Water Testing: What Makes Water Good?
We have been learning about the chemical makeup of water and how to test water for hardness, pH, copper and chlorine. We have read and discussed what it means if the water is hard or soft, or if the pH is neutral, alkaline or acidic. Now we are going to consider the question: What makes water good?
In this investigation, your team will conduct and compare several tests on different water samples to try to determine what makes each water sample “good” and which one is the best for you to drink, to wash with, etc. Use your data to draw your conclusions.
Approximately five 45-minute class sessions, including four lead-up activities to learn how to do each water test.
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Suggested materials
Items to have available during investigations include different brands of bottled water, tap water, measuring cups, small paper cups and Hach water test strips. Additional investigations might use different kinds of dish soap and distilled water.
This inquiry was part of our unit on water. To introduce the science concepts and skills in testing water for hardness, pH, copper and chlorine, students participated in four guided activities that explored the quality of water. Each day, we completed one test on the water in our classroom using Hach Water Quality Test Strips. Students had a one- to two-page overview of each test and directions to follow using the test strips. After each test, we recorded the results and discussed what the data meant. For example, because most water pipes are made of copper, you would not want a pH to be acidic (below a pH of 7) since copper would dissolve from the pipes and go into the water. Pipes that dissolve would also begin to leak over time.
Each activity explained a little about one characteristic of water and guided students in how to conduct the tests. As the students worked, they built their understanding of the nature of water as a solvent and improved their ability to apply some of the skills of scientific investigation. Students were then encouraged to compare different water samples in order to answer the question: What makes water good?
This task provides information to the teacher about whether students are building an understanding of the characteristics of water and can use scientific reasoning to evaluate the quality of water for particular uses (for example, if water is hard, soap does not work well; however, it is not bad for drinking).
This task demonstrates how well students can use scientific tools to collect data and show evidence of conceptual understanding in how they interpret data. Students have the opportunity to communicate their results to the class and learn from what other students have investigated.
After the guided investigations with the whole class, teams of two to four students then repeated the tests with the classroom water and chose a second water sample from several choices of bottled water to conduct the four tests on. Finally, students interpreted their data and discussed conclusions based on the data collected. Each group shared their investigation and results with the class. Class sharing is a way that teachers can check understanding, make connections to prior knowledge, and extend thinking to the next lessons in the unit and/or introduce new science vocabulary.
Note: For teachers who have little background in conducting these tests, the Hach Water Quality Test Strips and teaching materials are user-friendly and written for easy conceptual understanding, even for third graders.
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Students may want to conduct further investigations with other water samples, such as the water they have at home. Concepts learned in these activities can build a foundation for more in-depth investigations with solvents and solutions.
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Understandings about the Nature of Science (see appendix)
Guiding questions to ask students before, during and after investigations might include:
- What makes water hard or soft? What does that mean? How does water become hard? (Water, a solvent, picks up impurities easily. When water flows over rocks, it can pick up calcium and/or magnesium salts from limestone or other rocks. These salts dissolve in the water, making it hard.)
- Why doesn’t soap work well in hard water? (Soaps contain long chain-like molecules that have two distinct ends. One end interacts with water and the other end interacts with oil and grease. If there is too much magnesium or calcium in the water, the end of the soap molecule that is supposed to react with the dirt and oil ends up reacting instead with the magnesium or calcium. This makes soap scum – bunched-up soap molecules.)
- How can hard water hurt pipes? (It can accumulate and clog the pipes.)
- What is pH? What does it mean to be neutral? acidic? alkaline? (pH is a way to measure how acidic or alkaline a solution is. The pH scale goes from 0 to 14. If there is an equal number of hydroxide and hydroxyl ions, the solution is neutral and has a pH of 7. Because a water molecule is made of one of each ion, pure water is neutral. If the solution is less than 7, it is acidic; greater than 7, it is a base, or alkaline.)
- Why do some water suppliers (like the town) add alkaline substances to the water to raise the pH? (To slow or prevent acidic water from “eating” the pipes from the inside.)
- Why are there sometimes blue-green stains in your bathtub? (Copper has dissolved in the water.)
- What do your results tell you? What did you learn from your results?
- What conclusions can you make based on your results?
- What new questions do you have about water?
- Have you learned anything that surprised you?
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Exemplars Rubrics
In this investigation, students need to list materials used, conduct the four tests and record their data for two water samples (tap water and bottled water). Then, students should be able to write an explanation for which water sample they think is better, based on the data they collected.
This student completes the testing but fails to list the materials used. Only one result is discussed, but four tests are conducted. The numbers for hardness are questionable. There is little evidence of conceptual understanding.
This student completes the testing and lists the materials used. Results reflect information read in class rather than an analysis of the test results or comparisons between the two samples. There is some evidence of conceptual understanding.
This student includes a list of materials used and collects all of the data from the four tests. The discussion of results compares each quality tested and demonstrates conceptual understanding that builds on prior knowledge and experience.
This solution includes a list of materials and data from all tests conducted. Because the students are curious, they also test seltzer – extending thinking beyond earlier investigations. The discussion of results include information learned in class, information learned through the testing, and comparisons of the data. There is clear evidence of conceptual understanding.