This project allows you to play a little with chemistry at home. You will make your own acid-base indicator using red cabbage and use it to test the acidity of several household chemicals. You will be required to write a short lab report.
It has been known for over 400 years that certain colored plants contain a colored substance which changes color depending on the kind of chemicals it is exposed to. These colored substances are called indicators because they indicate what their surroundings are like. Red cabbage can be used to make an indicator that shows whether a substance is acidic or basic. Something that is acidic is sour like vinegar or lemon juice. Something that is basic (also called alkaline) is caustic and bitter like soapy water or ammonia. The categories of acids and bases are very useful since many things we eat, drink or use in the home are either acidic or basic.
- a ½ cup finely chopped
red or purple cabbage
Do not use radicchio;
it may not work
- a ½ cup water
- four clean, recycled glass jars w/lids
(old jam or pickle jars, for ex.)
- a microwave
- small disposable cups
(white inside or clear)
- kitchen strainer
- baking soda
- more water
- distilled (white) vinegar
- clear carbonated beverage
- 3 other household items of your choice
Throughout the procedure keep a lab notebook handy to jot down observations and results as you work. Also write down any modifications you make to the procedure.
Part I: Making the Indicator
- Finely chop some of the red cabbage so that you have about ½ cup; you can use a blender or food processor for this
- Put the chopped cabbage and the ½ cup of water into one of your jars and microwave for about 30 s; it should not get hot enough to scald or boil
- Crush the slightly cooked cabbage in the water to get more of the juice out then strain the purple-colored solution into a clean jar; the resulting solution is your acid-base indicator
- Store this solution in a tightly capped jar…for obvious reasons (the smell)
Part II: Making Standard Solutions
- Pour ½ cup tap water into one of your jars
- Pour ½ cup of distilled (clear) vinegar into another one of your jars
- Measure about ½ cup of water into a different jar and add a tablespoon of baking soda; mix thoroughly
- Add about 1 tablespoon of the indicator solution to each of these jars; what color do you see in each jar?
- Keep these solutions handy for comparison with other substances
Water is neither an acid nor a base: the color the indicator has in water is the color it has when a solution is neutral (that is, neither acidic nor basic).
Vinegar is an acid: the color the indicator turns in vinegar will always indicate an acid.
Baking soda is sodium bicarbonate; in water it breaks into sodium ions and bicarbonate ions and the bicarbonate is a base: the color the indicator turns in baking soda solution will always indicate a mild base. Stronger bases may cause the color to change yet again.
Part III: Testing Household Chemicals
- Use a disposable cup to perform the following tests
- To test a substance, pour it into a cup and add about 1 teaspoon of your indicator solution
- Compare the color you see to the jars containing water, vinegar, and baking soda solution and determine whether the substance is acidic, basic or neutral
- First perform tests with the clear soda and the ammonia (ammonia is a very strong base and may cause the indicator to go beyond the color of the baking soda water)
- Identify at least three other substances you can test this way and test them (you could try lemon juice, other food items, household cleaners, or anything that you can make into a reasonably clear solution)
- Safety note: Do not mix chemicals when you do not know what the results will be. In particular never mix two household cleaners because many of them will react with each other to produce poisonous gases. One combination to avoid at all costs is a mixture of bleach and ammonia. Do not taste any of the materials!
Part IV: An Experiment
- Fill one of your disposable cups a little less than halfway with the vinegar that contains the indicator solution
- Pour some of the baking soda solution that contains the indicator into the same cup a little at a time until the cup is full
(Note: there will be some fizzing as you do this. If you do it slowly enough you will not overflow the cup. Try not to let it overflow since you need to observe the color change. This fizzing is fun but is not the point of this experiment.)
- What happened to the color in the cup that holds the vinegar as you added first a little and then more and more of the baking soda solution?
- What other observations could you make? What does the color of the resulting solution tell you about what happens when you mix and acid and a base?
A Few Questions
Answers to these questions should be in the Analysis section of your report. Research is required for your answers to these questions. Several paragraphs may be required for each answer! A bibliography is required.
- What is the definition of neutral in the context of this lab?
- What is the pH scale and what does it measure?
- Why is baking soda sometimes used as a remedy for heartburn?
- Use your indicator to find out whether some of the foods in your house are basic or acidic? Try fruits, vegetables, and several other items of your choice. What foods are acidic? What do acidic foods taste like? Are there any foods with a basic pH? Why or why not?
- There are four areas for taste on the human tongue: salt, sweet, bitter, and sour. Early humans did not have supermarkets or even farms so deciding whether something was good to eat was very important. People need salt to survive so the salt taste is clearly important. When something tastes sweet it means that it is good to eat (usually, as in ripe fruit). These two tastes clearly contribute to survival. What, do you think, do the bitter and sour taste areas on the tongue contribute to human survival?
For this project you must write a full formal lab write-up. See the description of this format that your teacher gave you earlier in the year. Also, you may have been provided with a sample lab report. Try to format your report the same way and use the content of each section to guide you as to what should be included in your own report.
You may work in groups to do the experiment but you must write your own report. Each individual student must write their very own report. No group reports will be accepted.
More Possible Experiments
For instance, what effect does dilution of the acid or base have on the color of the indicator in acids and bases? That is, can you make something closer to the neutral color by diluting with water? Or, can you quantify how much baking soda water it takes to make a given amount (say 1/4 cup) of vinegar completely neutral? Or, can you determine which substances are more acidic or more basic than others? What basis for comparison will you use? Write up the results of these or other experiments as part of the lab write-up. Any experiments not described here must receive prior approval from your instructor.