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Standards-based assessment and Instruction

Science 6-8


















Time Required: Approximately 45 minutes

What Is Causing the Changes in pH?

On the table in front of you are samples taken from a pond over the course of several years.

  • Use the equipment provided (wide-range pH paper, pH test kits) to find the pH of each sample. Record your results.
  • Use the computer to plot and label a bar graph of these data.
  • Analyze the data and state any trends/patterns you see.
  • Give a justifiable explanation as to what might be causing any changes in pond pH.
  • Predict what effects these changes might have on plants and animals in the pond’s ecosystem.
  • What things could be done to reverse these changes?

Grade Level: 6–8

Big Ideas and Unifying Concepts

  • Cause and effect

Physical Science Concept

  • Properties and changes of properties in matter

Science in Personal and Societal Perspectives Concept

  • Populations, resources and environments

Inquiry Process Skills

  • Communicate scientific procedures and explanations
  • Design and conduct a scientific investigation
  • Develop descriptions, explanations, predictions, and models using evidence
  • Identify questions that can be answered through scientific investigations
  • Recognize and analyze alternative explanations and predictions
  • Think critically and logically to make the relationships between evidence and explanations
  • Use appropriate tools and techniques to gather, analyze, and interpret data
  • Use mathematics in all aspects of scientific inquiry

Mathematics Concepts

  • Comparison of attributes or effects
  • Data collection, organization, analysis
  • Graphs, tables, representations
  • Measurement

Suggested Materials

I prepared a station with eight or nine beakers of “pond water” for every five or six students. I labeled the beakers with dates from about 1965 through 1996 - so as to mimic samples being taken every four years or so. Starting with a pH of about 7 for 1965, I prepared solutions of decreasing pH (using dilute hydrochloric acid) to about pH 3 for 1988 and then increasing to about pH 5 for the last sample. This was done to imitate the effects of acid rain. I provided wide-range pH paper and HACH company pH indicator kits for each station. Computers with a spreadsheet program were also made available.


This inquiry task was given at the end of a unit examining streams and water quality. During the unit, we investigated the water quality of a local stream using (among other tests) a colorimetric test of pH. We used results of our tests and data from tests of previous years to learn to graph data using a spreadsheet program. Finally, we used wide-range pH paper to examine the pH of a variety of common liquids (cola, vinegar, milk, lye, etc.) and studied the effects of acid precipitation on lakes and streams.

Instructional Stages

Engagement: Students access prior knowledge and engage with phenomena.

Exploration: Students explore ideas and phenomena using inquiry to clarify their understanding of concepts.

Explanation: Students construct explanations of concepts and phenomena.

Elaboration: Students apply learning to new situations.

What the Task Accomplishes

This task was originally designed to assess partial achievement of (Vermont) standards related to appropriate data representation, graphing, skills using computers and conducting pH testing and human resource demands on environmental systems. Students are asked in this task to make predictions, observe trends/patterns revealed in the data and justify their explanations and interpretations of data.

How the Students Will Investigate

Students first need to determine which of the two methods they will use to determine pH. Either is acceptable and both were used previously in class. Next, students need to carefully record their pH measurements for each year's sample (eight or nine samples spanning from about 1965 to 1995). Students then need to use a computer with a spreadsheet or other graphing program with which they are already familiar. Next, students need to identify the trend (rapid decrease followed by gradual increase) in the data. Finally, students need to use prior learning about the causes and effects of acid precipitation to explain the observed changes, state the effects on natural (plant and animal) systems, and list possible steps (human activities) to ameliorate the effects.

Interdisciplinary Links and Extensions

Social Studies/Health: Public environmental policy issues related to acid rain and public health are natural links between science and social studies. Subsequent student actions, such as public awareness campaigns, presentations to local select boards, and local stream cleanup days, could easily link to this unit of study.

Mathematics: Graphing and trend identification are already a part of the task. Investigation of the pH scale could provide a smooth introduction to powers of ten, exponents and logarithmic functions.

Teaching Tips and Guiding Questions

Be careful when working with the more acidic solutions. I require my students to wear protective eyewear whenever performing pH tests. Also, monitor the pH of the solutions as the tests are performed. I found that the pH “drifted” considerably while students were working. I assumed this was due to students contaminating samples by transferring liquid between samples on the tweezers used to hold the pH paper. (I also could not rule out downright tampering! This is something which happens in the real world of science from time to time and is worth discussing with students.) Students sometimes were unclear as to what “state any trends/patterns you see...” meant. I prompted them by asking:

  • What do you see the pH doing over the years? Can you describe it?
  • Can you recall anything we have studied previously which would be similar to what you see here? Remember to first describe what you actually see, then interpret or give it meaning based on what you have learned. It is also a good idea to remind students to check their accuracy during data collection – either by verifying with lab partners or repeating tests to get the same (within experimental error) results.

