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Lesson plan Grade 8 Engineering & Design

Morley Cross Hard Water: Choosing How to Soften a Supply

A 50-minute grade 8 lesson where students weigh central lime softening, ion exchange, and household rebates against hardness, chloride, cost, and fairness criteria.

Objectives

  1. Students will explain what water hardness is and what each softening method does to the water.
  2. Students will build measurable criteria that include an effect felt outside the water system.
  3. Students will recommend one softening option and name the trade-off the choice forces.

Vocabulary

Ion exchange
A water treatment method that swaps unwanted charged particles in the water for harmless ones held on tiny resin beads. It is used to remove PFAS and to soften hard water.
Brine
The extra-salty water left over after desalination pulls fresh water out of seawater. Plants must dispose of it carefully so it does not harm sea life.

Also: hardness, calcium carbonate, lime softening, chloride, scale

Teacher overview

Hard water carries dissolved calcium and magnesium, which leave scale inside water heaters and pipes and make soap harder to lather. Students act as engineers for the fictional town of Morley Cross, whose wells deliver water at a hardness of 320 mg/L, measured as calcium carbonate, and choose among central lime softening, central ion exchange, and a rebate that leaves softening to individual households. The misconception worth naming at the start is that softened water is simply cleaner or purer water. Softening is aimed at calcium and magnesium. It is not a general purification step and does not disinfect the water, and ion exchange works by swapping sodium into the water for the calcium it takes out, sending a salty brine down the sewer. Conventional wastewater treatment does not remove chloride, so it travels through the plant and into the river, which is why the choice is not only about what comes out of the tap.

Procedure

Hook (5 min): Show a photograph or a real scaled fixture and ask what the white crust is. Tell students that the wells serving the fictional town of Morley Cross deliver water at 320 mg/L hardness and that residents have asked the town to do something about it. Record two or three student suggestions on the board without judging them.

Hardness background (12 min): Explain that hardness is mostly dissolved calcium and magnesium that came from the rock the groundwater moved through. Describe the two central methods. Lime softening adds lime, which makes the calcium come out of solution as a solid that settles and is hauled away. Ion exchange passes the water through resin beads that hold sodium and release it as they grab calcium and magnesium, so the treated water carries more sodium and the resin must later be rinsed with salt, producing a brine. Then say where the brine goes: to the sewer, through the wastewater plant, and out to the Morley River, because chloride passes through conventional treatment unchanged. Ask the class what that means for anyone downstream, and let the answer sit before moving on.

Build the criteria table (8 min): Tell the class what Morley Cross cares about: water that does not scale fixtures, a river that does not get saltier, a total cost the town and residents can carry, and a result that is fair across households. Model turning the first concern into a criterion with a unit and a weight from 1 to 5, then let each group write the remaining three. Require that the river criterion name a measurable quantity rather than a feeling.

Group decision (15 min): In groups of three, students score all three options against their criteria on the handout, total the weighted points, and write their recommendation with the trade-off it forces. Circulate and press any group that ignores the chloride row to explain who bears that cost.

Share-out and exit ticket (10 min): Two groups present their recommendation and the criterion that decided it. As an exit ticket, each student writes one sentence explaining why softer water at the tap can still be a worse outcome for the river. Collect and check that at least three quarters of the class names brine or chloride specifically; if fewer do, reteach the path from resin rinse to sewer to river at the start of the next class.

Morley Cross Hard Water: Choosing How to Soften a Supply — Grade 8 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

You are engineers for the fictional town of Morley Cross. The town's wells deliver water at a hardness of 320 mg/L. About 1,600 households are connected. The wastewater plant discharges to the Morley River, and chloride is not removed by that plant.

Option A: central lime softening Option B: central ion exchange Option C: household rebates
Hardness delivered to the tap About 90 mg/L to every home About 60 mg/L to every home 320 mg/L, except in homes that install their own softener
Added chloride at the river None About 150 mg/L added to the plant discharge About 60 mg/L added if uptake reaches the town's estimate
What the town builds A softening basin and sludge handling at the plant Resin vessels and a brine system at the plant Nothing at the plant
Cost to build $3,400,000 $2,100,000 $0
Cost to run each year $110,000 $180,000 $400 rebate per household, to an estimated 40 percent of homes over five years
Waste produced Lime sludge hauled away by truck Salty brine to the sewer Salty brine to the sewer from each household unit

Answer every question using the table above.

  1. Which option delivers the lowest hardness to every home in the town, and what is that hardness?
  2. Explain why Option B lowers hardness at the tap while making the river saltier.
  3. Which option leaves some households with hard water, and why does that happen?
  4. Write the criterion your group used for the river, including the quantity and the unit you would measure.
  5. Which option does your group recommend, and which criterion decided it?
  6. Name one trade-off your recommendation forces the town to accept, and say who bears that cost.

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: Option B, at about 60 mg/L. Option B delivers the lowest hardness to every connected home. Option A reaches about 90 mg/L, and Option C leaves most homes at 320 mg/L.
  2. Answer: Because ion exchange moves the hardness rather than destroying it. Accept any response explaining that the resin holds the calcium and magnesium and releases sodium in their place, and that rinsing the resin with salt produces a brine that goes to the sewer, through a plant that does not remove chloride, and into the river.
  3. Answer: Option C. Accept any response noting that under a rebate only the households that choose to install a softener get soft water, so families who cannot afford the remaining cost, or who rent, keep water at 320 mg/L.
  4. Answer: Accept any criterion that names a measurable quantity and a unit, such as added chloride at the wastewater plant discharge in mg/L, with a stated ceiling the group chose. Do not accept "keep the river clean" or "protect fish" without a measurable quantity attached.
  5. Answer: Accept any of the three options when the recommendation follows from the group's own weighted totals. Groups weighting the river heavily commonly choose Option A, groups weighting tap hardness heavily commonly choose Option B, and groups weighting the town's construction cost heavily commonly choose Option C.
  6. Answer: Accept any real trade-off matched to the choice with the party who bears it named. Option A costs the most to build and produces sludge that must be trucked away, paid for by all ratepayers. Option B sends the most chloride to the river, borne by people and wildlife downstream. Option C leaves hard water in homes that do not take the rebate, borne by those households.

Differentiation

Support

Give these students the criteria table with all four criteria and their weights already filled in, so their work is scoring the options rather than writing the criteria. Highlight the hardness row and the chloride row on the option sheet, and provide a sentence frame for the exit ticket.

Extension

Ask students to propose a fourth option the town did not list, such as blending the hard wells with a softer source or softening only part of the flow, and to explain which criterion that option improves and which it leaves untouched. Strong responses recognize that partial softening trades a smaller benefit at the tap for a smaller load on the river.

ELL supports

Provide a labeled diagram of an ion exchange column showing resin beads, incoming hard water, softened water leaving, and the brine line to the sewer, with the key terms glossed in the student's home language. Offer sentence frames such as "Option ___ lowers hardness, but it sends ___ to the ___" and "I recommend Option ___ because the ___ criterion matters most to ___."

Extensions

Have students find out whether their own community's water is hard or soft by reading the annual water quality report their utility publishes, then write a short paragraph on which of the three options, if any, their utility appears to have chosen. Comparing the class findings usually shows that neighboring systems made different choices for defensible reasons, which is the point of the criteria table.