Using a Universal Indicator to Identify Acids and Bases

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Objective

Students will be able to use a universal indicator to determine the approximate pH of various solutions, and to connect the pH to whether something is acidic or basic.

Big Idea

Acid-base indicators, such as red cabbage juice, turn different colors in acidic and basic solutions due to a pigment molecule called anthocyanin.

Introduction

In this lesson students will be able to use a universal indicator to determine the approximate pH of various solutions. They will mix a pre-made red cabbage indicator with various solutions and powders, record the color, change, and determine from their colors and a color key what the pH of the solution is. At this point, they will also be able to determine if they have an acid or a base.

It aligns to the NGSS Practices of the Scientist of Planning and carrying out investigations because, while the investigation is planned out for them, students will have to conduct the investigation at the lab bench. It also aligns to the NGSS Practice of Analyzing and interpreting data; once students investigate they will use the colors to determine pH, and they will then be able to name whether they have an acid or a base.

It aligns to the NGSS Crosscutting Concept of Cause and Effect:Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system. In this case, the cause of the color change is the pH, a nanoscale phenomenon.

In terms of prior knowledge or skills, students could do the lab portion of this as an opening investigation into acids and bases. My students have already had an introduction to acids and bases at this point in the unit. They have performed a simple neutralization reaction, studied the properties of acids and bases, compared the Arrhenius theory of acids and bases with the BrØnsted-Lowry definition, and they studied the pH scale and begun to learn how to convert between hydronium and hydroxide concentrations and pH.

The materials needed for this lesson include the following:

  • Universal indicator (I make my own from red cabbage (here are directions for this) or you can buy a universal indicator such as this one)
  • Solutions to measure pH: Alka-seltzer, Vinegar, ammonia, lemon juice, lemon-lime soft drink, 1M KOH (drain cleaner), detergent, baby shampoo
  • test tubes and racks
  • graduated cylinders
  • pipettes

 

 

 

Do Now/Activator

15 minutes

Do Now: Students begin class by comparing the answers of yesterday’s homework questions with a partner. Yesterday’s lesson was about converting [H+] and [OH-] into their corresponding pH values, and I want to assess how students are doing with this material today at the start of the lesson. By bringing students back to the point that we left off at the last class I am providing a logical sequence of events for students. This consistency of linking one class to the next is important for students because it helps them to make connections between discrete topics.

Activator: I greet my students and ask them how they did on the homework. I reteach if students ask me to. I answer questions students have, and then I ask them to take a pop pH quiz so that we can all be clear about how they are doing. Each student gets his or her own unique H+ and OH- concentration assignment and they are asked to calculate pH.

I have chosen this approach because at this point in the year I am really trying to drive home the point that students have to ask questions, and be intellectually honest with themselves about what they know and what they do not know.

The quiz results vary widely. Some students know how to convert concentration to pH and some do not. I use part of the start of class to reteach this skill, and I use the students who know this skill as a resource for helping students who do not know the skill.

Mini-lesson

10 minutes

Mini-lesson: I then pass out the Universal Indicator Lab directions for today’s class and ask students to read them. I then ask students to explain what the lab is about with the following questions:

What goes in test tube #1? Number 2? Number 3? (baking soda solution, vinegar, ammonia)

About how much of each solution goes in the test tubes? (5 ml)

How much indicator should you use in each test tube? (until color changes)

When I am satisfied that students understand the procedure, I point out where the materials are for the lab.  We discuss lab safety and the waste disposal procedure.

This instructional choice reflects my desire to have students be able to quickly work through this lab. I want them to experience the color changes and I believe that if they thoroughly understand the lab ahead of time they will conduct it in a more efficient manner.

Application

25 minutes

Student Activity: Students use the indicator to test the various solutions found in their data table. While they are doing this I walk around and make observations and watch to insure that students are conducting the lab safely.

I want students doing this work because I want them to experience the idea that indicators change colors based on the ingredient they are interacting with.

Debrief

10 minutes

To wrap this lesson up I have students share data and explain what the pH of the different materials is as shown in this interpreting the results video. I ask for the class to agree or disagree with the presenter. I ask students to assess which chemical was the most acidic and which was the most basic. I ask them to assess which chemicals were dangerous based on their pH. This student work contains values typical of what we see in the lab.

I then ask them to give a shout-outs for something their partner did that made him or her a good lab partner. Finally, I discuss the data I gathered from the quizzes we had at the start of class.

Ending class this way allows me to address both content and character skills that were the learning objectives for today’s class.