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Lesson plan Grade 11 Health & Consumer Science

Reading the Marsh Hollow Water Report: Chlorine and pH

A grade 11 health lesson where students read a fictional water district's quality report and check its chlorine residual and pH against federal limits.

Objectives

  1. Students will compare a water report's chlorine residual to the federal maximum residual disinfectant level.
  2. Students will write and use a compound inequality for the acceptable pH range of drinking water.
  3. Students will judge whether a household's readings fall inside the ranges public water systems maintain.

Vocabulary

Chlorine residual
Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.

Also: maximum residual disinfectant level, compound inequality, secondary standard

Teacher overview

Every community water system publishes an annual water quality report, and reading one closely is a consumer skill students will use long after graduation. In this lesson students work through the report of the fictional Marsh Hollow Water District, checking two of the most common lines a household sees: the chlorine residual, measured against the federal maximum residual disinfectant level of 4.0 mg/L, and the pH, measured against the secondary standard range of 6.5 to 8.5. A common misconception this lesson addresses is that any chlorine in tap water is a warning sign; a measured residual well under the limit is exactly what a system aims for, because it shows disinfectant is still active in the pipes.

Procedure

Hook (5 min): Ask students what they would look for if a water report landed in their mailbox, and whether they would know which numbers were good and which were a problem. Collect a few answers, then explain that most reports compare each reading to a published limit, and that today they will read two of those lines.

Direct instruction (12 min): Introduce the chlorine residual and the federal maximum residual disinfectant level of 4.0 mg/L, and note that systems usually keep the residual between 0.2 and 2.0 mg/L in the distribution system. Then introduce the secondary standard for pH, which runs from 6.5 to 8.5. Model one check using a fictional sample reading of 3.1 mg/L chlorine: it is below the 4.0 mg/L limit (compliance is judged on a running annual average of distribution samples, so one reading is a check rather than a verdict), though it sits above the usual 2.0 mg/L top of the typical range.

Guided practice (12 min): Distribute the handout for the fictional Marsh Hollow report and work items 1 through 3 together, having students state out loud whether each reading is above or below the limit before they subtract. Confirm that they divide by the limit, not by the reading, when finding a percent of the limit.

Independent practice (18 min): Students complete items 4 through 8 on their own, including the compound inequality in item 5 and its two applications in items 6 and 7. Circulate and check that students write the pH condition as a single inequality with the reading in the middle rather than as two disconnected statements.

Formative check and close (8 min): As an exit ticket, students answer one prompt on a card: "A neighbor sees a chlorine residual of 1.8 mg/L and wants it to read 0. Using item 8, explain in two sentences why a small residual is what the system wants." Scan responses for students who connect the residual to continued protection in the pipes.

Reading the Marsh Hollow Water Report: Chlorine and pH — Grade 11 Health & Consumer Science
Name: ____________________________ Date: ______________

Student Handout

The fictional Marsh Hollow Water District sends each household an annual report. Two limits apply below. The federal maximum residual disinfectant level (MRDL) for chlorine is 4.0 mg/L, and systems typically keep the residual between 0.2 and 2.0 mg/L in the distribution system. The secondary standard for drinking-water pH runs from 6.5 to 8.5.

  1. A sample taken at the Larkin family's tap reads a chlorine residual of 1.8 mg/L. The MRDL is 4.0 mg/L. By how many mg/L is the reading below the MRDL?
  2. What percent of the 4.0 mg/L MRDL is the 1.8 mg/L chlorine residual? Round to the nearest whole percent.
  3. Systems typically keep the chlorine residual between 0.2 and 2.0 mg/L in the distribution system. Is the 1.8 mg/L reading inside that typical range? Explain.
  4. A second sample, from the Okoye family's tap, reads 2.4 mg/L chlorine. Is it above or below the 4.0 mg/L MRDL, and is it inside the 0.2 to 2.0 mg/L typical range?
  5. The secondary standard for pH runs from 6.5 to 8.5. Write a compound inequality using p for a pH reading that meets this standard.
  6. The Larkin family's tap has a pH of 8.2. Use your inequality from item 5 to decide whether 8.2 meets the secondary standard.
  7. The Okoye family's tap has a pH of 8.9. Does 8.9 meet the secondary standard, and if not, by how much does it miss the range?
  8. Explain why a chlorine residual that is present but well under the 4.0 mg/L MRDL is what a water system aims for, rather than a reading of 0 mg/L.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 2.2 mg/L. 4.0 mg/L minus 1.8 mg/L equals 2.2 mg/L below the MRDL.
  2. Answer: 45%. 1.8 divided by 4.0 is 0.45, or 45 percent of the limit.
  3. Answer: Yes. 1.8 mg/L is between 0.2 and 2.0 mg/L, so the reading sits inside the typical distribution range.
  4. Answer: Below the MRDL, outside the typical range. 2.4 mg/L is below the 4.0 mg/L limit but above the 2.0 mg/L top of the typical range.
  5. Answer: 6.5 ≤ p ≤ 8.5. A reading meets the secondary standard when p is at least 6.5 and at most 8.5.
  6. Answer: Yes. 8.2 satisfies 6.5 ≤ p ≤ 8.5 because 8.2 falls between 6.5 and 8.5, so it meets the standard.
  7. Answer: No; it is 0.4 above the range. 8.9 is greater than 8.5, and 8.9 minus 8.5 equals 0.4 above the upper limit.
  8. Answer: A measurable residual shows the disinfectant is still active through the pipes, guarding against contamination, while staying under the MRDL keeps it within the safe limit; a reading of 0 mg/L would mean no protection remained in that part of the system.

Differentiation

Support

Provide the two limits printed at the top of a worked template, with item 1 already set up as 4.0 minus 1.8 so students see the structure. Allow a calculator throughout, and accept the pH condition in item 5 written as two separate statements before students combine them into one inequality.

Extension

Ask students to redesign the fictional Okoye report so both the chlorine residual and the pH just meet every standard, stating a residual inside the typical range and a pH inside 6.5 to 8.5, then justify each choice against the published limits.

ELL supports

Pre-teach "residual," "limit," "range," and "inequality" with a simple number-line diagram showing the acceptable band shaded between 6.5 and 8.5. Provide sentence frames for items 3, 6, and 8, such as "The reading is ___ the limit because ___."