Lab / experiment Grade 6 Science
Amberton's Cloudy Water: Building and Testing a Sand Filter
A grade 6 science lab where students build a layered bottle filter, run muddy water through it, rate cloudiness on a 0-to-10 scale, and calculate the percent removed.
Objectives
- Students will build a layered filter and run muddy water through it.
- Students will rate water cloudiness before and after filtering on a 0-to-10 scale.
- Students will calculate the percent of cloudiness their filter removed.
Vocabulary
- Filtration
- Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.
- 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.
Also: sediment, cloudiness
Teacher overview
The fictional town of Amberton pulls raw water from a muddy creek, so its plant must remove the cloudiness before the water reaches homes. Cloudiness, measured by scientists as turbidity, is one of the first things a plant treats. Real plants must keep finished water at or below 0.3 NTU in nearly all samples, a level far clearer than a classroom can measure. This lab lets students build a simple sand filter, rate the cloudiness before and after, and find the percent their filter removed.
Procedure
Work in groups of three or four. Wear goggles for the whole lab.
Step 1. Turn the cut bottle upside down so the neck points into a collecting cup. It works like a funnel.
Step 2. Build the filter from the neck up. Push a cotton ball into the neck. Add a layer of gravel, then a layer of sand, then a coffee filter on top.
Step 3. Stir your muddy water sample so the dirt mixes in. Rate its cloudiness on the 0-to-10 scale, where 0 is clear and 10 is very cloudy. Write the score as the "before" score for Trial 1.
Step 4. Slowly pour 250 mL of the muddy water through the filter. Pour gently, because pouring fast can push dirt through the layers.
Step 5. When the water finishes dripping, rate the collected water on the same 0-to-10 scale. Write it as the "after" score for Trial 1.
Step 6. Pour a second 250 mL sample from the same muddy water and repeat the rating for Trial 2, writing the before and after scores.
Step 7. Safety. Do not drink or taste any water in this lab. Wipe up spills right away so no one slips. Wash your hands when you finish.
Data & Analysis
| Trial | Cloudiness before (0-10) | Cloudiness after (0-10) |
|---|---|---|
| 1 | ||
| 2 |
What was your cloudiness score before filtering in Trial 1?
What was your cloudiness score after filtering in Trial 1?
Find the drop in cloudiness for Trial 1 by subtracting the after score from the before score. What does a bigger drop tell you about the filter?
Suppose a group measured a cloudiness of 8 before filtering and 2 after. What percent of the cloudiness did their filter remove? Use percent removed equals the change divided by the before score, times 100.
Use your own before and after scores from Trial 1 to find the percent your filter removed, with the same formula.
Which layer, the gravel, the sand, or the cotton, do you think trapped the smallest bits of dirt? Use what you saw.
Real plants must reach 0.3 NTU or lower, which is much clearer than this scale can show. Could your filter alone reach that level? Why or why not?
If you poured the same water through your filter a second time, would the cloudiness go down more, stay about the same, or go up? Explain.
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: Accept the student's own before score, which should be a high number because the water is muddy.
Answer: Accept the student's own after score, which should be lower than the before score if the filter worked.
Answer: The drop is the before score minus the after score. A bigger drop means the filter removed more cloudiness. Accept any answer that matches the student's own two scores and explains the difference.
Answer: 75%.
The change is 8 minus 2, which is 6. Then 6 divided by 8 is 0.75, and 0.75 times 100 is 75%.Answer: Accept the student's own value. It equals the student's before score minus the after score, divided by the before score, times 100.
Answer: Most groups will say the sand trapped the smallest bits. The water meets the coffee filter first, which catches clumps, then the sand, which holds much of the finer dirt. The gravel and cotton below mainly keep the sand from washing out. Accept any answer supported by what the student observed.
Answer: No. A classroom filter removes larger bits of dirt but not the tiny particles measured in NTU. Real plants add finer steps and constant testing to reach 0.3 NTU, so a bottle filter alone cannot meet it.
Answer: Most students should predict the cloudiness goes down more, because more filtering usually traps more dirt. Accept a "stay about the same" answer if the student reasons that the first pass already removed most of what the filter can catch.
Differentiation
Support
Build one filter as a class demonstration before groups build their own, and give a simplified data table with only Trial 1. Provide the percent formula worked once with different numbers so students have a model for questions 4 and 5.
Extension
Have students test a second filter design, such as adding a charcoal layer or a second sand layer, run the same muddy water through it, and compare the percent removed with their first filter. Ask them to state which design worked better and why.
ELL supports
Provide picture cards showing each layer (gravel, sand, cotton, coffee filter) labeled with its name, and a printed cloudiness scale with sample shades. Allow students to record scores with the numbered scale alone and to answer questions 6, 7, and 8 with short phrases.