Writing prompt Grade 6 Science
Does Thornmere Pond Freeze Solid? A Winter Argument
A grade 6 science argument prompt: students use a fictional winter temperature profile to explain why a pond freezes from the top down and stays liquid below.
Objectives
- Students will make a claim about whether a pond freezes to the bottom in winter.
- Students will support that claim with depth and temperature readings from a winter record.
- Students will explain why the coldest water in a pond sits at the top rather than the bottom.
Vocabulary
Key words: density, thermal energy, state of matter, depth profile, insulate
Teacher overview
Students write a short scientific argument about what winter does to a fictional pond, using a record of ice thickness and water temperature measured at four depths on three dates. The task asks them to reason from the pattern in the data rather than from a remembered rule, because the numbers show the coldest water sitting at the top and the warmest at the bottom, which is the reverse of what students expect in air. The misconception to name before students write is the belief that a pond freezes solid from top to bottom in a cold winter, and that fish either leave or die and return from eggs in spring. Ice forms a lid because solid water is less dense than the liquid beneath it, and that lid slows further heat loss. Students who hold the frozen-solid picture will read the depth profile as an error rather than as evidence.
Prompt
Background
Thornmere Pond sits in the fictional town of Kelsoe. Everything here about Thornmere and Kelsoe was written for this assignment. The pond is about five meters deep at its middle.
A neighbor told a sixth grade class that Thornmere "freezes solid every January." The class decided to check. On three winter days they drilled through the ice at the middle of the pond. Each time, adults from the county parks department measured the ice along the path before anyone walked out, and an adult ran the drill. They measured the ice thickness. Then they lowered a thermometer on a marked line and read the water temperature at four depths.
Here is the science they were given before the trip. Water is made of moving particles. Taking thermal energy away slows those particles down. For most of the cooling, the slowing particles pack closer together, so the colder water grows denser and sinks. Water is unusual, though. Once it cools past about four degrees Celsius, the particles begin to line up into a pattern that takes up more room instead of less. Water colder than that is less dense, so it floats. At zero degrees Celsius it changes state and becomes ice, and ice floats on the liquid water below it.
Table 1. Thornmere Pond winter record (fictional class data)
| Date | Air temperature | Ice thickness | 0.5 m deep | 1.5 m deep | 3 m deep | 4.5 m deep |
|---|---|---|---|---|---|---|
| December 15 | minus 4 degrees Celsius | 5 inches | 0.5 degrees Celsius | 2.0 degrees Celsius | 3.4 degrees Celsius | 4.0 degrees Celsius |
| January 20 | minus 14 degrees Celsius | 9 inches | 0.2 degrees Celsius | 1.8 degrees Celsius | 3.5 degrees Celsius | 4.1 degrees Celsius |
| February 24 | minus 9 degrees Celsius | 11 inches | 0.3 degrees Celsius | 2.1 degrees Celsius | 3.6 degrees Celsius | 4.0 degrees Celsius |
Two more observations from the same fictional trips
On January 20, the coldest day of the three, the drill went through the ice and then dropped into liquid water. The line hit the muddy bottom at 5.0 meters with water the whole way down. On February 24 the class caught two perch through the hole, looked at them, and let them go.
Your task
Write a scientific argument of four or five paragraphs. Answer both parts of this question: does Thornmere Pond freeze solid in winter, and why does the water near the bottom stay close to four degrees Celsius?
State a clear claim. Support it with at least three specific readings from Table 1 or from the two observations. For each reading, explain what it shows. Use the word density correctly at least once. Finish by naming one measurement the class should take next to test your explanation.
