Lab / experiment Grade 8 Science
Ravensworth Settling Column: How Time Clears Cloudy Water
A grade 8 lab where students track a settling column's clarity over 40 minutes, graph clarity against time, and connect the trend to sedimentation-basin design.
Objectives
- Students will collect clarity data from a settling column at set time intervals.
- Students will graph clarity against time and connect the pattern to how sedimentation basins work.
Vocabulary
- Sedimentation
- Sedimentation is the step where water sits almost still in a big basin so the heavy clumps of dirt can sink to the bottom. The cleaner water on top then moves on to the filters.
- Turbidity
- Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
- Detention time
- Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.
Also: settling
Teacher overview
Before a treatment plant filters water, it often lets the water sit still in a large basin so heavy particles can sink. That step is called sedimentation, and the time the water rests is the detention time. This lab lets students watch the same process in a jar, tracking how a cloudy sample clears as it sits. The sample stands in for raw water from the fictional Ravensworth source, and real basins are usually designed for 2 to 4 hours of settling.
Procedure
Make the raw water. Stir about 2 tablespoons of soil into a full jar of water until the water is evenly cloudy. This is the fictional Ravensworth raw-water sample. Do not drink or taste any water in this lab.
Record the start. Set the jar on the table and do not move it. Right away, hold the clarity card behind the jar and rate the water from 1 to 5, where 1 is very cloudy and 5 is clear. Write this down as the reading at 0 minutes.
Rate the water over time. Leave the jar still. Rate the clarity again at 5 minutes, 10 minutes, 20 minutes, and 40 minutes, using the same card each time. Keep the jar in one place so the settling is not disturbed.
Measure the settled layer. At 40 minutes, use the ruler to measure the height of the settled mud at the bottom in centimeters. Record it. Wash your hands after cleanup.
Data & Analysis
Fill in the clarity rating at each time. Then measure the settled layer at the end.
| Time (minutes) | Clarity rating (1 to 5) |
|---|---|
| 0 | |
| 5 | |
| 10 | |
| 20 | |
| 40 |
Settled layer height at 40 minutes: ____ cm
- Make a scatter plot of clarity rating (up) against settling time (across) using your five points. Describe the association: is it positive, negative, or none?
- Between which two time readings did the clarity improve the most? Use your data to answer.
- By how many clarity points did the water improve from 0 minutes to 40 minutes?
- Real sedimentation basins are designed for 2 to 4 hours of settling, far longer than your 40 minutes. Based on your graph's trend, would more time likely make the water clearer, level off, or get cloudier? Explain.
- A plant must reach 0.3 NTU, a much clearer reading than your best rating. Could settling alone, with no filters or chemicals, meet that standard? Why or why not?
- Sedimentation lets gravity pull particles down. Using your results, explain why a plant gives water hours of still time before filtering it.
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: Positive association.
As settling time goes up, the clarity rating goes up, so the scatter plot slopes upward. Accept "positive" with any wording that says clarity increases over time. - Answer: Usually the earliest interval, from 0 to 5 or 5 to 10 minutes.
The largest, heaviest particles drop fastest, so clarity often jumps most early. Accept the interval shown by the group's own data. - Answer: Accept the group's own difference between their 0-minute and 40-minute ratings, with the subtraction shown.
The value equals the 40-minute rating minus the 0-minute rating and depends on each group's soil, water, and card readings, so there is no single correct number. Most groups record an increase of about two to four points. Any positive difference that matches the group's recorded ratings is correct. - Answer: Clearer, but leveling off.
The trend keeps rising, so more time helps, yet the curve flattens because the finest particles settle very slowly. Accept "clearer" or "levels off" with reasoning tied to the graph. - Answer: No.
Settling removes only the larger particles; the tiny particles that keep water above 0.3 NTU need coagulation and filtration. Accept any answer explaining that settling alone leaves fine particles behind. - Answer: More still time lets more and finer particles sink.
Their own upward clarity trend shows that longer settling drops more sediment, which lightens the load on the filters that follow. Accept any answer linking longer settling to clearer water.
Differentiation
Support
Build one demonstration column as a class and rate it together at 0 and 5 minutes before groups start, so students practice using the clarity card. Provide a simplified data table with only three time readings (0, 10, and 40 minutes) for groups that need fewer data points.
Extension
Have students run a second column with twice as much soil and compare how the two settling curves differ. Ask them to predict, from their graph, the clarity rating at 60 minutes and explain how they extended the trend.
ELL supports
Give each group a picture card showing a cloudy jar and a clear jar side by side, labeled with the numbers 1 and 5. Provide sentence frames such as "At ___ minutes, the clarity was ___." Allow students to record ratings with the numbered card alone rather than written descriptions.