Concepts to Be Assessed

(Unifying concepts (big ideas) and science concepts to be assessed using the Science Exemplars Rubric under the criterion: Science Concepts and Related Content.)

  • Science in Personal and Societal Perspective — Populations, Resources and
    Students recognize that human activities have an impact on natural systems and that changing human behaviors can lessen the impact on ecosystems.
  • Physical Science – Properties of Matter: Students use the terms pH, acid, base and alkaline appropriately and describe cause-effect relationships with some justification, using data and prior knowledge.
  • Mathematics: Students make precise measurements and apply the concepts of compare and contrast. Students collect, organize and analyze data and use graphs, tables and representation appropriately.

Possible Solutions

Students need to identify a trend (rapid decrease followed by gradual increase) in the data and give a plausible (justifiable) reason for the observed changes, state the effects on natural (plant and animal) systems and list possible steps/human activities to ameliorate the effects. I expected students to stick with acid rain as the explanation for the data obtained; however, other reasonable explanations would be acceptable – for example, that acidic waste from a nearby landfill was leaching into the pond. Similarly, any reasonable suggestions for amelioration of the problem – reducing emissions from coal-fired power plants and driving less – would be acceptable.

Student Anchor Papers and Task-Specific Assessment Notes

Task Specific Rubric/Benchmark Descriptors
Click on a level for student example.
Novice The student is able to collect appropriate data but is unable to proceed further. The student draws (rather than using a computer) a bar graph that shows the data, but axes are unlabeled
in the representation. The trend the student identifies simply states that the pH values went down, not that they also fluctuated from 1982 – 1995.
The explanation lists several possible causes (pollution, farms and acid rain) but makes no attempt to explain the data, indicating that the student probably has little understanding of how each of these causes might affect the pH of the water. The stated cause-effect relationship (“it would kill them”), a sweeping generalization, indicates again that the student lacks any real understanding of how pH is linked to plant and animal life or of the varied effects possible. The student suggests treatments but does not explain how or why these treatments might be used to increase the pH levels.
Apprentice The student is able to collect and plot the data correctly, appropriately using the computer and labeling the graph. The student is able to state some aspects of the data gathered but is unable to clearly articulate a trend or connection between the data. (“pH leveled out in the end. The water is very acidic in 1982.”) A partial and unclear explanation (“If a farmer used a different kind of fertilizer and it leaked into the pond”) does not show a cause-effect relationship and is not consistent with the available data.
The effects on plant and animal life that are described (“It wo uld make too much nutrients and the algae would grow rapidly and kill the animal life in the pond”) and the things listed that could be done to reverse these changes (“To stop the fertilizer and reduce the nutrients in the pond, I would put chlorine into the water to reduce the nutrients”) are both consonant with the explanation the student gives – even though not consistent with the data gathered. This indicates some understanding of the human impact and of pH but an inability to link data to the cause-effect reasoning.
Practitioner The computer-generated graph is an accurate, appropriately-labeled representation of the data. The trend is correctly identified. (“As the years get later Acidity gets greater but in 1995 Acidity is almost normal.”) The student correctly identifies a relationship between human activities and acid rain (“probably just started getting monitored in 1995 so something was done about it, but before that all that acid rain was going into this pond and there was no way to neutralize it.”), although the effect is somewhat overstated. (“All living things would probably die. At least the majority of them.”) The student gives a reasonable (if partially unrealistic) explanation of how the effects of acid rain can be reduced. (“Local industries that are burning combustion fossil fuels could either raise their smoke-stacks or cut down all together.”)
Expert The computer-generated graph is an accurate, appropriate representation of data; pH measurements are more precise (e.g., 7.5 rather than 7, when appropriate). The trend is correctly identified. (“I see the pond water getting more and more acidic, until it has been treated with alkali to neutralize the effects.”) The student gives a detailed explanation of the relationship between human activities and acid rain (“...In 1995, someone has probably treated it, or eliminated some of the source of acid”) and clearly explains some of the interrelationships among pond organisms. (“When it gets more acidic, all life in the pond fails. Microscopic bugs die – then larger bugs die – fish feed on the bugs, so they die – until there is no life left in the pond. Plants just dry up and die.”) The student gives two possible and plausible solutions to the problem. (“You could treat the water with alkaline, to neutralize the acid. Or just eliminate the acid source.”)

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Here's What People Are Saying

'We need to start,' said my students last week. They told me to hurry up, read the problem and pass out the paper. This is a noticeable change in student attitudes toward problem solving. They are no longer meek, frightened, inhibited problem solvers.

Karen McKee's first grade classroom Easthampton, NY

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