In this experiment modified from The Best of WonderScience, p282-283, we will be using a chemical indicator to determine whether certain household substances are acids or bases. For an example of a natural indicator, look at your face: Happiness can be indicated with a smile, and sadness with a frown! The indicator we will be using in this experiment is a substance that changes color when it reacts with an acid or a base.

An acid is a substance that has the ability to accept an extra proton (H+) from a chemical. A base is a substance that has the ability to donate an extra proton (H+) to a chemical. When some chemicals lose or gain a proton, their chemical structure changes so that they absorb different wavelengths of light. When a solution absorbs the red wavelengths of light, it shows up as the complementary color (green). When a solution absorbs the yellow and blue wavelengths, it looks red: these are called indicators. The indicator we will be using is from red cabbage leaves. When an acid is added to this indicator, it turns pink. When a base is added, it turns blue to green. These color changes signify a change in the pH of the solution. The pH scale is used to judge how acidic or basic a solution is; the scale goes from 1 to 14, with acids having pHs less than 7, and bases having pHs over 7. Water is found to be neutral, with a pH of approximately 7. The way this experiment is set up allows students to take the initiative for inquiry into the properties of acids and bases, using the indicator solution as a guide and test.

As a review of some of the basic chemistry involved in this experiment, have the students discuss what acid is and give examples of it (e.g. the Joker's little accident in Batman, or the acid found in certain foods that gives stomach aches to certain people). Explain that bases are opposite from acids; they differ in their molecular structure, as well as their behavior when reacting with other substances. Discuss examples (Mylanta, household cleaners). You can help them to see the importance of reactions between acids and bases through an example like treating a stomach-ache (acid) with Mylanta (base) to counter-effect the work of the acid in our stomachs! Give the students the opportunity to work on how they would choose to test for the presence of acids or bases in household substances, using an indicator such as the one that has been described above, before they read the directions and engage in this experiment.

Acid

base

chemical

indicator

neutralize

proton

This is a very easy indicator to make. Cut the red cabbage into small pieces and place in a large jar of EXTREMELY HOT tap water for approximately 15 minutes. The amount of water and cabbage is dependent on how much indicator you will need per student (approximately 300mL per set-up), and how dark you want the indicator to be. Using hot tap water gives a light indicator, which works fine. If you are unable to procure hot tap water, or prefer not to wait 15 minutes, it also works well if you boil the cabbage in water for approximately 5 minutes. This gives a very dark indicator, which works more quickly.

There are two kinds of indicators at work here. The most common red-blue one is found in cabbage (red when the pH is less than 7, blue when it is higher than 7, and a combination of the two (purple) when it is neutral at 7). The second is colorless-yellow, and changes at a much higher pH level. For this indicator, if the pH is above 9 or 10, then it will turn yellow, below that it remains colorless. You will see that if you add enough base to the cabbage juice, it will turn green due to the combination of yellow indicator and blue indicator. If you do not have access to red cabbage, there are several indicators that will respond to acids and bases in a similar manner. Cranberry juice, grape juice, raspberry juice, cherry juice, blueberry juice, and violet petals are a few of the substances that will indicate the presence of acid and base. Reference The Best of WonderScience p292 for other ideas.

Place all materials needed for this experiment in an accessible place, divide the students into cooperative groups and give them the following instructions:

1. Put three clear plastic cups on a white plate, and label them #l, #2, and #3.
2. Add approximately 1 inch ( 1 ") of indicator from the labeled bottle (see Procedure for indicator), making sure that all three cups have equal amounts.
3. Using the pipette or dropper, add 1 drop of vinegar to cup #1 and swirl. What does the indicator tell you? Compare this cup to the other cups of indicator solution, and write these observations in your lab notebook.
4. Continue adding drops of vinegar and swirling until the color stops changing. (I found that it took approximately 12 drops of vinegar to change to a pink color, and that further additions of vinegar did not change the hue of the solution noticeably at all, which suggested a plateau effect. This level depends on how strong the vinegar is. Some old vinegars have microorganisms growing in them that make them more acidic, causing them to react more quickly or strongly.)
5. Add a small scoop (less than a teaspoon) of detergent to cup #2 and swirl (I found that it took much less detergent than I expected for the indicator to change color and plateau).What does the indicator tell you? Compare this cup to the other cups of indicator solution, and write these observations in your lab notebook.
6. Now hypothesize as to what could make these indicators return to their original purple color. Design your own experiment to test this out, making sure that you have a control, and a way to measure your results. (I found that adding acid to the basic mixture, and base to the acid mixture does return the indicator color to nearer its original hue, although not very precisely.)
7. Perform your own experiment to test another substance for its acidity or basicity, keeping clear lab notes so that if anyone needs to refer to your work, they will be able to follow your instructions.
8. What are your results? Did the indicators behave as you thought they would? Write down your observations in your lab notebook.

Discuss with the class their results, making sure they understand the vocabulary. Have them name acids and bases that they use every day (e.g. citrus fruits are acidic, household cleaners are basic). Make sure they understand what it means to neutralize a solution and have them propose a way to figure this out using the scientific method. * MAKE SURE NOT TO MIX BLEACH AND AMMONIA IN ANY FORM IN THE LAB. This can be extremely dangerous.

Students also need to list, or discuss, some of the common characteristics of substances in each group (e.g. a lot of acids have a strong smell, or taste strong, while a lot of bases taste more bland). Make certain that students know never to taste chemicals in the lab, or to take a whiff of strong acids or bases.

Have each group submit a lab report that includes their original hypothesis, observations, results, and conclusions.

This experiment is important to understand because acid-base interactions do occur in our everyday lives. Discuss physiological effects of heartburn (stomach acid) and how people can take Mylanta (a basic solution) to calm heartburn, or to neutralize it. Discuss other ideas or suggestions. Further extensions could be to look into the effects on the indicator of adding different household solutions, e.g. milk, cleaners, eggs, juice, etc. to determine whether they are acidic or basic. These could be tested using a simple paper indicator made by soaking the paper in the cabbage juice, or other indicator. Students could also make an indicator for only bases using Turmeric. A project more related to art could be to work on making a rainbow in the glass jars using indicators with acids and bases, to leave up for a day as decoration. See also the lab enclosed in this packet entitled "Must it Rust?" for other examples of chemical reactions.

SCIENCE CONTENT STANDARD #1: NATURE OF SCIENCE

This experiment satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."

This experiment also satisfies Benchmark #2 for grades K-4: "investigate questions by following scientific procedures (e.g., making and recording observations)."

Chemical reactions are occurring all the time, all around us. This provides many opportunities to connect other disciplines to the use of this experiment. Culturally and historically, it is interesting to look at the types of food that people eat in different countries and environmental areas, and what reasons there may be for this (e.g. Using more vinegar and curing spices in humid places, so that food does not rot as quickly). See other labs in this packet that teach more about chemical reactions, such as "Must it Rust?", "The Egg-citing Egg-speriment", and "A Green penny?".