4 Experimenting with pH

By Daniell DiFrancesca

Provided by Kenan Fellows Program.

In this lesson, students will practice their critical thinking skills in an inquiry-based experiment. They will explore the effects of volume and concentration on pH as well as the relationship between acids and bases. This lesson is written using the 5E learning model, which includes five phases: engage, explore, explain, elaborate, and evaluate.

Learning outcomes

• Students will define the relationship between pH, volume, and concentration.
• Students will understand the pH scale.
• Students will increase their inquiry skills.

Teacher planning

Time required

Approximately 6 class periods (about 50 minutes each) are needed, however, some things can be assigned as homework to decrease the time spent in class.

Materials needed

For the Engage portion of the lesson:

• materials to make a cabbage juice pH indicator
• blank note cards or filter paper
• scissors
• dry storage (Ziploc bags or containers with lid)
• several known pH solutions (pH 2 through 13) in order to create a scale using the cabbage juice indicator
• Optional: Instead of making the paper you can also purchase pH Hydrion paper with a wide scale to use in the experiment.

For the Explore and Explain portions of the lesson:

• 1 piece of citrus fruit per group (it is best if the groups have different types of citrus fruit)
• plastic knives
• pH paper (made in the Engage section, can also be bought)
• fruit juicer
• 5 equal-sized containers per group
• distilled water
• graph paper
• overhead of a pH summary chart

For the Extend portion of the lesson:

• various types of peppers
• pH paper
• paper to collect and organize data
• student-requested materials

For the Evaluate portion of the lesson:

• fruit juice from the Explore stage
• peppers from the Extend stage
• pH paper

Student handouts

Technology resources

Overhead projector

Pre-activities

Students should be introduced to the basic pH scale. Do not give too much information before they begin the exploration.

Activities

Engage

What is an indicator?

(Time: 20 minutes for discussion and soaking paper/Time: 50 minutes to cut paper and make a scale.)

1. Teacher and class will discuss:
   • What is an indicator?
   • How do we use an indicator when testing pH?
2. Make cabbage juice pH indicator. Directions for this can be found several places online. Try these directions from the MadSci Network.
3. Students should soak their paper (either blank note cards or filter paper) in the juice. They should then allow the paper to dry overnight. Students should make an excess of paper for the experiment. After the paper is dry the students should neatly cut the paper into strips for pH testing. The paper should be stored in a dry container. A Ziploc bag or a container with a lid will work. The containers should be labeled so the students can use their own paper.
4. The students need to make a chart of the pH scale using their pH paper. Give each group a large note card. Ask the students to draw the pH scale. Wikimedia Commons has a nice model but you will want to hide the examples of each level and eliminate the shadings for now. Using their pH paper, have the students test their indicator in several known pH solutions. Allow the paper to dry before taping or gluing it to the note card below the appropriate pH. The students are creating a key to allow them to measure pH using their own pH paper.
5. The scale card should be properly labeled and stored in a Ziploc bag to prevent moisture exposure.

Note: If you are using purchased pH paper please introduce students to indicators in another way.

Explore

Experimenting with pH, concentration, and volume

(Time: 50 to 70 minutes)

The students will complete the lab worksheet. This lab has three pieces and may need to be spread into a second period. Please allow students to store their labeled containers of juice until complete. The 100 percent concentrated juice will need to be stored for use in the Evaluate section of the lesson. Please make arrangements with your students for the labeling and storage of the juice.

Explain

pH experiment follow-up

(Time: 50 minutes)

• Complete the lab questions on the lab worksheet.
• Students will need to summarize the class/team data and graph the pH of various types of fruit. Create a pH summary overhead to gather and share data with the students. Provide graph paper if necessary.

Elaborate

How does pH vary within the pepper family?

(Time: 50 minutes to plan; 50 minutes to complete)

1. The students will now develop their own experiment to test the variation of pH within the pepper family. Peppers are basic rather than acidic so it will give them an opportunity to explore the other side of the pH scale.
2. Provide the students with the experimental design graphic organizer. The students will need a class period to plan their experiment and create a data table for their information.
3. Students can bring in various types of fresh peppers and will only need a small piece of each. The peppers can be used throughout the day. You can allow them to create a common list of peppers for the class to use and each group can volunteer to bring in one type of pepper.
4. If students are cutting pieces of peppers themselves be sure they wear gloves with the hot peppers.
5. Allow the students to complete their experiment and organize their data.

Evaluate

What happens when we mix acids and bases?

(Time: 50 minutes)

1. Provide the students with a copy of the pH evaluation worksheet.
2. The students will need their 100 percent concentration juice from the Explore section and a piece of pepper from the Elaborate section.
3. Allow the students to make predictions about what will happen when an acid and base are mixed. Be sure they explain the reasoning behind their prediction.
4. The students should then test their hypothesis by mixing equal parts fruit juice and pepper juice. They should test the pH and record their observations.
5. The students should complete the questions in the evaluation worksheet.

Assessment

See the Evaluate portion of lesson.

Modifications

• The experimental design graphic organizer can be edited for any motor-skill deficiencies by making it larger, or making it available to be typed on.
• All basic modifications can be used for these activities.

Supplemental information

This is a list of all the attachments used in this lesson.

Critical vocabulary

• acid
• base
• neutral
• pH
• concentration
• volume

Comments

This lesson is part of the Critical Thinking in Science unit and relies on the inquiry skills and vocabulary practiced in the first two lessons (Introduction to Experimental Design and The Story of Pi). This lesson should be used while teaching Goal 4 of the science curriculum for grade 8 in the North Carolina Standard Course of Study, which focuses on chemistry. Students are designing their own experiments to improve their ability to approach problems and questions scientifically. By developing their ability to reason through problems they are becoming critical thinkers.

• Next: Water quality

Contents: Critical thinking in science

• Next: Water quality
• Previous: The story of pi

(Image source. More about the photograph)

Learn more

• Cabbage Juice pH Indicator Directions for creating a pH indicator using red cabbage from the MadSci Network.

Related topics

• Search LEARN NC for more resources on acid, base, chemistry, ph, and science.

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