In this activity, students discover two ways to detect acids. Then they test condiments and other foods to figure out which ones are acids.
Step 1: Think ahead
For a class or large group, we suggest students work in groups of four at a workstation where they can share supplies. Within each group, students will work with a partner. Students working at home can work on their own or with a partner.
Before buying supplies, figure out how many groups you’ll have, so you can set up supplies accordingly.
Step 2: Gather equipment and supplies
To prepare for class, you’ll need:
- measuring spoons and cups
- a sharpie or permanent marker
- 1 cup of dry black beans or 2 cups of chopped raw purple cabbage
- 2 cups of water
- a container that can hold at least 4 cups of water (for making purple indicator liquid; see Step 4 for instructions)
- 20 drinking straws (you’ll be modifying these so students can use them for transferring liquids; see Step 4 for instructions)
- at least two foods or drinks that students can test for acidity (Anything in the kitchen is fair game. You can keep it simple and have students all test the same foods, or give every group something different to test. We recommend choosing at least one acid, such as lemonade, ketchup, mustard, pickle juice, yogurt, or sour cream, and one non-acid, such as mayonnaise, milk, or soy sauce. You’ll need about 1 cup of each for a class of 32.)
For each group of four students (or each solo student), you’ll also need:
- newspaper or plastic to cover work areas
- baking soda (about 1 tablespoon; 8 groups need about half of a 1-pound box)
- baking powder (about 1 tablespoon; 8 groups need about 7 ounces)
- vinegar (about 2 tablespoons; 8 groups need about 2 cups)
- water (about 2 tablespoons; 8 groups need about 2 cups)
- 2 craft sticks or spoons (for spooning out powders; 8 groups need 16 craft sticks or spoons)
- 7 small paper cups: 5 for testing known substances, and 2 for testing unknowns (We used 3-ounce bathroom cups, also known as Dixie cups. For 8 groups, you’ll need a total of 56 cups.)
For each pair of students working together (or each solo student) you’ll need:
- a sheet of Press’n Seal sealable plastic wrap; you can also substitute taped-down waxed paper (For 8 groups, you’ll need 16 sheets.)
- a few toothpicks (for mixing samples)
- paper towels (for cleanup)
Step 3: Print out materials
Each table will need:
Each pair of students will need:
Each student will need:
Here is a "Results Answer Key" for you to reference if needed.
Step 4: Prepare for class
Cut straws in half to make short straws. Students will use these to transfer drops of liquid. (Full-length straws are likely to tip over cups.) For a class of 32, we suggest you cut 20 full-sized straws in half. That gives you 5 short straws per group—3 for initial testing and 2 for testing unknowns.
Make the purple indicator liquid. For a class of 32, you’ll need about 1 cup. (If you’re using black beans, put 1 cup of beans into 2 cups of water and leave them for at least an hour. The beans will soak up some water and turn the rest purplish brown. If you’re using purple cabbage, put 2 cups of chopped cabbage in 1½ cups of water. Leave it for at least an hour, stirring occasionally. The cabbage will turn the water purple.)
To distribute each group’s materials quickly and easily, we recommend you set up three supply stations.
Station A: Acid Reaction Supplies
- “Acid Reaction Supplies” sheets (1 per group)
- a cup containing 2 tablespoons of water (1 per group)
- a cup containing 2 tablespoons of vinegar (1 per group, labeled “V” with marker so students can quickly tell it from the water)
- short straws (2 per group)
Station B: Testing Supplies
- “Testing Supplies” sheet (1 per group)
- a cup containing 2 tablespoons of baking soda (1 per group; note that you may want to tell students that baking soda is different from baking powder)
- a cup containing 1 tablespoon of baking powder (1 per group, labeled “BP” with marker so students can quickly tell it from the baking soda)
- a cup containing 1 tablespoon of purple indicator liquid (1 per group)
- craft sticks or spoons (2 per group)
- short straw (1 per group)
Station C: Unknowns
- a cup containing 1 tablespoon of each unknown substance used for testing (we recommend two test substances per group, each in its own cup)
- a short straw for each test cup
- a toothpick for each test cup
The purple liquid that you prepare from the cabbage is called an indicator. There’s a pigment in purple cabbage that changes color when it reacts with an acid or base. You and your students should notice that the color of the cabbage juice changes to a reddish/pink when you add it to any of the acids (e.g. vinegar). You can then use this information to test unknown liquids. If the liquid turns pink, then it’s an acid. You can learn more about cabbage juice indicator here.
You and your students will also notice that when baking soda is mixed with vinegar, there is fizzing that indicates an acid-base reaction. Baking powder will also fizz with vinegar. But you will notice that baking powder will also slightly fizz when water is added. This is because baking powder is actually a mixture of baking soda (base) and cream of tartar (acid). This is why it reacts with both water and vinegar.