Lab / experiment Grade 8 Engineering & Design
Crayfen Ridge Pond Covers: Testing Three Evaporation Shields
A grade 8 lab in which teams run four-day evaporation trials on three cover designs, score each against stated criteria, and build a combined cover from the winners.
Objectives
- Students will measure evaporation loss from four identical containers over four days.
- Students will score three cover designs against stated criteria for loss, shade, and handling.
- Students will combine the best features of the tested covers into one design and predict its result.
Vocabulary
- Evaporation
- Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.
Also: control, shade, algae, criterion, iteration
Teacher overview
An open pond loses water to the air all summer, and a garden or a small irrigation system that stores water outdoors can lose a noticeable share of it before a single plant is watered. Covers reduce that loss, yet a cover that seals the surface completely also shades out light, changes how the water warms, and has to be moved by hand every time someone needs the pond. In this lab teams test three cover designs against an uncovered control for the fictional Crayfen Ridge Community Garden, score each against three stated criteria, then build a combined cover from the features the data supports. The design question is which cover a working crew would keep using, because a cover that performs well in a trial can still be too awkward to handle every day.
Procedure
Safety. Do not drink any water used in this lab. An adult cuts the foam sheet and shade cloth before class; students do not cut with blades. Keep the tray away from electrical outlets, wipe spills at once, and wash hands at the end.
Session 1, set up the four containers (20 min). Label the four containers A, B, C, and D with tape. Pour exactly 600 mL into each one using the graduated cylinder, then stand the ruler inside each container and record the starting depth in millimeters. Container A stays open and is your control. Container B gets a floating layer of ping pong balls or bottle caps covering the whole surface. Container C gets the foam sheet floating flat on the water. Container D gets the shade cloth stretched over four craft sticks so it sits about 3 cm above the water and does not touch it.
Session 1, agree on the protocol (15 min). Set all four containers on the same tray, in the same place, away from a window or vent that would hit one container harder than another. Write down three rules the whole team will follow: who reads the ruler, what time of day the reading is taken, and how the ruler is placed each time. A reading taken at a different spot on a tilted container is the most common source of error in this lab.
Session 1, run the light check (10 min). Slide the printed light-check card under each container in turn, look straight down through the cover in ordinary room light, and record whether the small text can be read. This is your measure of how much light each cover blocks, which matters because shaded water grows less algae.
Daily readings (5 min each day, four days). At the same time each day, read the depth in each container to the nearest millimeter and record it. Do not top up any container. If a container is bumped or spilled, write that in the notes column rather than estimating.
Session 2, analyze and score (20 min). Complete the data table, find the total depth lost by each container, and compare each cover's loss to container A's loss. Score every cover against the three criteria below.
Session 2, build the combined cover (20 min). Decide which feature of each tested cover your data supports keeping, and build one combined cover using those features. Predict what it should do, set it on a refilled container next to a fresh control, and start a second four-day run.
Clean up (15 min). Leave the combined-cover container and its fresh control on the tray. Pour the water out of the other containers, dry them and any unused cover pieces, return spare shade cloth and sticks, and wipe up any spills.
The criteria. A cover passes only if it meets all three. It must cut the depth lost by at least 40 percent compared with container A over the same days. The small text on the light-check card must not be readable under it, so algae growth is slowed. One person must be able to lift it off and set it back without tipping the container or dropping anything into the water.
Data & Analysis
| Container | Cover design | Day 0 depth | Day 1 | Day 2 | Day 3 | Day 4 | Total depth lost | Light-check result | Notes |
|---|---|---|---|---|---|---|---|---|---|
| A | Open, no cover | ||||||||
| B | Floating balls | ||||||||
| C | Floating foam sheet | ||||||||
| D | Raised shade cloth | ||||||||
| Combined | Built in session 2 |
- How much depth did the open control, container A, lose over the four days?
- Which of the three cover designs lost the least depth over the four days?
- Using your own numbers, what share of container A's loss did your best cover lose? Give the share as a percent and show the division you used.
- Which covers met the criterion of cutting the loss by at least 40 percent compared with container A?
- Which covers met the shade criterion? Cite your light-check result for each one.
- Which covers met the handling criterion? Describe what happened when you lifted each cover off and set it back.
- Which single cover met all three criteria? If none did, name the criterion each one failed.
- Describe the combined cover your team built, and name which feature came from which container.
- Which criterion do you expect the combined cover to fix, and what measurement in your table would show that it worked?
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: Varies by team
Accept any value read from the team's own table. Most classrooms see the open container lose several millimeters over four days, and a room with moving air loses more. A team recording no loss at all should check whether the ruler was read at the same spot each day.
Answer: Usually container C, the floating foam sheet
Accept whichever cover the team's data supports. A floating solid sheet normally loses the least because it touches the water and blocks nearly the whole surface, while the raised shade cloth allows air to keep moving across the water.
Answer: Varies by team
The division should be the cover's total loss divided by container A's total loss, written as a percent. A team whose best cover lost 2 mm while container A lost 8 mm reports 25 percent. Check that the student divided by the control's loss and not the other way around.
Answer: Varies by team
A cover meets this criterion when its loss is 60 percent or less of container A's loss, since losing at least 40 percent less means keeping no more than 60 percent of the control's loss. Accept any list that follows correctly from the team's own numbers.
Answer: Varies by team
Accept any list matching the team's recorded light-check results. The floating foam sheet and a dense shade cloth usually block the text, while ping pong balls and a loose-weave cloth often leave gaps that let enough light through to read it.
Answer: Varies by team
Accept any description grounded in what the team actually did. Teams commonly report that the foam sheet lifts cleanly, that the balls spill when the container is tipped, and that the raised cloth needs two hands because of the sticks.
Answer: Varies by team
Outcomes vary. The foam sheet often passes loss and shade, and it passes all three if the team's lift test went cleanly. The balls usually fail shade and often fail handling if they spill, and the raised cloth often fails the loss criterion. Require that each failure names a specific criterion rather than saying a cover did not work.
Answer: Varies by team
Accept any combined design whose parts trace to the tested covers, such as a floating foam sheet cut into two lighter halves with a fabric tab for lifting, or floating balls held under a light shade panel. Each feature must be matched to the container it came from.
Answer: Varies by team
Accept a single named criterion paired with the column that would show it. A team fixing the handling criterion should say so and point to a repeat of the lift test, while a team fixing shade should point to the light-check column of the combined row.
Differentiation
Support
Give these teams the containers pre-labeled and pre-filled, and a data table with the day headings and the control row already started from a teacher demonstration. Assign the roles of ruler reader, recorder, cover handler, and timekeeper, and keep the same roles for all four daily readings so each student practices one measurement well.
Extension
Ask the team to predict, before the second run begins, the depth its combined cover will lose, then compare the prediction to the result and explain the gap. Teams may also test whether the tray's position matters by running one duplicate control in a different part of the room, which shows how much of the difference between covers is really a difference in location.
ELL supports
Provide a labeled side-view diagram of each cover on a container, with the words floating, raised, control, and shade marked. Pair the light-check card with a three-step picture sequence showing slide, look, record. Offer sentence frames such as "Container ___ lost ___ millimeters, which is ___ than container A" and "I kept the ___ from container ___ because the data showed ___."
Extensions
Have teams scale their recommendation up to the garden's real pond by measuring the surface area of a container, comparing it to a stated pond area, and describing how many cover sections the crew would need and how they would be stored in winter. The storage question usually changes the recommendation, because a cover that performs best in the data is not always one a volunteer crew can handle twice a season.