Lesson plan Grade 7 Science
Why Ferrisby Adds Orthophosphate: Corrosion Control and Lead
A 50-minute grade 7 science lesson using a fictional bench test of lead released at five pH values to show how corrosion control lowers lead without digging up pipes.
Objectives
- Students will interpret a bench test showing how much lead dissolves at different pH values.
- Students will calculate percent decreases in lead released after a treatment change.
- Students will explain how a protective coating lowers lead at the tap while lead pipes stay in place.
Vocabulary
- Corrosion control
- Treatment that keeps water from slowly eating away at the inside of pipes. It helps stop metals like lead from getting into drinking water.
- Orthophosphate
- A chemical that utilities add to water in very small amounts. It coats the inside of pipes so lead cannot easily get into the water.
- pH
- pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.
- Service line
- A service line is the small pipe that connects the water main under the street to the plumbing inside one home or building. It is the last leg of the trip water makes to your tap.
- Action level
- A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.
Also: corrosion
Teacher overview
Lead reaches a home's tap because water sitting in a lead service line slowly dissolves the metal, and how much dissolves depends on the chemistry of the water itself. This lesson uses a bench test from the fictional town of Ferrisby, where lead coupons sat in water at five different pH values, to show that the same pipe can release very different amounts of lead. A common misconception in this lesson is that lead at the tap means lead in the source water; the bench-test data corrects that, because the source water in this scenario carries no lead at all and every trace of it comes off the inside wall of the pipe. Federal rules set the lead action level at 15 ppb, dropping to 10 ppb in 2027, and the secondary standard for drinking water pH runs from 6.5 to 8.5, so both of the treatment levers in this lesson stay inside real limits.
Procedure
Hook (5 min): Pass around a short piece of pipe and ask students where lead in tap water comes from if the reservoir has none. Collect answers on the board and leave them visible for the closure.
Direct instruction (14 min): Define corrosion, corrosion control, pH, and service line. Explain that water with a low pH is more aggressive toward metal, and that utilities raise pH or add orthophosphate so a thin, stable coating forms on the inside of the pipe. Emphasize that the coating is the treatment, not the pipe, and that disturbing a pipe can break the coating loose. Note for the town of Ferrisby that federal rules set the lead action level at 15 ppb and lower it to 10 ppb in 2027, and that the secondary standard keeps drinking water pH between 6.5 and 8.5, which limits how far operators can push the pH lever.
Guided practice (12 min): Work handout items 1 through 3 with the class, reading values off the bench-test table together and naming what stays the same across the five test jars. Have students state in their own words what the table shows before computing the percent in item 3.
Independent practice (12 min): Students complete handout items 4 through 9 alone, then compare with a partner. Ask partners who disagree to point at the row of the table they used.
Closure and exit ticket (7 min): Students write one sentence answering: "Ferrisby raises its pH and lead at the tap drops. Are the lead pipes gone?" Collect and sort the responses. The expected answer is that the pipes remain and only the coating protects the water, so the result depends on the plant holding that pH steady. Reteach the coating idea with a diagram if more than a few students say the pipes were replaced.
Student Handout
Ferrisby is a fictional town, and the bench-test results below were written for this exercise. Each jar held an identical piece of lead pipe in water for 24 hours at the pH shown.
| Test jar pH | Lead released (ppb) |
|---|---|
| 6.5 | 48 |
| 7.0 | 34 |
| 7.5 | 21 |
| 8.0 | 12 |
| 8.5 | 7 |
- How many ppb of lead did the jar at pH 6.5 release?
- How many fewer ppb of lead did the jar at pH 8.0 release than the jar at pH 6.5?
- The lead released fell from 48 ppb at pH 6.5 to 12 ppb at pH 8.0. What is the percent decrease?
- Ferrisby's plant currently sends water out at pH 7.0. If operators raised it to pH 8.0, how many fewer ppb of lead does the table predict?
- Starting in 2027 the lead action level is 10 ppb. Using the table, which tested pH values released 10 ppb or less?
- A second bench test added orthophosphate to water held at pH 7.0. Lead released fell from 34 ppb to 11 ppb. What is the percent decrease?
- Ferrisby has 1,850 service lines, and 370 of them are lead. What percent of the town's service lines are lead?
- Explain how orthophosphate can lower the lead at a kitchen tap even though the lead service line is still buried in the ground.
- A resident says the lower bench-test numbers prove Ferrisby's water is safe from now on. Give one reason to disagree, using what you know about the coating.
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: 48 ppb.
The table row for pH 6.5 lists 48 ppb of lead released after 24 hours. - Answer: 36 ppb.
The jar at pH 6.5 released 48 ppb and the jar at pH 8.0 released 12 ppb, and 48 minus 12 equals 36 ppb. - Answer: 75 percent.
The drop is 48 minus 12, or 36 ppb. Dividing 36 by the starting value of 48 gives 0.75, a decrease of 75 percent. - Answer: 22 ppb.
The table shows 34 ppb at pH 7.0 and 12 ppb at pH 8.0, and 34 minus 12 equals 22 ppb less lead released. - Answer: Only the jar at pH 8.5.
The pH 8.5 jar released 7 ppb, which is below 10 ppb. Every other tested pH released 12 ppb or more. - Answer: About 68 percent.
The drop is 34 minus 11, or 23 ppb. Dividing 23 by 34 gives about 0.676, which rounds to a 68 percent decrease. - Answer: 20 percent.
370 lead service lines divided by 1,850 total service lines equals 0.2, which is 20 percent. - Answer: Orthophosphate builds a coating inside the pipe.
The additive reacts with the pipe wall and forms a thin, low-solubility layer between the metal and the water. Water then touches the coating instead of bare lead, so much less lead dissolves even though the pipe has not been removed. Accept any answer describing a protective layer or barrier that separates water from the pipe metal. - Answer: The coating can be lost if the water chemistry changes.
The protection lasts only while the plant keeps the treatment steady. A change in source water, a break in treatment, or physical work that shakes the line can strip the coating loose and release lead again, which is what happened in Flint when a new source was used without corrosion control. Accept any answer noting that the pipe is still there and the coating is not permanent.
Differentiation
Support
Give students a copy of the table with the two rows needed for items 2 and 3 highlighted, and a filled-in model of one percent-decrease calculation using numbers not on the handout. Allow calculators throughout.
Extension
Ask students to explain why a utility might prefer orthophosphate over raising pH, considering that the secondary standard holds drinking water pH between 6.5 and 8.5 and that high pH can cause other problems in the distribution system. Have them write a short recommendation to a plant supervisor.
ELL supports
Pre-teach "corrosion," "coating," and "dissolve" by leaving a nail in water overnight and comparing it with a coated nail. Provide the sentence frame for item 8: "Orthophosphate lowers lead because it makes a ___ between the water and the ___."