Acids are materials that have certain properties in common. Bases (also called alkalis) are other substances with a different set of properties. In these experiments, you will investigate some of these properties with materials that are found around your home. In addition, you will learn how chemists use the pH scale to describe acids and bases.
The most striking property of both acids and bases is their ability to change the color of certain vegetable materials. A common vegetable whose color responds to acids and bases is red cabbage. The first step in this experiment is to prepare an extract of red cabbage, so you can investigate its color changes. Place about 500 milliliters (2 cups) of red cabbage cut into 2.5-centimeter (1-inch) cubes into a blender or food processor. Add about 250 milliliters (1 cup) of water and blend the mixture until the cabbage has been chopped into uniformly tiny pieces. Strain the mixture by pouring it through a sieve. This strained liquid, the red-cabbage extract, will be used for exploring acids and bases.
Examine the label of a bottle of white vinegar. The label probably says that it contains acetic acid. This indicates that vinegar is an acid and has properties of an acid. Let's see what effect an acid has on the color of the red cabbage extract. Pour 125 milliliters (½ cup) of vinegar into a colorless drinking glass. Add 5 milliliters (1 teaspoon) of red cabbage extract, stir the mixture, and note its color. What is the color of the mixture. (Write your answer in the box.)
The color of the cabbage extract with vinegar is the color the extract has when it is mixed with an acid. Save the mixture in this glass to use as a reference in the rest of the experiment.
Now examine the effect of laundry ammonia on the color of red cabbage. Pour 125 milliliters (½ cup) of laundry ammonia into another colorless drinking glass. Add 5 milliliters (1 teaspoon) of red cabbage extract and stir the mixture. Write the color of this mixture in the box.
Laundry ammonia is a base (alkali). The color of this mixture is the color of cabbage extract when it is mixed with a base. The color of cabbage extract indicates whether something mixed with it is an acid or a base. Cabbage extract can be called an acid-base indicator. Save the mixture in this second glass to use as a reference.
Now test the properties of a solid, baking soda. Place 5 cubic centimeters (1 teaspoon) of baking soda in a glass and add 125 milliliters (½ cup) of water. Stir the mixture until the baking soda has dissolved. Then, add 5 milliliters (1 teaspoon) of red cabbage extract to the solution. Write the color of the mixture in the box.
The color obtained with baking soda is different from the color obtained with vinegar and from the color obtained with ammonia.
Red cabbage extract can indicate whether a substance is an acid (like vinegar) or a base (like ammonia). It can also show how strong an acid or a base a substance is. Chemists use the pH scale to express how acidic (like an acid) or basic (like a base) a substance is. A pH value below 7 means that a substance is acidic, and the smaller the number, the more acidic it is. A pH value above 7 means that a substance is basic, and the larger the number, the more basic it is. Red cabbage extract has different colors at different pH values. These colors and approximate pH values are:
|color of extract:||red||purple||violet||blue||blue-green||green|
Based on this information, what is the approximate pH of vinegar?
What is the approximate pH of ammonia?
What is the approximate pH of the baking soda mixture?
Use the instructions for testing vinegar and ammonia to test the pH of several other nearly colorless liquids, such as lemon-lime soft drink (Sprite or 7-Up) and lemon juice. Record your observations. Liquids that are white, such as milk, can be tested in the same way. You can also test solids that dissolve in water by following the instructions for baking soda. This will also work with viscous liquids such as liquid detergents. Test other substances around the house, such as sugar, table salt, shampoo, hair rinse, milk of magnesia, antacid tablets, and aspirin.
|Material||Extract color||pH||Material||Extract color||pH|
CAUTION: Some household products can cause skin irritations. Do not allow these to contact skin; rinse thoroughly with water if they do.
|For additional information, see CHEMICAL DEMONSTRATIONS: A Handbook for Teachers of Chemistry, Volume 3, by Bassam Z. Shakhashiri, The University of Wisconsin Press, 2537 Daniels Street, Madison, Wisconsin 53704.|