Q1.
Valli goes for a nature walk with her Dadi and collects some water from a pond in a container. She observes some unwanted substances in it. Design and create a working model of water filter using low-cost materials.
Answer
Sample answer — a three-layer bottle filter. This is a design you can build in the classroom; write your own observations after testing it.
Things you need: one empty 2-litre plastic bottle, a clean cotton cloth or old cotton wool, washed coarse sand, washed fine sand, small pebbles or gravel, powdered charcoal from a burnt-out chulha, a rubber band and a glass to catch the water.
How to make it:
- Cut off the bottom of the bottle and turn it upside down, so the cap end points into the glass. Remove the cap.
- Tie the cotton cloth over the mouth with the rubber band. This is the finest layer and it goes at the bottom.
- Now fill, from the bottom upward: a layer of cotton → charcoal powder → fine sand → coarse sand → pebbles on top.
- Pour clean water through two or three times first, to wash the layers.
- Now pour the pond water in slowly at the top and watch what collects in the glass.
| Layer (top to bottom) | What it removes | Why |
|---|---|---|
| Pebbles / gravel | Leaves, twigs, insects | Largest gaps — stops only big things |
| Coarse sand | Grit and larger dirt particles | Smaller gaps |
| Fine sand | Fine mud, making the water clear | Very small gaps between grains |
| Charcoal | Colour and bad smell | Charcoal holds these on its surface |
| Cotton cloth | Anything still left; also holds the layers in | Finest pores of all |
Why the layers go coarse-on-top and fine-at-the-bottom: the big pieces are removed first, so the fine sand below does not choke immediately. This is exactly how a municipal water works treats river water — screens, then settling tanks, then sand beds.
Important: water from this filter looks clean but is not safe to drink. Filtration removes only what is big enough to be trapped; germs are far too small. Always boil the water afterwards.