How Much Is Too Much?

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Indiana Expeditions Show

Life Science

Overview

We each make choices and take risks every day.  Some of us are willing to assume more risks than others.  In this activity, students will decide how much risk they are willing to assume as it relates to drinking polluted water.  (Note: Students do not actually drink the “polluted” water in this activity.)  A teacher will demonstrate a serial dilution to show students what happens to the appearance of a polluted body of water as the pollutant is diluted from a “pure” concentration to a concentration of 1ppm (part per million). Afterwards students will select which diluted, polluted solution they would be willing to drink and explain the reasons for their choice. 

Background information

Waste products from agricultural, industrial and municipal sources can easily end up in bodies of water.  Many of these toxic or synthetic chemicals cannot be broken down by natural processes.  Thus, once present in water, the waste products do not simply disappear, but may just be diluted.  Even in the small amounts of sometimes less than 1 part per million, these waste products can cause problems for the plants, animals, and humans that depend on water for survival. 

Because it is impractical to remove all pollutants form a body of water, the Environmental Protection Agency has established levels at which a substance can be safely consumed for long periods of time.  These levels are usually expressed as parts per million (ppm) or parts per billion (ppb). 

Parts per million and parts per billion are rather abstract concepts for most students.  In this activity, a demonstration showing a serial dilution to 1 ppm helps make these abstract concepts more real for students.  To further personalize the concept of ppm or ppb, students are asked to choose and explain their choice of a level of dilution at which they would feel comfortable risking taking a drink of polluted water.

Connections to the Indiana Academic Standards for Science

            3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.2.2, 3.2.4, 3.2.6, 3.4.6, 3.4.8, 3.5.1, 3.5.3, 4.1.3, 4.1.5,

            4.2.4, 4.2.5, 4.5.4, 4.5.5, 5.1.3, 5.1.6, 5.2.4, 5.2.6, 5.3.6, 5.4.5, 5.5.1, 6.1.2, 6.2.5,

            6.2.6, 6.3.8, 6.3.16, 6.5.4, 7.1.2, 7.1.4, 7.1.8, 7.2.7, 7.4.14

Science Process Skills

• Calculating
• Communicating
• Forming operational definitions
• Hypothesizing and predicting
• Measuring
• Observing
• Posing questions
• Sequencing
• Using graphs

Estimated Time Requirement

One 45- minute session

Materials

• Red food coloring
• Graduated cylinder (at least 50ml.)
• One unopened, clear plastic .5 liter bottle of water
• One “polluted,” clear plastic .5 liter bottle of water (prepared as described below)
• Six additional clear plastic .5 liter bottles with caps, each containing 450ml of water
• One funnel
• One permanent marker

Objectives

Students will be able to

• explain what parts per million represents.
• select and explain their reasons for selecting a personal level of comfortable risk  as it relates to drinking polluted water.

Procedure

Preparation prior to the lesson:

• Remove the labels from the six .5 ml bottles with caps and fill each bottle with 450ml of water. Be sure to use clear plastic bottles rather than bottles that might be tinted since students will need to be able to see the contents.
• Prepare a “polluted” bottle of water by filling it with water and then adding several drops of red food coloring to make the water appear dark red.  Replace the cap on the bottle.  Hide the bottle within easy reach for the anticipatory set of the lesson.

Anticipatory set:

• View the Life Science Indiana Expeditions segment
• Open a fresh bottle of water and take a drink from it.  Ask the students if any of them would also like a bottle of water and when some of them say “yes” pull the “polluted” bottle out of its hiding place. Open it, and ask them again if they would still like a drink.

Lesson sequence:

• Introduce the idea that for most of us, the water we drink has been cleaned prior to coming out of our faucet.  Discuss with students where our water comes from and how chemicals can get into bodies of water.  Discuss what this means for the plants, animals, and humans that depend on that water for survival.
• Mention that most water contains some pollutants and that the EPA has established levels at which a substance can be safely consumed for long periods of time.  These levels are usually expressed as parts per million (ppm) or parts per billion (ppb). 
• Tell students that today you are going to demonstrate, with their help, how your polluted bottle of water will look if it is diluted to 1ppm.
• Conduct the serial dilution demonstration as follows. 
• Point out to students that you will first dilute the polluted water to a 1 part per 10 solution.  Get a student volunteer to measure out 50ml of polluted water and using the funnel, pour it into one of the bottles containing 450ml of water.  Replace the lid, shake the bottle to mix the contents and then label the bottle: 1/10.
• Let the students know that now you are ready to dilute the polluted water even more.  Get another student volunteer to remove 50ml of water from the 1/10 bottle and place it in another one of the bottles containing 450ml. of water.  Cap, shake, and label the bottle as 1/100.  Line up the three bottles in the following order: original sample of polluted water, 1/10 solution, 1/100 solution.
• Ask the students what they notice about the color of the water in the new bottles compared to the original bottle of “polluted” water.
• Dilute the water again by having another student volunteer remove 50ml of water from the 1/100 bottle and place it into another one of the bottles containing 450ml of water. Cap, shake, label this bottle as 1/1000, and place it in the line of polluted water samples.
• Continue the serial dilution process in this manner, making a 1/10,000 dilution, a 1/100,000 solution and finally, a 1/1,000,000 solution.  Cap, shake, label each solution, place them in order, and ask students between each dilution what they notice.
• Discuss whether the 1ppm sample is still polluted or if it is safe to drink.
• Separate the bottles by spreading them out across the front of the classroom.
• Ask students to line up behind the bottle from which they would be willing to take a drink.
• Explain to the students that they have just created a histogram.  Replicate the histogram on the board.
• Discuss what the histogram represents; the concept of risk, and what, in the minds of the students, constitutes an acceptable risk.
• Ask students to write about or describe what they learned from this activity.

Closure

• Provide students with a drink of clean water and ask them to think about what is and is not acceptable with regard to what is in their drinking water.

Suggested Student Assessment

Objective #1:

Provide the students with 10 drops of a “pollutant” and ask them to demonstrate

or describe and draw pictures to illustrate how they would get it to a 1ppm

solution.

Objective #2:

            The written/description activity from the lesson sequence can be used to assess

            this objective.

Extending the Lesson

• Have students research and report on some substances for which the EPA has established levels at which a substance can be safely consumed for long periods of time. 
• Have students research water quality issues in their own town or neighborhood and then develop an action plan to either improve or maintain their local water quality.

Source of Lesson

Rob Hedges – Science Teacher

Creekside Middle School, Carmel, Indiana

Deb Sachs
Director, Office of Professional Development
School of Education
University of Indianapolis