Kenan Fellows Program Logo and page header graphic

Critical Thinking in Science

Part 3: Experimenting with pH


Students will experiment with the relationship between pH, volume, concentration, and the relationship between acids and bases while practicing their critical thinking skills in an inquiry based experiment. This lesson is written using the 5E Learning Model.

Learning Outcomes:

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

Curriculum Alignment:

1.01 Identify and create questions and hypotheses that can be answered through scientific investigations.

1.02 Develop appropriate experimental procedures for:

  • Given questions.
  • Student generated questions.

1.04 Analyze variables in scientific investigations:

  • Identify dependent and independent.
  • Use of a control.
  • Manipulate.
  • Describe relationships between.
  • Define operationally.

1.05 Analyze evidence to:

  • Explain observations.
  • Make inferences and predictions.
  • Develop the relationship between evidence and explanation.

1.06 Use mathematics to gather, organize, and present quantitative data resulting from scientific investigations:

  • Measurement.
  • Analysis of data.
  • Graphing.
  • Prediction models.

1.08 Use oral and written language to:

  • Communicate findings.
  • Defend conclusions of scientific investigations.
  • Describe strengths and weaknesses of claims, arguments, and/or data

4.01 Understand that both naturally occurring and synthetic substances are chemicals.

4.03 Explain how the periodic table is a model for:

  • Classifying elements .
  • Identifying the properties of elements.

4.06 Describe and measure quantities related to chemical/physical changes within a system:

  • Temperature.
  • Volume.
  • Mass.
  • Precipitate.
  • Gas production.

Classroom Time Required:

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

Materials Needed:


  • 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.

**** Instead of making the paper you can also purchase pH Hydrion paper with a wide scale to use in the experiment.


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 Engage stage- can also be bought), fruit juicer, 5 equal sized containers per group, distilled water, copies of pH experiment worksheet, graph paper, overhead of pH summary chart


Experimental Design Graphic Organizer copies, Various types of peppers, pH paper, paper to collect and organize data, student requested materials


Fruit juice from Explore, peppers from Extend, pH paper, copies of Evaluate worksheet

Technology Resources:

  • Overhead Projector

Pre-Activities/ Activities:

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


  • What is an indicator? (Time: Day 1=20 minutes for discussion and soaking paper; Day 2= 50 minutes to cut paper and make a scale)
  • Teacher and class will discuss: “What is an indicator?” and “How do we use an indicator when testing pH?”
  • Make cabbage juice pH indicator. Directions for this can be found online.
  • Students should soak their paper (blank notecards 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 zip lock bag or a container with a lid will work. The containers should be labeled for the students to use their own paper.
  • The students need to make a chart of the pH scale using their pH paper. Give each group a large notecard. Ask the students to draw the pH scale. (see example) Using their pH paper, have the students test their indicator in several know pH solutions. Allow the paper to dry before taping or gluing it to the notecard below the appropriate pH. The students are creating a key to allow them to measure pH using their own pH paper.
  • The scale card should be properly labeled and stored in a Ziploc bag to prevent moisture exposure.
  • ****If you are using purchased pH paper please introduce students to indicators in another way.


  • Experimenting with pH, concentration, and volume (Time: 50 to 70 minutes)
  • The students will complete the pH Experiment lab (See Worksheet 1). 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% concentrated juice will need to be stored for use in the Evaluation section. 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 (See Worksheet 1).
  • Students will need to summarize the class/team data and graph the pH of various types of fruit. Use the 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)

  • 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.
  • Provide the students with the Experimental Design Graphic Organizer (See Worksheet 2). The students will need a class period to plan their experiment and create a data table for their information.
  • 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.
  • If students are cutting pieces of peppers themselves be sure they wear gloves with the hot peppers.
  • Allow the students to complete their experiment and organize their data.

Evaluate: What happens when we mix acids and bases? (50 minutes)

  • Provide the students with a copy of the pH Evaluation worksheet (See Worksheet 3).
  • The students will need their 100% concentration juice from the explore section and a piece of pepper from the elaborate section.
  • 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.
  • 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.
  • The students should complete the questions.


See Evaluate piece of Activities Section.


EDGO 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.

Critical Vocabulary:

  • Acid
  • Base
  • Neutral
  • pH
  • Concentration
  • Volume


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 How important is a decimal place?). This lesson should be used while teaching Goal 4 of the North Carolina Standards of Learning (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.