pH indicators and tests for acids and bases

Students will create various pH indicators and test unknown solutions to determine if they are an acid or a base.

A lesson plan for grades 9–12 Science

By Richard Beckham

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Learning outcomes

Students will:

• Correctly measure and mix the components.
• Create a table and record data correctly.
• Compare data and draw conclusions.
• Identify unknown solutions as acids or bases using the indicators they created.
• Consider how the scientific community verifies data.
• Engage in a matching game of acid/base, and pH information at a designated web site.

Teacher planning

Time required for lesson

2 hours

Materials/resources

• triple beam balance
• multiple dropper bottles
• heating element (hot plate)
• distilled water
• rubbing alcohol
• one large red beet
• 0.1 gram bromthymol blue powder
• one head of red cabbage
• two coffee filters
• four to six laxatives tablets that contain phenolphthalein
• one packet of turmeric from the spice section of the grocery store
• four percent sodium hydroxide
• ammonia
• vinegar

Technology resources

• computer access

Pre-activities

This lesson should be preceded by lessons on lab safety, scientific law and scientific method. The pH scale and values for acids and bases should be covered. Knowledge of scientific measurement and equipment should be covered.

Activities

Beet Juice Indicator Solution

Wash and slice a fresh beet. Place about four slices of beet into a pan containing one cup of water. Heat until boiling and continue boiling for about five minutes. Remove the beet slices and allow the red liquid to cool. Store in dropper bottles. Beet juice is red in acidic solutions and blue in basic solutions.

Bromthymol Blue Indicator Solution

Dissolve about 0.1 grams of bromthymol blue powder in one liter (1 quart) of distilled water. Stir well to dissolve. If the solution is green, it is neutral. Add a four percent solution of sodium hydroxide drop-by-drop until the color changes to deep blue. Store in dropper bottles. Bromthymol blue solution (BTB) is blue in basic solutions and yellow in acidic solutions. (Caution: BTB stains hands and clothing. Have students wear safety goggles when using BTB. Sodium hydroxide is corrosive. If spilled, flush affected areas with water.)

Red Cabbage Indicator

Tear five leaves of red cabbage into small pieces. Place the cabbage pieces in a small pan. Add four cups of hot water. Let the leaves soak for about half an hour until the water is a deep purple and cooled to room temperature. Strain the liquid into a storage bottle. Cover and store in the refrigerator.

Cabbage Indicator Paper

Pour one cup of cabbage indicator (above) into a bowl. Dip one or two coffee filters into the indicator. Place the wet filter paper on a cookie sheet or flat pan. Continue to soak the paper until saturated. Allow the paper to dry (this will take more time than your class time, so use it the next day or for another activity). The paper will be pale blue. Cut the dry papers into strips about 1.25 by 7.5 centimeters (0.5 by 3 inches). Store the strips in a zip-lock plastic bag. Cabbage paper turns green in the presence of bases and pink to red in the presence of acids.

Phenolphthalein Indicator Solution

Purchase any laxative that contains phenolphthalein. With the back of a spoon, mash four to six tablets in a saucer. Pour the powder into a small cup. Add about ten milliliters of rubbing alcohol. Let this mixture stand for fifteen minutes. Pour off the liquid and store in a dropper bottle. Phenolphthalein is purple in very basic solutions and colorless in acidic solutions.

Turmeric Indicator Solution

Obtain a package of turmeric form the spice section of the grocery store. Add ¹/₄ teaspoon of turmeric to four tablespoons of rubbing alcohol. Stir to mix. Store in dropper bottle. Turmeric solution stays yellow in the presence of acids and changes to purple-brown in the presence of bases. Turmeric solution can also be made into indicator paper (see Cabbage Paper). Dry turmeric paper is bright yellow and changes to red in the presence of bases.

Test the Unknown Solution

Pour the ammonia and the vinegar into unidentified dropper bottles. Each group of five or six students will use these solutions to test their indicators. Place two to three drop of each indicator solution in a small dish and test the unknown solution and record the color change.

Assessment

1. Students should be directed to play the matching game. The game will reinforce the basic information and knowledge gained during the activity.
2. Upon completion of the game activity the students should be directed to take the quiz (Note: the quiz website may require registration or subscription). The instructor can choose to print the quiz as a formal activity or let the students take the quiz on their own for self-assessment.
3. The compilation of class data and student scores should indicate successful mastery of the objectives.

Supplemental information

Attachments:

• pH Indicator Template
• pH Indicator Solution

Comments

You might consider doing one indicator at a time if your time is limited. Save your indicator paper for future use when identifying acids and bases.