The Class Water Filter Challenge
The class must build a filter that meets two criteria: (1) it makes muddy water clear enough that a coin lying at the bottom of the output cup can be seen from above, and (2) it lets 200 mL of water pass through in under 5 minutes. Each filter may use at most three materials, in layers. Available materials: gravel, sand, cotton cloth, activated charcoal, small stones. Four teams tested their designs once each.
| Team | Layers (top to bottom) | Time for 200 mL | Coin visible? | | A | gravel, sand, charcoal | 12 min | yes | | B | gravel, sand | 4 min | partly | | C | sand, cotton | 1 min | no | | D | gravel, charcoal, sand | 4.5 min | yes |
1. Which team's filter best meets the challenge? Use both criteria in your answer, and explain why the other teams fall short.
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Team D is the only design that meets both criteria: 4.5 minutes is under the 5-minute limit, and the coin was visible, so the water was clear enough.
Team A produced clear water but took 12 minutes, more than twice the time limit, so it fails the second criterion however clean the water is. Team B was fast enough at 4 minutes but the coin was only partly visible, so it fails the clarity criterion. Team C was very fast at 1 minute, but its water was not clear at all, so it fails badly on clarity.
A design is only successful if it satisfies every stated criterion at once. Speed and clarity pull against each other - more layers and finer materials trap more particles but slow the flow - and D found the balance the rules asked for.
2. Team B wants clearer water without spending much more time. Suggest one change to their filter and predict what it would do to clarity and to flow time.
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Add one thin layer of activated charcoal or a layer of cotton cloth beneath the sand, keeping the total to no more than three materials.
Charcoal has a huge internal surface area that traps tiny particles and dissolved colour that sand lets through, so clarity should improve from 'partly' toward 'coin visible'. Cotton cloth acts as a final screen, catching fine sand and silt that slips past the gravel and sand.
Both changes will slow the flow, because finer material and extra layers resist water. With a thin layer of charcoal or a single cloth, the time may rise from 4 minutes to roughly 5-6 minutes. That is a risk: the team gains clarity but may break the 5-minute rule, so they should test a thin layer and, if needed, use coarser sand on top to keep the flow fast.
3. A student concludes from the table that charcoal is the best filtering material. Explain why the table does not support that claim, and describe the test that would.
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Charcoal only appears in teams A and D, and in both it was combined with other materials in different orders and thicknesses. Team A also had sand above it; Team D had sand below it. Team C filtered poorly using sand and cotton alone, but that says nothing about charcoal by itself. Each team tested once, so we cannot tell whether a result was typical or a lucky run.
To test charcoal fairly, keep the filter identical except for one layer: build two filters with the same gravel and the same mass and depth of sand, with charcoal in one and no charcoal in the other, and change nothing else. Run each three times with the same muddy water recipe and measure both clarity and flow time. Only then can the effect of charcoal be separated from the rest of the design.