Writing prompt Grade 6 Science
Why Dunwilloe's Water Mains Break in February
A grade 6 science argument prompt using a fictional town's break log, frost depths, and repair notes to explain why water main breaks peak in late winter.
Objectives
- Students will make a claim about why water main breaks cluster in late winter.
- Students will support that claim with at least three pieces of evidence from a record.
- Students will describe what happens to water particles when thermal energy is removed.
Vocabulary
- Water main
- A big underground pipe that carries drinking water beneath the streets to a neighborhood. Smaller service lines branch off it to reach each building.
Also: frost depth, cast iron, particle, expand
Teacher overview
Students write a short science argument explaining a pattern in a fictional town's water main break records, using both the data and a particle-level account of what freezing does to water. The misconception this prompt is built to catch is the common belief that a water main breaks because the water inside it freezes and bursts the pipe. Crews almost never find ice inside a broken main, because water in a working main is moving and is well below the frost line. The ice forms in the wet soil around and above the pipe, and the frozen soil swells and shifts, bending a pipe that has no give left in it. Expect several students to write the burst-pipe explanation on their first draft; the repair notes in the prompt are included so they can correct it themselves.
Prompt
Background. Dunwilloe is a fictional town, and every record below was written for this assignment. Dunwilloe owns 58 miles of water main. Colvern Street has the oldest pipe in town. Last year the town wrote down every break and what the crew found.
Dunwilloe's break log:
| Month | Breaks | Average low temperature (degrees F) | Frost depth (inches) |
|---|---|---|---|
| November | 2 | 34 | 0 |
| December | 6 | 26 | 8 |
| January | 11 | 18 | 24 |
| February | 15 | 15 | 32 |
| March | 8 | 29 | 16 |
| April | 3 | 41 | 2 |
| July | 1 | 62 | 0 |
What the repair crews wrote down:
- Of the 58 miles of main, 41 miles are cast iron laid before 1955. Cast iron is strong but brittle, so it cracks instead of bending.
- The mains sit about 42 inches below the street. In February the frost reached 32 inches, so the soil directly above the pipes froze.
- Dunwilloe's soil is a heavy clay that holds water. Wet clay swells when it freezes.
- In 12 of the 15 February breaks, the crack ran all the way around the pipe, like a ring. Crews call that a bending break.
- No crew found ice inside a broken main all winter. Water kept moving through the pipes.
- System pressure stayed steady near 62 psi from November through April.
Your task. Write an argument that answers this question: why do most of Dunwilloe's water main breaks happen in February?
Your argument must:
- state a clear claim in the first paragraph,
- support the claim with at least three pieces of evidence from the log or the crew notes,
- include one paragraph that describes what happens to the water particles in the soil when thermal energy is removed, and why the ice takes up more room than the water did,
- end with a sentence explaining what the town should watch for next winter.
Write four to six paragraphs. Do not use outside sources. Everything you need is above.
Planning Organizer
Fill in every row before you draft. Each row becomes at least one sentence in your essay.
| Part of the argument | What I will write |
|---|---|
| My claim (one sentence answering the question) | |
| Evidence 1 (from the break log) | |
| What evidence 1 shows | |
| Evidence 2 (from the crew notes) | |
| What evidence 2 shows | |
| Evidence 3 (a crew note that rules out a wrong idea) | |
| What evidence 3 rules out | |
| Particle paragraph: what happens to water particles as heat leaves | |
| Particle paragraph: why the ice needs more room | |
| Ending: what the town should watch next winter |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response
Most of Dunwilloe's water main breaks happen in February because the soil around the pipes is frozen deepest that month, and frozen soil bends old cast iron until it cracks.
The break log shows the pattern clearly. February had 15 breaks, the most of any month, and it also had the deepest frost at 32 inches and the coldest average low at 15 degrees. July had the warmest temperatures, no frost, and only one break. The number of breaks rises and falls with the frost depth, month by month.
The crew notes explain how the frost does the damage. Dunwilloe's soil is a heavy clay that holds water, and wet clay swells when it freezes. The mains sit 42 inches down, and in February the frost reached 32 inches, so the ground right above the pipes turned to frozen, swollen clay. That soil pushes on the pipe from above and does not push evenly, so the pipe bends a little. In 12 of the 15 February breaks, the crack ran all the way around the pipe like a ring, which is the shape a pipe makes when it is bent rather than pushed from inside. The pipe is also 70-year-old cast iron, which is brittle and cracks instead of flexing.
Some people think the water inside the main freezes and bursts the pipe. The crew notes rule that out. No crew found ice inside a broken main all winter, and the water kept moving the whole time. Pressure also stayed steady near 62 psi, so nothing pushed harder from the inside in February than in November.
Freezing still matters, though, because of what happens in the soil. Water is made of tiny particles that are always moving. When thermal energy leaves the wet clay, those particles slow down. Below the freezing point they lock into a fixed pattern instead of sliding past one another. That pattern holds the particles a little farther apart than they were in liquid water, so the same amount of water takes up more room as ice. The extra room has to come from somewhere, so the soil swells and lifts.
Next winter Dunwilloe should watch the frost depth, not just the air temperature. When the frost line passes about two feet, the crew should expect breaks on the oldest cast iron streets first, including Colvern Street.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No claim, or the claim does not answer the question | States a claim that is vague or only partly answers the question | States a clear claim that answers the question in one sentence | States a clear claim and previews the reason behind it without listing the whole essay |
| Evidence | Uses no evidence from the records | Uses one piece of evidence, or restates numbers without connecting them | Uses at least three pieces of evidence and explains what each shows | Uses at least three pieces, explains each, and uses one of them to rule out the burst-pipe idea |
| Particle reasoning | No particle explanation, or says only that water freezes | Says ice takes more room but does not describe the particles | Describes particles slowing and locking into a pattern, and connects that to ice taking more room | Describes the particle change accurately and links it to the swelling clay that bends the pipe |
| Organization and conventions | Paragraphs are missing or ideas are out of order | Has paragraphs, but the order makes the argument hard to follow | Four to six clear paragraphs in a logical order with few errors | Clear paragraphs, smooth transitions, precise word choice, and almost no errors |
Differentiation
Support
Give these students the break log with the February row highlighted and the frost depth column circled, so the pattern is visible before drafting. Provide the claim as a sentence starter: "Most of Dunwilloe's breaks happen in February because ___." Allow three paragraphs instead of four to six, and let the particle paragraph be a labeled drawing of liquid water particles and ice particles with one or two sentences underneath.
Extension
Ask students to rank the six crew notes from most useful to least useful for answering the question, and to write a short paragraph defending the note they placed last. A second option is to graph breaks against frost depth on grid paper and describe in writing what the graph shows that the table alone did not.
ELL supports
Pre-teach "water main," "frost depth," "cast iron," "brittle," and "expand" with a labeled cross section showing the street, the frozen layer, and the pipe. Provide the essay frame with the first sentence of each paragraph written out. Allow students to talk through the organizer with a partner before writing, and accept the particle paragraph with a drawing plus two sentences.
Extensions
Have students find their own city or town's winter break count if it is posted online, or ask a public works office how deep local mains are buried and how deep frost usually reaches. Students can then write one paragraph comparing that burial depth with Dunwilloe's and predicting whether their town would see the same February peak.