Testing for pH

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Objective

SWBAT use a chemical test kit to determine the pH in a water sample.

Big Idea

pH is a key indicator of water quality as it determines whether the water can support life.

Introduction

10 minutes

To begin the lesson, I ask students to review the water quality test graphic organizer that they created in our lesson about water quality tests. For this lesson we focus on the test for pH.

To help students understand pH, I create a color coded pH scale on our class whiteboard. I use the colors which best align with the test strips that are included in our test kits so that students can link the color on the chart with the results that they get from their test. I show examples of various familiar items including bleach, lemon juice, milk, and drinking water. I have students guess at the pH of each item and then we add the item to the pH scale. Using familiar items helps students make connections with the pH scale.  

A key feature of this lesson is student safety. Since students will be working with chemicals during their pH test, we review the safety expectations of the science classroom. These include following directions, reporting accidents, asking clarifying questions as needed, and handling materials with caution. A more in-depth discussion of student safety, including a student safety contract, can be found in the reflection section of this lesson.

Guided Practice

10 minutes

To ensure that students are able to safely and accurately conduct a pH test, I demonstrate the proper method for conducting this test before distributing materials to students. I begin by showing the students the materials included in the test kit (sample cup, indicator solution, and vials used for comparison). I then review the step-by-step directions included with their test kit. After all student questions are addressed, I distribute the materials and ask students to conduct their own test.

Independent Practice

10 minutes

After each student group has received their materials, I ask each group to begin their test for pH. I remind students about the importance of following all directions; both those included with their test kit and those given by the teacher. I also remind students about the importance of accurately recording and labeling their results. 

A video of a student testing for pH can be found here.

Closing

10 minutes

To conclude the lesson, I provide time for each student group to share their results with the class. I record each group's results on the whiteboard so that they are accessible to all students. After recording all test results, I lead a discussion on whether our results are expected or unexpected. I ask students to provide potential explanations for any outlier data and remind students of the importance of including all data, even data which does not match our prediction, when recording and sharing results. A a class, we average the data and record it for use in a future lesson.