackberry litmus paper will have a purple color when you’re done and will turn a deep purple when dipped in a base (alkaline) solution and a pinkish red when dipped in an acid. If you are comparing several substances the relative darkness of the colors will indicate the relative amount of acid or base in the substance being tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
CLICK HERE if you want to know why red cabbage changes color.
Want to learn even more about pH, in more detail?
Here’s a site that good a good overall explanation of pH:
And here is a really good tutorial on pH:
Books with related materials:
Bath, John B. and Mayberry, Sally C. Kitchen Chemistry, Carson-Dellosa, 1994.
A step-by-step series of science books containing simple science experiments for kindergarten through six grade students.
Chem Fax, Flinn Scientific, 1990. This is an excellent source of teacher demonstrations and classroom activities for students.
Gardner, Robert. Science Projects About Kitchen Chemistry, Enslow, 1999. Contains ideas for science projects using various foods or other materials found in the kitchen.
Herron, Frank, Sarquis, J., Sarquis, M., Schrader, and Kulka, Chemistry, D.C. Heath and Company, 1996. This unique textbook includes macro and micro laboratory investigations in each of the chapters. It also has many “Consumer Chemistry” sections throughout the book.
Hillman, Howard. Kitchen Science: A Guide to Knowing the Hows and Whys for Fun and Success in the Kitchen, Houghton Mifflin, 1989. A very enjoyable and easy reading book that answers many common questions dealing with the science of foods and cooking.
LeMay, Beall, Robblee, and Brower. Chemistry: Connections to Our Changing World, Prentice Hall, 1996. This book is used by many high schools as a first year general chemistry textbook. Contains approximately thirty “Chemistry in Action” activities which relate chemistry concepts to the real world.
Loeschnig, Louis V. Simple Chemistry Experiments With Everyday Materials, Sterling, 1995. This book is ideal for elementary and middle school students but many activities could also be used at the high school level to demonstrate a specific concept.
Mandell, Muriel. Simple Kitchen Experiments: Learning Science With Everyday Foods, Sterling, 1994. A great book packed full of ideas to teach many science concepts using food products.
McGee, Harold. On Food and Cooking: The Science and Lore Of the Kitchen, Simon and Schuster, 1984. A great reference book dealing with the science of food and cooking. Includes a lot of historical and background information on most food products.
McGee, Harold. The Curious Cook: More Kitchen Science and Lore, Macmillan, 1990. This is the second of two books written by the author. In this book he tries to answer more questions relating foods and cooking. It is written in story form and contains less historical information than the first book.
Phillips, John S., Victor S Strozak and Cheryl Wilstrom. Chemistry: Concepts and Applicatons, Glencoe/McGraw-Hill, 1997. A general chemistry textbook containing “MiniLabs” and “ChemLabs” throughout each chapter.
Wilbraham, Staley, and Matta. Chemistry, Addison-Wesley, 1997. This is the textbook currently being used by the Pittsburgh Public Schools for all general chemistry classes as well as the scholars chemistry classes. It contains many simple activities, sections dealing with consumer chemistry and other helpful topics. A very well written text which students find easy to comprehend.
Williams, Tammy K. Science Experiments Chemistry and Physics, Mark Twain Media, 1995. Science experiments written specifically for middle school students that can easily be adapted to fit any high school curriculum.
Zumdahl, Steven S. Introductory Chemistry: A Foundation, D. C. Heath, 1996. A textbook suitable for high school students. It contains interesting “Chemistry in Focus” sections throughout each chapter.