Testing for Dissolved Oxygen

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Objective

SWBAT use a chemical test kit to determine the level of dissolved oxygen in a water sample.

Big Idea

Dissolved oxygen is a key indicator of water quality as high dissolved oxygen is necessary to support life.

Introduction

10 minutes

To begin the lesson, I ask students to review the water quality test graphic organizer that they created in the previous lesson, focusing on the test for dissolved oxygen.

I then conduct a brief demonstration to help students understand the unit of measurement that will be used in the dissolved oxygen test that they will conduct later in the lesson. Dissolved oxygen is measured in parts per million. To demonstrate how small this unit of measure is, I bring in a liter of water and a milliliter of food coloring. I review with students that a liter of water contains 1,000 mL. We discuss how many liters of water would be needed to show 1 million milliliters of water. I then add the mL of food coloring to the liter of water and show how the dye diffuses through the water and changes the color only a small amount. We discuss what would happen if I added the same amount of food coloring to 1,000 liters of water (and why it is impractical to do so in class).  This helps students better understand the small unit of measure utilized in the dissolved oxygen test. We also discuss the level of dissolved oxygen (7 or more ppm) needed to sustain life in an aquatic ecosystem. 

Guided Practice

10 minutes

To ensure that students are able to safely and accurately conduct a dissolved oxygen 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 dissolved oxygen (DO). 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. 

I use the document camera as a technology tool to assist students in this lesson. When students demonstrate the DO test to their peers, the can use the document camera to show other students each step in the test procedure and can receive immediate feedback about both their performance and their conclusions. 

A video of a student testing for dissolved oxygen can be found here. In this demonstration, one student is showing his peers how to conduct the DO test and read the results using comparison samples.

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.