Planning Organizer
Fill in every row before you draft. Each row matches one part of the rubric.
| Part of the argument | What to write | My notes |
|---|---|---|
| Claim | One sentence answering whether the pond freezes solid, and one naming what holds the bottom near four degrees Celsius. | |
| Evidence 1 | An ice thickness reading with its date, and what the drill found underneath. | |
| Reasoning 1 | What does liquid water under thick ice tell you about the neighbor's claim? | |
| Evidence 2 | A full set of four depth readings from one date. | |
| Reasoning 2 | Where is the coldest water, and where is the warmest? Why is that the opposite of what happens in a room? | |
| Evidence 3 | A comparison across the three dates, either of ice thickness or of the deepest reading. | |
| Reasoning 3 | The air got much colder in January. What changed at 4.5 meters, and what does that say about the ice? | |
| Conclusion | What all this means for the perch, and one measurement the class should take next. |
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 (written at grade 6 level)
Thornmere Pond does not freeze solid. Ice forms a lid on top, the water under it stays liquid, and the water near the bottom stays close to four degrees Celsius because that is the densest water in the pond and it sinks and stays there.
The clearest evidence is what the drill found. On January 20 the ice was nine inches thick, and the air was the coldest of the three trips. The drill went through the ice and into water, and the line reached the mud at five meters with water all the way down. If the pond froze solid, there would have been nothing to lower the thermometer into.
The depth readings show a pattern that fits the science. On January 20 the water was 0.2 degrees Celsius just under the ice, 1.8 degrees at 1.5 meters, 3.5 degrees at 3 meters, and 4.1 degrees at 4.5 meters. The coldest water is at the top and the warmest is at the bottom. In a room the warm air rises and the cold air sinks, so this looks backward. It is not backward for water. Water is densest at about four degrees Celsius, so that water sinks to the bottom and the colder, less dense water floats above it.
Comparing the three dates shows how much the ice protects the water below. The air went from minus 4 degrees Celsius in December to minus 14 degrees in January, which is much colder. The ice grew from 5 inches to 9 inches. Yet the reading at 4.5 meters barely moved, from 4.0 to 4.1 degrees Celsius. The ice and the still water under it slow the loss of thermal energy, so the bottom stays about the same even when the air changes a great deal.
This is why the class could catch two perch in February. The fish had liquid water near four degrees Celsius to live in all winter. To test this explanation further, the class should measure the same four depths again in late March, after the ice melts. If the ice was holding the layers in place, the temperatures at all four depths should become much more alike once it is gone.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No claim, or the claim answers only one part of the question | Answers both parts but leaves the cause vague | Clearly answers both parts, naming the ice lid and the dense bottom water | Answers both parts and states the temperature at which water is densest |
| Evidence | No readings from the record | Uses one reading, or gives numbers with no dates or depths | Uses at least three specific readings with their dates and depths | Uses three or more readings chosen to test the claim, including a full depth profile |
| Reasoning | Restates a number without explaining it | Explains one piece of evidence | Explains what each reading shows about the pond in winter | Explains each reading, uses density correctly, and connects the ice lid to the steady bottom temperature |
| Organization and conventions | Hard to follow; frequent errors interrupt meaning | Some order; errors distract in places | Clear paragraphs, correct use of density, few errors | Clear paragraphs, precise vocabulary, a closing next measurement, and clean conventions |
Differentiation
Support
Give these students a blank pond cross section with the four depths already marked, and have them write the January numbers onto it before drafting. Provide sentence frames such as "On January 20 the water at 4.5 meters read ___, while the water just under the ice read ___." and "This shows that ___." Allow a three-paragraph response, and accept the labeled cross section in place of the final paragraph.
Extension
Ask students to predict what the profile would look like in late July, with reasons, and to say which single reading would change the most. A second option is to explain, in one paragraph, what would happen to pond life if ice sank instead of floated, using the record to say which layer would freeze first.
ELL supports
Pre-teach "density," "profile," "thermal energy," "state," and "insulate" with a labeled drawing of the pond in cross section, ice on top and arrows showing heat leaving. A clear cup with an ice cube floating in water makes the key point visible in seconds. Provide a bilingual glossary card, let students rehearse the organizer aloud with a partner, and accept a first draft that uses the sentence frames word for word.
Extensions
Have students find out how thick ice must be before their state considers it safe to walk on, and compare that number with the three thicknesses in the record. Make clear that students should never walk onto pond ice unless trained adults have checked that it is safe. A second option is to plan a summer trip to the same fictional pond: list the depths to measure, the time of day, and what pattern they expect to find.