Key concepts

You might have done experiments with well-labeled acids and bases in school, but have you ever wondered whether a certain food or chemical around the house is an acid or a base? You can find out using a red cabbage to make an indicator solution.

When two or more ingredients are entirely dissolved in one another, you have a solution. For example, mixing salt with water creates a clear solution, even though the salt is there and the solution tastes salty. When mixed with water, whether a chemical "donates" a charged particle (called an ion) to the solution—in this case, a hydrogen ion—or "accepts" one from it determines whether it's an acidic or basic solution. An indicator changes color when exposed to such a mixture, depending on whether the solution is acidic or basic.

Acids are solutions that lose hydrogen ions and usually taste sour. Some very common household solutions are acids, such as citrus fruit juices and household vinegar. Bases are solutions that pull hydrogen ions out of solution and onto themselves, "accepting" them, and usually feel slippery. Bases have many practical uses. For example, "antacids" like TUMS are used to reduce the acidity in your stomach. Other bases make useful household cleaning products.

To tell if something is an acid or a base, you can use a chemical called an indicator. An indicator changes color when it encounters an acid or base. There are many different types of indicators, some that are liquids and others that are concentrated on little strips of "litmus" paper. Indicators can be extracted from many different sources, including the pigment of many plants. For example, red cabbages contain an indicator pigment molecule called flavin, which is a type of molecule called an anthocyanin. Very acidic solutions will turn an anthocyanin red whereas neutral solutions will make it purplish and basic solutions will turn it greenish-yellow. Consequently, the color an anthocyanin solution turns can be used to determine a solution's pH—a measure of how basic or acidic a solution is.

•    A small red cabbage
•    Pot of boiling water
•    Strainer
•    Two large bowls or pots
•    Grater
•    Tablespoon measurer
•    Large spoon (optional)
•    Three or more small, white paper cups (small, white paper drinking glasses or dishes will also work)
•    Goggles or other protective eyewear
•    Lemon or lime juice
•    Vinegar
•    Bleach-based cleaning product
•    Other foods to test, such as clear soda pop, baking soda solution, egg whites, tomatoes, cottage cheese (optional)

•    Grate a small red cabbage. If you do not want to grate the entire cabbage, grating half of a cabbage should be enough. Put the fine, pulpy grated cabbage into a large bowl or pot.
•    Boil a pot of water. Use caution when handling the boiling water. Pour the boiling water into the bowl with the cabbage pulp until the water just covers the cabbage.
•    Leave the cabbage mixture steeping, stirring occasionally, until the liquid is room temperature. This should take at least half an hour. The liquid will become red or purplish-red in color.
•    Place a strainer over another large bowl or pot and pour the cabbage mixture through the strainer to remove the cabbage pulp. Press down on the pulp in the strainer, such as by using a large spoon, to squeeze more liquid out of the pulp.
•    In the bowl, you should now have only liquid that will either be purple or blue in color. This will be your indicator solution, which you will use to test the pH of different liquids.
•    Children should wear goggles or other protective eyewear and adults should supervise and use caution when handling bleach and vinegar, because they can irritate eyes and skin.

•    Fill a small, white paper cup, drinking glass or white dish with one tablespoon of your cabbage-indicator solution. What is the color of your indicator solution?
•    Add drops of lemon or lime juice to the indicator solution until you see the solution change in color. Gently swirl the solution and make sure the color stays the same. What color did the solution become?
•    The color of the solution will change depending on its pH: Red color indicates the pH is 2; Purple indicates pH 4; Violet indicates pH 6; Blue indicates pH 8; Blue-green indicates pH 10; Greenish-yellow indicates pH 12.
•    Based on its color, what is the pH of the lemon or lime juice solution?
•    In another small, white paper cup, add one tablespoon of your original cabbage-indicator solution. Add drops of vinegar until you see the solution change color. What color did the vinegar solution become? What is the pH of the solution?
•    In a third small, white paper cup, add one tablespoon of your original cabbage-indicator solution. Handling it with caution, add drops of the bleach cleaning product until you see the solution change color. What color did the bleach solution become, and what does this indicate about its pH?
•    If you want to test the pH of other foods, again add one tablespoon of your original cabbage-indicator solution to a small, white paper cup and add drops of the food until you see the solution change color. If the food is not in liquid form, crush it or dissolve it in a small amount of water before adding it to the indicator solution. What color did the solution become, and what does this indicate about its pH?
•    Extra: There are other vegetables and fruits that can be used to make pH indicators as well: red onion, apple skins, blueberries, grape skins and plums. Which different sources of pigment produce the best indicators?
•    Extra: You can use an indicator solution to write secret messages. Just use full-strength lemon juice to write an invisible message on paper and let the message dry. To reveal the message, paint cabbage-indicator over the paper with a paintbrush.

Observations and results
Did the indicator solution change color when you added the lime or lemon juice, vinegar and bleach? Did the solution color indicate that the lime or lemon juice and vinegar were acidic (had a lower pH) and that the bleach was basic (with a higher pH)?

A solution with a pH between 5 and 7 is neutral, 8 or higher is a base, and 4 or lower is an acid. Lime juice, lemon juice and vinegar are acids, so they should have turned the indicator solution red or purple color. Bleach is a strong base, therefore it should have turned the indicator solution a greenish-yellow color.

How basic or acidic a solution is depends on the amount of hydrogen ions in it. A basic solution accepts hydrogen ions (or donates electron pairs as hydroxide ions) whereas an acidic solution donates hydrogen ions (or accepts electron pairs). An indicator, like anthocyanin, responds to the levels of hydrogen ions in the solution. Anthocyanin and other biological pigments absorb certain wavelengths of light and reflect others, and it is the reflected light we see that makes them appear a certain color. Depending on the levels of hydrogen ions in the solution, the indicator pigment undergoes a chemical reaction that changes its chemical structure, making it reflect a different wavelength of light and thereby change color.

Dilute the bleach solution with water before pouring it down a drain. (Remember to keep your goggles on when you do this.)

More to explore
"Experiments with Acids and Bases" from Fun Science Gallery
"Acids and Bases Are Everywhere" from Rader's
"Acids, Bases, and the pH Scale" from Science Buddies
"Cabbage Chemistry" from Science Buddies

This activity brought to you in partnership with Science Buddies