Indicators For Acids And Bases
Marian Johnson Morton Career Academy
431 North Troy
Chicago IL 60612
(312) 534-6791
Objectives:
Fourth grade students will be able to complete the following:
1. Make cabbage water as an indicator and see the color it turns when
acids and bases are mixed in it.
2. Observe the colors of the pH scale that range from 0 to 14.
3. Associate certain colored litmus paper with acids or bases.
4. Use phenolphthalein or thymolphthalein indicators and observe how they
react with acids or bases.
Materials needed:
Activity 1
Teacher preparation:
1. Make cabbage solution.
Take about one cup of shredded red cabbage, and boil it in two cups of
water for five minutes in a nonaluminum pan. Strain out the cabbage and
keep the water. The water should turn purple. If the water changes to
pink, throw that batch away. Refrigerate any portion you don't use
immediately. (Can keep a day or two.) Frozen extract can be kept for
months.
2. Common substances:
Make each of the 16 common household solutions before class. (dissolve only
enough of the solids to slightly cloud the water.)
diluted ammonia dissolved aspirin
rubbing alcohol distilled water
dissolved baking soda white vinegar
dissolved baking powder lemon juice
detergent (liquid) dissolved cornstarch
soap (liquid) antacid
hydrogen peroxide all-purpose cleaner
window cleaner dissolved washing soda
Pour each solution into a jar labeled with solution name.
Also--pH scale, pH paper, and eye droppers.
Activity 2
Teacher preparation:
1. Make phenolphthalein indicator solution by dissolving 0.1 g phenolphthalein
in 50 mL of ethyl alcohol. Add 50 mL water.
2. Make thymolphthalein indicator solution by dissolving 0.1 g thymolphthalein
in 50 mL of ethyl alcohol. Add 50 mL water.
3. Make baking soda solution by mixing 1 g of baking soda per 100 mL of water.
4. Make washing soda solution by mixing 1 g of washing soda per 100 mL of
water.
Also--red litmus paper eye droppers plastic cups
blue litmus paper white vinegar plastic cup with rinsing water
Strategies:
Activity 1
Introduction to students:
Scientists never touch or taste a substance to see if it is an acid or
a base because both acids and bases can destroy the skin. A safe way to find
out if a substance is an acid or base is to use an indicator. Indicators are
a group of compounds that change color when added to acids or bases. The color
change indicates whether a substance is an acid or base.
We are going to begin our work with indicators that are made from natural
objects like red cabbage juice. We will also use chemical paper indicators
called pH paper, litmus paper, and chemical solution indicators called
phenolphthalein and thymolphthalein.
1. Organize your students in small groups and give each group a recording
chart. Identify the test substances and point out potential dangers. Then
distribute the test substances, a cup of cabbage juice, pH scale, pH paper,
and a dropper or a drinking straw to each group of students.
2. Before students test the substances, instruct them to write the names of
the solutions and the "original color" in appropriate column on their
chart.
3. Ask students to add two or three droppers of cabbage juice to each cup and
gently swirl the mixture. Have them note the color changes in the
appropriate column on their chart. Remind students to swirl the cups
rather than use the dropper to stir. Ask students why this is important.
(The dropper could contaminate the test sample.) If droppers are not
available, substitute straws. To use the straw, submerge a portion of it
in the cabbage juice. Place your finger over the other end. Hold the
straw above the test cup and remove your finger from the end of the straw.
Add two or three "strawloads" to each cup and swirl.
4. Explain to students that a pH of 7 is neutral (neither acidic or base).
The farther below 7, the more acidic the solution; the farther above 7, the
more basic. Students will complete recording chart, listing the solutions,
pH factors (pH paper color and pH number) and identifying acids from bases.
Safety tip:
Students sometimes want to stick their noses near the substances to smell
them. When they try this with ammonia they receive a rude awakening. To
demonstrate wafting, wave your hand over the cup and "pull" the air toward your
noise. This allows you to smell a small amount rather than an eye-watering
dose.
Activity 2
1. Add 2 eyedropper squirts of baking soda solution to each clean cup. Add
2 squirts of vinegar to one cup and 2 squirts of washing soda to the other.
2. Observe carefully and record observations on chart.
3. Place one drop of vinegar on one end of a piece of blue litmus paper.
Place one drop of washing soda on the end of another blue piece of litmus
paper. Record your observations on chart.
4. Repeat the test using a piece of red litmus paper. Record observations
on chart.
5. Place 2 squirts of vinegar into a clean cup. Add 3 drops of
phenolphthalein solution to the cup. Record observations.
6. Place 2 squirts of vinegar into another clean cup. Add 3 drops of
thymolphthalein solution. Record observations.
7. Repeat steps 5 & 6 with washing soda instead of vinegar. Again record
results.
Chart For Test With Vinegar and Washing Soda
Name of Test Acids (Vinegar) Bases (Washing Soda)
Baking soda _______________ ________________
Blue litmus _______________ ________________
Red litmus _______________ ________________
Phenolphthalein _______________ ________________
Thymolphthalein _______________ ________________
Remember:
1. The last step is cleanup. Instruct students to pour the substances down
the drain with the water running. Rinse and dry the cups and stack in an
appropriate location.
2. As a point of information bases are alkaline. Batteries, soaps and
shampoos all use the name alkaline which means they are basic solutions
above pH 7.
Performance Assessment:
Questions that would ascertain if students know that:
1. Bases turn litmus paper______(blue).
2. Acids turn litmus paper______(red).
3. Phenolphthalein turns a base______(red).
4. Phenolphthalein is clear in an______(acid).
5. Thymolphthalein turns an acid______(red).
6. Thymolphthalein is clear in a______(base).
7. Cabbage juice is blue and turns pink or lavender in ______(acid).
8. Name some common liquids that are acids or bases.
9. Basic solutions are sometimes called__________(alkaline).
10. A solution with a pH of 7 is called a_________(neutral) solution.
11. A solution with a pH of 9 is a called a_______(base or alkaline).
Through observation by teacher, students should orally relate how they
determined:
1. If a solution is an acid or base.
2. Compare or contrast.
3. Recorded data.
4. Shared information and drew conclusion.
References:
Guzdziol, Ed. Demonstration given during SMILE, Summer, 1993
Burns, Joe. (1989, Feb.) The Cabbage Caper. Science Scope pp. 28-31.
Hanshumaker, William. (1987, Nov./Dec.) A Head For Chemistry.
Science and Children pp. 24-26.
Phillips, Donald. (1986, Jan.) The Magic Sign: Acids, Bases. and
Indicators. Science and Children pp. 120-123.
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