Water filter challenge: what filtering removes and what it cannot · Concept Studio

The idea

A layered filter traps suspended particles by size, so muddy water comes out clearer, but dissolved substances pass straight through because their particles are far smaller than any gap in the filter.

Safety card

Low riskAn adult supervises

Hazards

cut plastic edges and scissors
soil can carry bacteria and fungi
hot basin during evaporation

Controls

bottles cut by the teacher; tape over cut edges
use garden soil rather than bagged potting mix, which carries a Legionella warning in Australia; gloves on and hands washed afterwards
the filtrate is never tasted or drunk: it has not been disinfected
evaporate over a water bath or on a warm windowsill; tongs for hot basins

Note

No hazardous chemicals; the risks are cuts, soil organisms and heat. Basic risk assessment per CSIS Section 1.7; Sydney Water's lesson plan includes a class risk assessment for this activity.

What you need

1 L and 2 L plastic soft-drink bottles (the teacher cuts them), screen mesh, cotton fabric, coffee filters, cotton balls, gravel and washed sand
250 mL of dirty water per trial: one tablespoon of garden soil and 5.0 g of table salt stirred into 250 mL of water (the salt stands for dissolved pollutants); a salt-only control of 5.0 g of table salt in 250 mL of clean water
measuring cups, clear plastic cups, stop clock, a 250 mL measuring cylinder over a printed black cross for the clarity test
two evaporating basins, balance reading to 0.01 g, water bath or warm windowsill, safety glasses and gloves

How to do it

Design a filter with at least three layers in the cut bottle and draw it. The challenge set by the NSW Department of Education: filter 250 mL of dirty water in less than a minute.
Pour the 250 mL through, time it, and record the volume collected in 60 seconds.
Clarity test: pour filtrate into the measuring cylinder standing on the cross until the cross can no longer be seen from above; record that depth. Do the same with the unfiltered water after stirring.
Evaporate 10.0 mL of filtrate and 10.0 mL of the salt-only control in weighed basins and weigh the residues.
Change one layer, run a second trial and compare speed, clarity and residue with the first.

What you should see

The filtrate is much clearer than the dirty water, so the cross stays visible through a much greater depth; finer layers give clearer water but a slower flow, the trade-off every filter design faces. The dissolved salt passes through untouched: 5.0 g in 250 mL is 20 g/L, so 10.0 mL of the salt-only control leaves 0.20 g of white residue and 10.0 mL of filtrate leaves about the same, a little more if minerals have dissolved out of the soil or fine clay has passed through. The unfiltered water is not used for this comparison because its suspended soil would add to the residue. The learner knows it worked when the filtrate is clearer by the cross test yet its residue is close to the 0.20 g left by the control, which shows filtering removes suspended particles but not dissolved ones.

What changes

What you change: filter design (choice and order of layers)
What you measure: volume filtered in 60 s, clarity depth, residue from 10.0 mL
What you keep the same: same dirty-water recipe
volume poured
bottle size
timing method

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Water that looks clear is clean and safe to drink.
A good enough filter can remove dissolved salt.
The more layers a filter has, the faster it works.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-SOL-01 SC4-WS-07
Australian Curriculum v9AC9S7U06AC9S7I02

Sources

The pages the author read to write this activity.