🔬 Filter Dirty Water
Build a layered filter from stones, sand, and cotton wool to clean muddy water — and learn how water treatment plants work!
🧰 What you need
Safety note: Ask an adult to cut the bottle — the cut edge can be sharp. Filtered water is NOT safe to drink.
🔬 Steps
- 1
Ask an adult to cut the plastic bottle in half. Flip the top half upside down and place it inside the bottom half as a funnel.
- 2
Layer your filter materials from top to bottom: small stones first (top of funnel), then gravel, then fine sand, then cotton wool balls at the very bottom.
💡 Each layer removes different-sized particles: stones catch big debris, sand catches smaller particles, cotton catches the finest. - 3
Mix a tablespoon of soil into a cup of water to make muddy water.
- 4
Slowly pour the muddy water through the top of your filter. Watch the layers capture the dirt!
- 5
Collect the filtered water that drips out. Compare it to the original muddy water — how much cleaner is it?
💡 Notice the colour of each layer after filtering — the top stone layer catches the biggest dirt, cotton catches the finest.
🧠 The Science
Physical filtration works by passing water through materials with tiny gaps. Large particles are trapped by the stones and gravel. Medium particles are caught by the sand. The finest particles are trapped by the cotton wool fibres. Each layer acts as a sieve for smaller and smaller particles. Real water treatment plants use the same principle on a huge scale, plus additional steps: chemicals to make particles clump together (coagulation), settling tanks, sand filters, and finally chlorination to kill bacteria. Our filter removes dirt, but the water still isn't safe to drink without that chemical treatment!
📚 Related Lessons
- 🏔️ Water Sources — Water comes from rivers, lakes, groundwater, rain, and dams — it travels a long journey to reach our taps!
- 🚰 Safe Water — Not all water is safe — germs, chemicals, and sewage contaminate water, but boiling, filtering, and chlorination make it safe!
- 🏭 Types of Pollution — Air, water, and soil pollution harm all living things — factories, vehicles, and waste are the main causes!