Lesson plan Grade 10 Science

Nettleton Plant Breakpoint Chlorination: Reading the Curve

A 50-minute grade 10 chemistry lesson where students read a breakpoint chlorination curve, calculate chlorine demand, and find the dose for a target residual.

Grade: Grade 10
Subject: Science
Material: Lesson plan
Time: 50 min
Prep: No prep
Materials: Projected or printed breakpoint curve plotted from the handout table, Printed student handout

Standards

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Students carry milligrams per liter through the demand calculation (dose minus residual) in handout items 1 and 5 and read the dose-residual curve to answer items 2 and 3.

All materials for HSN-Q.A.1 →

HSN-Q.A.3 Common Core Math

Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.

How this material aligns: Handout item 6 asks students to report a chlorine residual only to the precision the test kit supports, avoiding false digits in a logged treatment result.

All materials for HSN-Q.A.3 →

HS-ETS1-3 NGSS

Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

How this material aligns: Handout items 3 and 4 evaluate candidate chlorine doses against two competing criteria, holding a free-chlorine residual past the breakpoint while staying under the regulatory MRDL ceiling.

All materials for HS-ETS1-3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will calculate chlorine demand as applied dose minus measured residual.
Students will identify the breakpoint on a dose-residual curve and explain why the residual dips there.
Students will determine the dose needed to hold a target free-chlorine residual.

Vocabulary

Key words: chlorine demand, combined chlorine, free chlorine, breakpoint, residual

Teacher overview

When chlorine is added to water that contains ammonia, the measured residual does not simply rise with the dose. It first climbs as combined chlorine (chloramines) forms, then falls as more chlorine destroys those chloramines, and finally rises again as free chlorine appears. The low point of that curve is the breakpoint, and operators dose past it to hold a reliable free-chlorine residual. This lesson uses the fictional Nettleton Water Plant and its invented jar-test data to teach the curve and the demand calculation. A common misconception this lesson corrects is that adding more chlorine always raises the residual; at the breakpoint, adding chlorine lowers it.

Procedure

Hook (5 min): Project the breakpoint curve for the fictional Nettleton plant, showing measured residual on the vertical axis and applied dose on the horizontal axis. Ask students to describe the shape in one sentence, and note aloud that the residual dips in the middle even as the dose keeps rising.

Direct instruction (12 min): Define applied dose, measured residual, and chlorine demand, where demand is the chlorine used up by reactions: demand = dose - residual. Explain combined chlorine (chloramines) versus free chlorine, and walk through why the residual first rises, then falls to the breakpoint, then rises as free chlorine. State the regulatory ceiling: the maximum residual disinfectant level for chlorine is 4.0 mg/L, and distribution systems usually hold 0.2 to 2.0 mg/L.

Worked example, teacher-modeled (8 min): Using a table row that the handout items do not ask about, model the demand calculation: at an applied dose of 2.0 mg/L the measured residual is 1.7 mg/L, so demand = 2.0 - 1.7 = 0.3 mg/L. Emphasize that demand is what the water consumed, not what is left.

Guided practice (10 min): In pairs, students locate the breakpoint on the projected curve and mark the region where free chlorine is present. Circulate and check that pairs point to the lowest residual, not the highest.

Independent practice (10 min): Students complete the numbered handout below on their own, using the printed data table.

Closure and exit ticket (5 min): On an index card, students compute the demand for an applied dose of 4.0 mg/L with a measured residual of 1.2 mg/L. Collect the cards and confirm students subtracted to get 2.8 mg/L; reteach the demand formula the next day if more than a quarter miss it.

Nettleton Plant Breakpoint Chlorination: Reading the Curve — Grade 10 Science

Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Nettleton Water Plant ran a breakpoint jar test and recorded the measured total chlorine residual at each applied dose. All values are invented for this scenario.

Applied dose (mg/L)	Measured residual (mg/L)
1.0	0.9
2.0	1.7
3.0	2.4
4.0	1.2
5.0	0.4
6.0	1.3
7.0	2.3

At an applied dose of 7.0 mg/L the measured residual is 2.3 mg/L. Calculate the chlorine demand.
Using the table, at which applied dose is the measured residual lowest? This dose is the breakpoint.
The plant wants to hold a free-chlorine residual of at least 1.0 mg/L, which appears only after the breakpoint. Using the table, what is the lowest applied dose past the breakpoint that gives a residual of at least 1.0 mg/L?
The maximum residual disinfectant level for chlorine is 4.0 mg/L. Do any residuals in the table go above that ceiling?
At an applied dose of 3.0 mg/L the measured residual is 2.4 mg/L. Calculate the chlorine demand.
The test kit reads residual only to the nearest 0.1 mg/L. Explain why recording a residual as "1.30 mg/L" claims more precision than the kit provides.
Explain why the measured residual dips at the breakpoint even though the plant is adding more chlorine.

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

Answer Key

Answer: 4.7 mg/L. Demand = dose - residual = 7.0 - 2.3 = 4.7 mg/L of chlorine consumed by reactions in the water.
Answer: 5.0 mg/L. The lowest residual in the table, 0.4 mg/L, occurs at an applied dose of 5.0 mg/L, which marks the breakpoint.
Answer: 6.0 mg/L. Past the breakpoint, the 6.0 mg/L dose gives a residual of 1.3 mg/L, the first dose that meets the 1.0 mg/L free-chlorine target.
Answer: No. The highest residual in the table is 2.4 mg/L, which is below the 4.0 mg/L ceiling.
Answer: 0.6 mg/L. Demand = 3.0 - 2.4 = 0.6 mg/L.
Answer: The kit resolves only tenths of a milligram per liter, so the true value is known to about 0.1 mg/L. Adding a hundredths digit implies the plant measured more finely than the kit allows, which overstates the precision.
Answer: Near the breakpoint the added chlorine reacts with and destroys the combined chlorine (chloramines) already formed, so those reactions consume chlorine faster than new residual builds. The measured residual falls until the chloramines are used up, after which free chlorine appears and the residual rises again.

Differentiation

Support

Provide the demand formula (demand = dose - residual) printed at the top of the handout with one worked row already completed, and let students use a calculator. Highlight the breakpoint row on the table before they begin.

Extension

Ask students to sketch how the curve would shift if the water started with twice as much ammonia, and to predict whether the breakpoint would move to a higher or lower applied dose, with a one-sentence justification.

ELL supports

Pre-teach "dose," "residual," "demand," and "breakpoint" with a labeled version of the curve in English and, where possible, students' home languages. Provide the sentence frame "The demand is ___ because the dose is ___ and the residual is ___."